AKARI FIS Data User Manual
Version 1.3
Eva Verdugo1, Issei Yamamura2 and Chris Pearson1,2
with contributions from:Mai Shirahata2 , Shuji Matsuura2 , Yoko Okada2 , Sin’itirou Makiuti2 ,Noriko Murakami3 and Mitsunobu Kawada3
1 European Space Astronomy Centre (ESAC), ESA
2 Institute of Space and Astronautical Science (ISAS), JAXA
3 Nagoya University, Japan

September 14, 2007

Version 1.3 (September 14, 2007)

Revision Record
Date
Mar 2007
Mar 2007Jun 2007
Sep 2007

Revision
Version 1.0
Version 1.1
Version 1.2
Version 1.3

Comments
Release of version 1.0
Updated toolkit setup instructions in Appendix A
Updated processing options explanation
FTS sections updated.
FIS 01/02 sensitivity numbers updated
Additional information about ghost added
Appendix A updated according to the new pipeline versioniContents
1 Introduction
1.1 Purpose of this document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Relevant information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
1
2

2 Instrument and AOT Overview
2.1 Hardware Speciﬁcation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Optics and Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Detector System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4 The Fourier Transform Spectrometer (FTS) . . . . . . . . . . . . . . . . .
2.1.5 Instrument Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 The FIS Pointed Observations: AOTs . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 FIS01: Slow-Scan Observations for Photometry / Mapping of Small Area
2.2.2 FIS02: Slow-Scan Observations for Mapping of Large Areas . . . . . . . .
2.2.3 Performance of FIS01 & FIS02 . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Saturation Limits for FIS01 & FIS02 . . . . . . . . . . . . . . . . . . . . .
2.2.5 FIS03: FTS Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6 Performance of FIS03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4
5
5
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8
9
12
13
13
15
17
18
19
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3 Instrumental characteristics
21
3.1 Imaging Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.1 Point Spread Function (PSF) . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.2 Cross-talk and ghosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.3 Leakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.4 Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.5 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.6 Stray Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Detector and instrument responsivity . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 Dead pixels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.2 Responsivity and Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.3 Excess Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2.4 Ramp curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.5 Saturation After Eﬀect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.6 Transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.7 Spectral Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Charged Particle Hits (Glitches) . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4 Diﬀuse and Point Source Confusion . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4.1 Confusion Due to Diﬀuse Background Emission . . . . . . . . . . . . . . . 33
3.4.2 Confusion Due to Point sources . . . . . . . . . . . . . . . . . . . . . . . . 33
3.5 FTS mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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Version 1.3 (September 14, 2007)
3.5.1
3.5.2
3.5.3
3.5.4
3.5.5
3.5.6

iii

Transient Eﬀects on the FTS data . . . . . . . . . . . . . . . . . . . . . .
Detector Response Inhomogeneity . . . . . . . . . . . . . . . . . . . . . .
Spectrum Reproductivity . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fringes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interference from the Cryocooler . . . . . . . . . . . . . . . . . . . . . . .
Detector position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35
35
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37

4 Data processing
4.1 AKARI All-Sky Survey (ASS) Pipeline . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Out-line of the data ﬂow . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 Green Box Modules used in the Slow-Scan data reduction process . . . .
4.2 Slow-Scan processing and calibration: FIS01 and FIS02 . . . . . . . . . . . . . .
4.2.1 Detecting glitches of cosmic ray-hits . . . . . . . . . . . . . . . . . . . . .
4.2.2 Calculation of signal current . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Dark signal subtraction and ﬂat-ﬁelding . . . . . . . . . . . . . . . . . . .
4.2.4 Conversion to surface brightness . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 Background oﬀset subtraction . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.6 Making images by co-adding multi-pixel data . . . . . . . . . . . . . . . .
4.2.7 Image processing for multiple pointings . . . . . . . . . . . . . . . . . . .
4.2.8 Photometric calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 FTS spectroscopy processing: FIS03 . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Brief description of each process . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 Further limitations in the functionality of the current version of the FTS
toolkit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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39
40
41
41
41
41
42
43
44
45
45
48
48
49
50

5 Caveats in the Data Processing
51
5.1 Treatment for transient eﬀects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.2 Straylight correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3 Photometric calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3.1 Scan speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3.2 Calibrators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.4 Attitude Determination During the Pointed Observations . . . . . . . . . . . . . 52
5.5 Pixel position table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.6 Projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
6 Instrument Related Data Products
6.1 Time Series Data (TSD): Overview and structure . . . . . . . . . . . . . . . . . .
6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Physical ﬁle format of TSD . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Nomenclature of Data Type . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.4 Time stamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.5 Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.6 TSD: detailed description . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Calibration Constant Files (CCF) . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 CCF Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2 List of CCFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53
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54
54
55
55
73
73
76

iv

AKARI FIS Data Users Manual

A FIS Slow-Scan Toolkit Cookbook
77
A.1 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
A.1.1 Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
A.1.2 Distribution package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
A.1.3 Setting and Starting up ﬁsdr under Unix environment . . . . . . . . . . . 78
A.1.4 Setting and Starting up ﬁsdr under the Windows environment . . . . . . 80
A.2 The Slow-Scan tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
A.2.1 Running ss run ss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
A.2.2 Available options for ss run ss . . . . . . . . . . . . . . . . . . . . . . . . 83
A.2.3 Tips and suggestions for Slow-Scan data reduction . . . . . . . . . . . . . 88
A.2.4 Behind ss run ss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
A.3 The FIS Data Visualizer (FISv) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
A.3.1 Main Functions: Input/Output . . . . . . . . . . . . . . . . . . . . . . . . 93
A.3.2 Main Functions: Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
A.3.3 Main functions: Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 99
A.4 Contribution tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
A.4.1 Making mosaic images from multiple observations . . . . . . . . . . . . . 100

Chapter 1

Introduction
1.1

Purpose of this document

This manual is intended to give the FIS observer all necessary information to produce their data
products.
The FIS IDUM should make the observer familiar with the FIS instrument, its operations,the output data, the calibration and the processing. To achieve this the following subjects are
addressed:• The FIS instrument and its operations• Some characteristics of the instrument that have implications for the data
• An overview of the data processing• The calibration of the data and the accuracy• A guide to the data products• How to start with the data
• A cookbook for the usage of the processing tools
Since this manual is intended to give the observer up to date information on the processing
and the calibration of the FIS instrument, it will be updated regularly. Every change in the FIS
data reduction software, the calibration procedures or the ﬁle descriptions will be reﬂected inthis manual.

1

AKARI FIS Data Users Manual2

1.2

Relevant information

AKARI Observer’s Web
The ISAS Web page contains the most up to date information:
URL:http://www.ir.isas.jaxa.jp/AKARI/Observation/
The ESAC page also includes up to date information:
URL:http://akari.esac.esa.int/observers/
Helpdesk
Any questions and comments on AKARI observations and user support are addressed to
the AKARI Helpdesks:
iris help@ir.isas.jaxa.jp
http://akari.esac.esa.int/esupport/

Version 1.3 (September 14, 2007)

3

Chapter 2

Instrument and AOT Overview
The Far-Infrared Surveyor (FIS ) Instrument is designed primarily to perform the All-Sky Survey
in four wavelength bands. The instruments are operated such that data acquisition is continuously made as the telescope scans the sky, resulting in sets of strip data of sky brightness.
This operation can also be used for pointed observations in a Slow-Scan mode. The FIS is also
equipped with a Fourier Transform Spectrometer (FTS) that enables imaging spectroscopy overthe full FIS wavelength range. FTS observations are only made as pointed observations.

4Version 1.3 (September 14, 2007)

2.1

Hardware Speciﬁcation

2.1.1

Overview

5

The speciﬁcations of the FIS instrument are summarized in Table 2.1.1. It provides four photometric bands between 50 and 180 μm, two broad bands and two narrow bands. Individual
detector systems are implemented for the two short and two long wavelength bands, respectively.
The FTS covers the entire FIS wavelength range with a resolution of ∼ 0.36 cm−1 (R = 450–150)
or ∼ 2.4 cm−1 (R = 75–23).
Table 2.1.1: Hardware Speciﬁcations of the Far-Infrared Surveyor (FIS)
Photometric mode
Band
N60
WIDE-S
WIDE-L
N160
Wavelength Range (μm)1 50–80
60–110
110–180
140–180
Central Wavelength (μm) 65
90
140
160
Band Width (μm)2
21.7
37.9
52.4
34.1
Detector
Monolithic Ge:Ga3
Stressed Ge:Ga
Array size
20 × 220 × 315 × 315 × 2
Operational Temperature∼ 2.0 K
∼ 2.0 K
Pixel size (arcsec)4
26.8
44.2
4
Pixel pitch (arcsec)
29.5
49.1
Readout
Capacitive Trans-Impedance Ampliﬁer (CTIA)
Spectroscopic mode
Spectrometer
Martin-Puplett type Fourier transform spectrometer
Wavelength (μm)
N/A
60–110
110–180
N/A−1
Resolution (cm )N/A
0.36 / 2.4
N/A
1 The detector responsivity is higher than 20% of the peak.
2 As deﬁned by the bandwidth equation below
3 The SW detector was manufactured by NICT.
4 The values may change as more accurate telescope parameters become available.

The Central Wavelength is given to be a representative number of the band near the weightcenter of the band proﬁle. The band width is then deﬁned by the following equation for a ﬂat
spectrum, νFν =const.


r(ν)T (ν)Fν (ν)dνr(ν)T (ν)dν/ν=.
Δν ≡r(νc )T (νc )Fν (νc )r(νc )T (νc )/νc

(2.1.1)

Together, the responsivity (r) and optical transmittance (T ) deﬁne the system responsivity(R, the relative spectral response function : RSRF.

AKARI FIS Data Users Manual

6

2.1.2

Optics and Filters

The FIS Optics
Figure 2.1.1 shows the bottom view looking out of the telescope (telescope and the sky are into
the page) from the FIS instrument. The light path is indicated by the dash-dotted arrows.
Optical elements are labeled. There are three moving parts, a shutter at the light entrance,
a ﬁlter wheel, and the moving mirror of the FTS. The ﬁlter wheel can be rotated to switch
between photometric and spectroscopic (FTS) modes. The shutter will be closed occasionallyto obtain instrumental dark levels.
Blocking Filters(σc~300cm-1 & 430cm-1)

Collimator Mirror

Bending Mirror

100mm

Input Aperture (with shutter)
Black-Body Source
MovableRoof-Top Mirror

Holeor
Input Polarizer

SCANNER

Mirror Transport Mechanism
Filter Wheel
Beam Splitter(Polarizer)

Dichroic Beam Splitter (σc~ 91cm-1)
or
Output Polarizer

Fixed
Roof-Top Mirror

SPECTROMETER

Camera Mirror

Camera Mirror

Low-Pass Filter (σc~90cm-1)
Low-Pass Filter (σc~70cm-1)

Weight: ~5.5kg

Calibration Sources

Low-Pass Filter (σc~220cm-1)
High-Pass Filter (σc~133cm-1)

Long Wavelength Detector(stressed Ge:Ga; < 1.8K)

Short Wavelength Detector(Ge:Ga; ~2.2K)

Figure 2.1.1: The optical paths within the FIS. Locations of the calibration lamps are indicated.

Filters and Photometric bands
Figure 2.1.2 describes the ﬁlter conﬁguration of the FIS photometric bands. The LW narrow
band ﬁlter was replaced in 2004 with a new ﬁlter with a cut-oﬀ at 70 cm−1 (143 μm). Thisextended the band proﬁle shortward and results in a better eﬀective responsivity. Following thischange, we now denote the longest FIS band as “N160” instead of the previous name N170.
Calibration Lamps
Two sets of calibration lamps are installed in the FIS. The Cal-A system consists of two lamps
placed in front of the SW and LW detector arrays. They can be operated individually so thatthe optimal calibration procedure for each detector array can be realized. An important roleof the Cal-A lamp is to produce a pulse ﬂash to simulate the passage of a real point source.
The pulse ﬂashes were planned periodically (e.g., every minute) to monitor and correct the time
variation of the detector responsivity1 . However, analysis of the PV-phase observations showedstrong after eﬀects due to slow transient response (see Section 3.2.6) thus the periodically pulse
ﬂashes (every minute) are now omitted for the pointed observations.
1
In the current calibration sequence the duration of pulse is 0.47 sec for the LW array and 0.24 sec for the SW
array

Version 1.3 (September 14, 2007)

7
Incident photons

Blocking Filter ( c=430cm-1 & 300cm-1 )Dichroic Beam Splitter ( c=91cm-1 )LPF ( c=90cm -1)

LPF ( c=220cm-1 )LPF ( c=70cm-1 )(~HPF ( c=133cm-1)

N160
WIDE-L-14 m) (~-110 m)stressed Ge:Ga array

WIDE-S(110-60 m)

N60( -50 m)Ge:Ga array

for FTS mode
dichroic BS is replaced by output polarizer

Figure 2.1.2: The ﬁlter conﬁguration of the FIS photometric bands. LPF is the Low Pass Filter.
Occasionally the lamps are continuously turned on to provide a stable irradiation. In addition, the Cal-B system is a lamp located in the middle of the FIS optical path. Its’ light passes
through the beam splitter reaching both detector arrays. This Cal-B system will be used as a
backup for the Cal-A system. Both of these two sets of lamps are used solely for monitoring therelative responsivity variation of the detectors. The absolute ﬂux calibration will be made withcelestial objects.
In addition to the calibration lamp sets, there is a black body source on the inner wall. It is
used for calibration of the FTS.
Shutter
There is a shutter at the entrance aperture of the FIS. The instrumental dark level can bemeasured by closing the shutter. During the All-Sky Survey, the shutter is closed periodically tomonitor the dark level. Leakage at the 1/200 (at every passage of the polar regions) level has been
observed in the pre-ﬂight measurement. No signiﬁcant leakage has been observed although adetailed quantitative evaluation is yet to be made. The internal calibration sequence, calibration
lamps and shutter operation for pointed observations are described in the AOTs (Astronomical
Observation Templates).

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2.1.3

Detector System

ArraysThe FIS has two kinds of detector arrays, a set of two monolithic Ge:Ga arrays2 for the SWchannel (50–110 μm) and a stressed Ge:Ga array for the LW-channel (110–180 μm). They areoperated at ∼ 2.0 K. Both the SW and LW channels consist of two photometric bands; i.e,
the FIS will have four bands in total, which are operated simultaneously in scan mode. In the
spectroscopy mode, only WIDE-S and WIDE-L are operated.
Figure 2.1.3 shows the FIS detector arrays in detail on the Focal-Plane. The upper detectors
with three rows are the WIDE-S and WIDE-L bands, and the lower ones with two rows are theN60 and N160 bands. The FIS detector arrays are tilted with respect to the scan direction by
26.5 deg such that the interval between the neighbouring scan paths of the detector pixels is one
half of the physical pitch of the pixels. As noted above, the SW and the LW detector arrays
observe almost the same area on the sky. The slight diﬀerence in the sky coverage is due to thediﬀerence of the projected array size.

Figure 2.1.3: Focal-Plane layout of the FIS detectors.

Detector Readout Modes
The readout circuit of the FIS is a Capacitive Trans-Impedance Ampliﬁer (CTIA). The signal
from each pixel is accumulated in the capacitor as the detector observes far-infrared photons.
The voltage at the capacitor is then read out at appropriate intervals. The charge level in thecapacitor is periodically reset in order to avoid saturation. The accumulated signal between
two resets is called a Ramp. Diﬀerentiation of the Ramps provides the data that should be
proportional to the ﬂux of the incoming radiation. The reset operation of the FIS detectors
2

The SW detector was manufactured by NICT.

Version 1.3 (September 14, 2007)9

is synchronous, i.e., resets are inserted with a set time interval, regardless of the signal levelof each pixel. Although the reset interval is carefully optimized to maximize the instrument
performance, bright objects passing through the FIS FoV will possibly cause saturation of thedetector signal.
Three diﬀerent detector readout modes are considered.
Simple sampling mode is the basic operation sequence. Each sampled signal is downlinked
to the ground after A/D conversion. This mode is used for the FTS observations.
Co-add (Nominal) mode samples the ramps at a higher rate, then the sums of every n sam√plings are transfered to the ground. This can reduce the noise by a factor of ∼ n while
preserving the data size, (n is ﬁxed as 6 in the current design). The Co-add mode isreferred to as the nominal mode and is generally used for both the All-Sky Survey andthe Slow-Scan modes (FIS01, FIS02) of the FIS.
CDS (Correlated Double Sampling) mode is designed to avoid saturation in very brightregions, such as the inner galactic plane. The detectors are reset at every two samplings,
and on board diﬀerentiation of these two samplings is retrieved as the signal. Therefore,
the downlinked data is already diﬀerentiated. This mode is occasionally used in the All-Sky
Survey and Slow-Scan mode (FIS01, FIS02).
Bias Light for the LW detectors
The Ge:Ga detectors generally have a strong transient response, i.e. the detector does not
respond instantaneously to the incoming ﬂux but rather has a delay of a few – hundreds seconds
time scale (see Section 3.2.6). In the All-Sky Survey, a point source passes across a detector pixelin 1/4–1/7 seconds. This is too short for the detectors to reach a steady level corresponding to
the source ﬂux. Accordingly the point source sensitivity decreases. This eﬀect is particularly
serious for the LW detectors (stressed Ge:Ga detectors), and in the low ﬂux condition (dark sky+ weak sources).
In order to reduce the impact of this eﬀect, we decided to add a ‘bias light’. This takes
the form of a set of thin stainless wires put in front of the LW aperture. During the survey a
constant current will be used to give a steady photon ﬂux into the detector array. The point
source sensitivity for the All-Sky Survey is expected to recover by a factor of a few with this‘bias light’.
The SW detectors do not require such a mechanism.

2.1.4

The Fourier Transform Spectrometer (FTS)

The FIS provides the opportunity for imaging spectroscopy over its full wavelength range (60–180μm) using the two wide-band arrays (WIDE-S and WIDE-L). The Fourier transform spectrometer of the FIS is of the Martin-Puplett type (Figure 2.1.4). This polarizing type Michelsoninterferometer uses input and output polarizers to create a linearly polarized beam, and anotherpolarizer to split the beams to a ﬁxed and moving mirror to make the interferogram. The input and output polarizers are mounted on the ﬁlter wheel. These polarizers reduce the opticaleﬃciency of the instruments by 1/4 (ideal case) compared to the photometric mode. The total
optical eﬃciency is thus 10–20%. The moving mirror, driven by an electromagnet, can shift±9.2 mm from its physical center. The optical center (zero path position) is displaced by 4.6mm from the physical center and the optical path length is asymmetric to the center.
Two preset operation modes are provided for observers; Full-resolution mode and SED mode
(Figure 2.1.5). They are in principle diﬀerent only in the mirror scan path length, and accordingly in the resultant spectral resolution. The Full-resolution mode uses the full physical path

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Fixed Roof-top Mirror
Camera Mirror

Movable Roof-top
MirrorCollimeter
Mirror90LPF

Main Beam 430BKF&
Splitter(Polarizer) 300BKF

Bending
MirrorLong Wavelength
Detector (WIDE-L)

Bending
Mirror

Liot Stop

Short Wavelength
Detector (WIDE-S)

Input Polarizer

Output Polarizer
218LPF

Bending Mirror
Camera Mirror

Figure 2.1.4: The optical diagram of the FIS Fourier Transform Spectrometer (FTS).
length, while the SED mode scans ±4.6 mm with respect to the optical center. The mirror
moving speed is always a constant 0.367 mm/sec for SED mode and 0.382 mm/sec for FullResolution mode. Taking an interferogram (scanning one-way) takes 48 sec in Full-Resolution
mode and 12 sec in SED mode.
Full-Resolution mode
9.2mm

= 1 / 2.76 cm

0.36 cm-1

9.2mm

0mm
0mm
48 sec-27.6mm-9.2mm

Optical PL

Mechanical PL

SED mode
4.25 mm
0 mm-4.25 mm

96 secDrive Speed = 0.367 mm/sec= 1 / 0.425 cm

2.4 cm-1

6.375 mm
2.125 mm
0 mm
48 sec96 secDrive Speed = 0.382 mm/sec

Optical PL

Mechanical PL

Figure 2.1.5: The mirror driving patterns of the FIS/FTS. There are two modes, Full-Resolution
mode and SED mode.
The spectral resolution, deﬁned as 1/L (L is the optical path length) is about 0.36 cm−1 for
the Full-Resolution mode and 2.4 cm−1 for the SED mode, respectively. Note that these values

Version 1.3 (September 14, 2007)11

assume apodization at the spectrum reconstruction. Better spectral resolution may be obtainedby optimizing the data reduction procedure. The speciﬁcation of the FTS mode is summarizedin Table 2.1.2.
In FTS mode, the WIDE-S and WIDE-L are operated simultaneously, with a higher sampling
rate than in the photometric mode in order to properly sample the interferogram. The narrow
band detectors, N60 and N160, are not used.
Table 2.1.2: Speciﬁcations of the FIS Fourier Transform Spectrometer (FTS) mode.
Wavelength (μm)
Operation mode
Mechanical Path Length (mm)Optical Path Length (mm)One scan time (sec)Resolution (cm−1 )Resolution λ/Δλ60–110 (WIDE-S), 110–180 (WIDE-L)
Full-ResolutionSED−9.2–+9.2+2.1–+6.3
27.6
4.25
48
12
0.36
2.4
450–150
75–23

The actual position of the ﬁelds of view (FOV) of the detectors for the FTS mode are oﬀset
from one another compared to the originally designed position and is shown in Figure 2.1.6.
Initial WIDE-S(pre-flight)

53"

30"

Initial WIDE-L(pre-flight)

66"
Scan Base(SW mode)

102"
62"

4"

Scan Base

61"

Ar
1
ra
2
y
Ba
21
3
se
22
4
41
23
5
Current
WIDE-S
1
42
24
6(post-flight)
43
7
2 25
16 44 45 236 27 8 917 46 4 428 2 10 1
FIS03_P3={0}18 7 48 95 30 1 1231
(Moderate mode)
19 49 50 361 32 13 1432
20 51 5 733 3 15 1
33
21 2 53 48 35 6 1734
22 54 55 396 37 18 1935
23 56 5 1038 3 2036
24 7 58 19140
37
25 59 60 12
3
8
FIS03_P3={1}

105"
Ar
ra
y

39

40

26

41

Current WIDE-L(post-flight)

27

42

Ba
se

13

28

43

14

29

44

15

30

45

FIS03_P3={2}Scan Direction(LW mode)

Figure 2.1.6: Actual schematic view of the FOV of the FIS detectors in FTS mode.

AKARI FIS Data Users Manual

12

2.1.5

Instrument Operation

The data acquisition process of the FIS is programmed into the hardware sequencer. When the
instrument begins an observation, a preset speciﬁed sequence pattern is repeatedly carried out
and the data are edited into telemetry packets and sent to the satellite’s main computer (Data
Handling Unit: DHU). The data sampling timing is as accurate as the FIS internal clock and isquite stable. The FIS data acquisition is continuous and reset of detector charge and calibration
lamps and the shutter are controlled by the FIS on-board software with an accuracy as good as
the readouts. They can alternatively be operated by commands if necessary. The status of the
shutter and calibration lamps are all recorded for the data processing.
The survey observation sequence and the AOTs (Astronomical Observation Templates) aredesigned by combining these elemental operation units.

Version 1.3 (September 14, 2007)

13

2.2

The FIS Pointed Observations: AOTs

2.2.1

FIS01: Slow-Scan Observations for Photometry / Mapping of Small
Area

The FIS01 AOT is designed for photometry of point sources and/or mapping of small areas up
to ∼ 25 × 10 arcmin2 .
Table 2.2.3: FIS01 summary.

Observing Mode
Band
Scan pattern
Sampling mode
Reset interval
Slow-Scan speed
Shift length∗ = Co-add mode

Fixed parameters
Photometry
N60, WIDE-S, WIDE-L, N160
Two round-trip scans with a cross-scan shift
User speciﬁed parameters
Nominal∗ or CDS
0.5 or 1 or 2 sec (for Nominal sampling mode)
8 or 15 arcsec/sec
70 or 240 arcsec[Instrument Operation]
The instrument is operated in the photometry/imaging mode; all four FIS photometric bands(N60, WIDE-S, WIDE-L, N160) retrieve data. The detector operation parameters are ﬁxed at
the recommended values provided by the instrument team. Two readout modes are possible:
Nominal and CDS.

[Scan Operation]
An observation consists of two sets of round-trip scans in the in-scan direction with a shift in
the cross-scan direction. The round trip scan is mandatory to ensure data redundancy. The
cross-scan shift is used to increase the redundancy or to observe a wider area of the sky. The
scan speed is either 8 or 15 arcsec/sec. These speeds are selected to maximize the performance(sensitivity) while enabling observations of a certain size area of sky.

[User speciﬁed parameters]
The following inputs are given by the observer.
Detector readout mode (Nominal / CDS) CDS mode should only be used for the brightest targets that would normally saturate the detector in Nominal mode. See Section 2.1.3
for the details of the readout methods.
Reset interval This parameter is only relevant for the Nominal readout mode and is chosen
based on the brightness of the targets. Generally, a longer reset interval improves sensitivity in the ampliﬁer but at the same time increases the risk of saturation. On the other
hand, a shorter reset looses more data points at the resets.

AKARI FIS Data Users Manual14
Step_scan
Shut_cls
975
Step 30s
Scan_p15630

Pre cal sequenceSettling time
Shut_opn 420.3

Scan_p15
Shut_opn
1005

Post cal sequence
Shut_cls 1351

Cal on Cal off
990
997.6

Scan area length:0.66 deg for 15”/sor
0.35 deg for 8”/s

On target
708.75896.251083.75

On target
1271.25

area centerorsourceEach scan 157.5s

Shut_cls 787.5

Turn 30s
w /cal sequence

Each scan 157.5s

Shut_cls
817.5
Shut_opn 1162.5

Shut_opn 1192.5

Cal_off 810.1

Cal_off 1185.1

802.5
Scan_m15
cal_on1177.5
Scan_m15
cal_onTurn 30s
w/ cal sequence

Figure 2.2.7: The scan sequence of the AOT FIS01 (numbers are in seconds).
Scan parameters Two numbers are given to deﬁne the scan pattern; A scan-speed and across-scan shift length. A scan-speed of 8 arcsec/sec balances depth and sensitivity withscan area for data redundancy and is used for photometry of small numbers of sources. A
higher scan speed of 15 arcsec/sec is used to observe larger areas to a slightly shallower
sensitivity.
A cross-scan shift length of 70 arcsec is used for point source photometry, and 240 arcsec
for small area mapping.
Target position The satellite is operated such that the target position is located at the centerof the scanned area.

Figure 2.2.8: Example of FIS01 in-ﬂight data from the PV phase. Time series raw signal withpre and post calibration sequences and the three w/cal sequences can be seen.

Version 1.3 (September 14, 2007)

2.2.2

15

FIS02: Slow-Scan Observations for Mapping of Large Areas

The FIS02 AOT is designed for the mapping of relatively large areas. An observation covers along strip of ∼ 1 degree.
Table 2.2.4: FIS02 summary.
Fixed parameters
Observing Mode PhotometryBand
N60, WIDE-S, WIDE-L, N160
Slow-Scan speed 15 or 30 arcsec / sec
Scan pattern
One round trip scan
User speciﬁed parameters
Sampling mode
Nominal∗ or CDS
Reset interval
0.5 or 1.0 or 2.0 sec
Slow-Scan speed 15 or 30 arcsec/sec∗ = Co-add mode

[Instrument operation]
The operation of the instrument with this AOT is the same as FIS01, except that FIS02 carries
out a single round-trip only, and that the choice of scan speed is 15 or 30 arcsec/sec.

[Scan operation]
The diﬀerence of this AOT from FIS01 is that an observation consists of only a single round-trip
scan. The scan speed is chosen from 15 or 30 arcsec/sec. The scan length is approximately 1
and 2 deg. depending on the scan speed.

[User speciﬁed parameters]
The following inputs are given by the observer.
Detector readout mode (Nominal / CDS) Same as FIS01.
Reset interval Same as FIS01
Target position The satellite is operated such that the target position is located at the centerof the scanned area.

AKARI FIS Data Users Manual

16630 Scan_p15

Pre cal sequenceSettling time

Post cal sequence

Shut_opn 420.3

Scan area length:1.44 deg for 15”/sor
0.77 deg for 8”/s

Shut_cls 1351

On target
802.5

On target
1177.5area centerorsourceEach scan 345s

Shut_cls 975
Turn 30s
w /calsequence

1005 Shut_opn
997.6 Cal_off
990 Scan_m15, Cal_on

Figure 2.2.9: The scan sequence of the AOT FIS02.

Version 1.3 (September 14, 2007)

2.2.3

17

Performance of FIS01 & FIS02

Photometry (imaging) observations by the FIS in pointed observation mode is always carriedout with the Slow-Scan operation. At the moment the performance of Slow-Scan mode has
not been systematically evaluated, because of various technical problem on the calibration anddata reduction. Therefore, the performance given in this section are estimated by scaling the
All-Sky Survey detection limits by factors which are functions of the scan speed and the reset
interval. However, because of the nonlinear responsivity, transient eﬀects, and other complicatedcharacteristics of the instruments, the scaling law is not always straightforward. Table 2.2.5
lists the sensitivities for the three scan speeds incorporated into these 2 FIS AOTs, Note that
confusion is not included in these estimates.
The numbers in the table are 5σ noise levels for a single scan. In principle, the sensitivity
should increase as the inverse square root of the number of scans such that four scans in FIS01
should improve the detection limit by factor of 2. On the other hand, this redundancy canalternatively be used to increase the reliability of the results. In such a case, the detection limit
remain unchanged. We leave this at the discretion of the user. As the “standard” rule of thumb,
we propose to apply the numbers√ per one round-trip scan. So for FIS01, which carries out tworound trips, a sensitivity of 1/ 2 of the value in the table can be assumed.
Following the resent results of careful analysis of the FIS in-orbit performance the sensitivityof the FIS has been further updated from the previous IDUM. In summary, the SW bands (N60,
WIDE-S) detection limits are worse than the pre-launch predicteions (i.e. as documented in the
Observer’s manual) by factor of 2–3. For the LW bands (N160, WIDE-L), various factors havebrought the sensitivity to a level a factor of four (0.5 sec integration) to eight (2 sec integration)worse with respect to the pre-launch values published in the Observers Manual.
It should be mentioned that these numbers are not the results of source extraction andnumber counting. Further technical charanges are needed to draw the maximum performanceof the FIS instrument.
Table 2.2.5: The 5σ detection limits for the FIS AOTs FIS01 & FIS02 (See Tables 2.2.3, 2.2.4
for relevant scan speeds for the two AOTs). Numbers are per scan, see text for details.

Scan speed
Reset
N60
WIDE-S
WIDE-L
N160
Scan speed
Reset
N60
WIDE-S
WIDE-L
N160

0.5 s
260
52
130
670

8 arcsec/sec
1.0 s 2.0 s
180
130
39
26
95
67
470
330

0.5 s
5.6
1.2
1.0
5.0

8 arcsec/sec
1.0 s 2.0 s
4.5
2.8
1.0
0.6
0.7
0.5
3.5
2.3

Point Source (mJy)
15 arcsec/sec
CDS 0.5 s 1.0 s 2.0 s CDS
3800
400
280
200
6100
890
84
60
42
1400
1600
200
140
100
2400
7560 1000 710
500 11000
Diﬀuse Source (MJy/sr)
15 arcsec/sec
CDS 0.5 s 1.0 s 2.0 s CDS
97
8.4
5.6
4.3
120
22
1.8
1.2
0.9
28
8.5
1.3
0.9
0.7
12
41
6.7
4.8
3.3
57

0.5 s
630
130
390
1900

30 arcsec/sec
1.0 s 2.0 s CDS
420
420
9800
92
92
2200
270
230
4400
1400 1100 21000

0.5 s
11
3.0
2.0
9.7

30 arcsec/sec
1.0 s 2.0 s CDS
8.4
8.4
170
1.8
1.8
42
1.3
1.1
24
6.8
5.5
110

The performance of the FIS detectors is inﬂuenced by cosmic ray hits. The induced glitches
in the detector signal (at a frequency of about of approximately one per minute) contributes tothe ﬁnal noise ﬁgure. After such hits, the noise increases by a factor of a few for around ten
seconds. This degrades the S/N ratio worse than the values listed in Table 2.2.5. Therefore,

AKARI FIS Data Users Manual18

we recommend for observers to repeat scans several times in order to obtain good sensitivity
close to the limits in Table 2.2.5 . The performance of the LW bands (Stressed Ge:Ga detector
array) is particularly worse than pre-launch prediction. This is attributed to two factors. Before
launch, we expected that the responsivity should increase at least by a factor of two in space, (asobserved in the case of IRAS, ISO, IRTS, and Spitzer). However, after launch, the bias voltage
had to be reduced in order to avoid unstable behavior. As the result of this, the responsivity
is, now, nearly equal to that on the ground and the detection limit has correspondingly been
degraded compared to the pre-launch expectations. In addition to this, it was assumed, before
launch, that the noise of the LW channels would depend on the integration time as the power of
t−1 , that is, S/N ∼ t−1 based on the ground test results. However, after launch, the observed
noise of LW arrays instead was found to follow a dependence of on the time as t−1/2 , that is,
S/N ∼ t−1/2 . As the result of these two factors, the LW sensitivities are degraded by a factor of
8 (= 2 x 4) in 2-second integration, and by a factor of 4 (= 2 x 2) in the 0.5-second integration
case.

2.2.4

Saturation Limits for FIS01 & FIS02

The saturation limits are functions of the reset interval only. These limits are deﬁned such thatthe integration ramps of the high sensitivity pixels reach a level where the reproductivity of
the ramp shape becomes bad. Beyond this level we still obtain data from the lower-sensitivity
pixels but photometric accuracy and quality of reconstructed image map may be signiﬁcantly
degraded. The in-ﬂight measurements of the saturation limits are yet to be assessed although
may be similar to those quoted in the Observer Manual. Table 2.2.6 gives the (Observers
Manual) saturation levels for these FIS AOTs.
Table 2.2.6: The saturation limits for FIS01 and FIS02 AOTs. Note that at present thesesaturation numbers have not been conﬁrmed in the ﬂight conﬁguration
Point Source (Jy)Reset (sec) 0.5 1.0
2.0
N60
280 14070
WIDE-S
60
30
15
WIDE-L120 5015N160
300 130
60
Diﬀuse Source (GJy/sr)
Reset (sec) 0.5 1.0
2.0
N60
4.0 2.0
1.0
WIDE-S
0.8 0.4
0.2
WIDE-L1.0 0.42 0.12N160
2.5 1.10 0.5

CDS
3600
700
1500
3500
CDS
50
10
13
30

Version 1.3 (September 14, 2007)

2.2.5

19

FIS03: FTS Spectroscopy

The FIS03 AOT is designed for spectroscopic observations with the FTS.
Table 2.2.7: FIS03 summary.
Fixed parameters
Observing Mode Spectroscopy
Band
WIDE-S, WIDE-L
Scan pattern
Staring (no Step-Scan)
User speciﬁed parameters
Sampling mode
Nominal (ﬁxed)
Reset interval
0.1, 0.25, 0.5, or 1.0 sec (see text)
Resolution
Full-resolution / SED
Target position
0, 1, 2 (see text)[Instrument operation]
The instruments are set up for the FTS mode. The ﬁlter wheel is rotated to switch from theempty hole to the polarizer. Detector readout mode is switched to high-frequency sampling
mode. Only data from the WIDE detector arrays are acquired. During the observation, themoving mirror is driven. Two path lengths, i.e., spectral resolutions, are available (See Section 2.1.4). The spectral resolution is ∼ 0.36 cm−1 for the Full resolution mode and ∼ 2.4 cm−1for the SED mode. The SED mode provides better quality spectra than the Full-resolution mode
spectra after being smoothed to a corresponding resolution. There are four choices for the reset
interval, similar to the other FIS AOT’s. However, CDS readout mode is not available.

[Scan operation]
The FTS spectroscopic observation is operated in a staring pointing mode. To minimize thedead time, No Step-Scan, Micro-Scan, nor Slow-Scan is carried out during a pointed observation.

[User speciﬁed parameters]
The following inputs are given by the observer. Target position is a new parameter added after
launch.
Resolution mode Full-resolution or SED. See the description above.
Reset interval Four choices from 0.13 , 0.25, 0.5, and 1.0 sec are available. As the CDS mode
for the brightest targets cannot be used for the FIS03 AOT, a shorter reset interval of 0.1
sec is alternatively provided. This shortest reset interval is available thanks to the 5–7
times faster detector sampling rate in this operation mode.
Target position (New) Due to the misalignment of the detectors (see Section 2.1.3 and Figure 2.1.6) three diﬀerent positions are deﬁned:
Position 0: Target at the overlapping region of the two detectors (pixel 7 for WIDE-L
and pixel 33 for WIDE-S)
3

The actual reset interval is 0.14 sec.

AKARI FIS Data Users Manual

20

Position 1: Target is located on the 2nd column of the WIDE-S detector array near pixel
28,29.
Position 2: Target is at the center of the WIDE-L array (pixel LW 23).

2.2.6

Performance of FIS03

The ﬂight sensitivity of the FIS03 (FTS) mode is known to be worse than the pre-ﬂight values
given in the Observers’ Manual. Although a quantitative evaluation of the ﬂight-performance isgiven below, users of FIS03 are still urged to contact the FTS team in Japan via Helpdesk.
The ﬂight sensitivity of the FIS03 (FTS) mode has been estimated from the signal-to-noise
ratio of bright sources for a portion of the observation modes and speciﬁc pixels. Noise is deﬁned
as the dispersion between diﬀerent scans in one pointed observation.
The sensitivity for the continuum emission is estimated from Neptune observed with the
AOT position parameter 0, i.e. it is derived only for the SW33 and LW07 pixels. The 5σ
detection limit is roughly 20Jy for 65–85 cm−1 (LW07), and 50Jy and 100Jy for 90–120 cm−1and > 120 cm−1 , respectively (SW33).
The sensitivity for the line emission is estimated from diﬀuse bright regions towards the
Galactic center. For [C II] 158 μm and [O III] 88 μm in LW30 and SW15, respectively, the 5σ
detection limit is estimated to be 7.3 × 10−8 and 8.0 × 10−7 W m2 sr−1 , respectively. Considering
the relative sensitivity among diﬀerent pixels (Fig. 3.5.20) and the estimate that the PSF in
spectroscopic mode is larger by ∼ 20% than that in photometry mode (section 3.1), these
correspond to 5σ detection limits of 3 × 10−15 W m−2 and 5 × 10−14 W m−2 for [C II] 158 μm
and [O III] 88 μm, respectively, with the AOT position parameter 0.
If the reset anomaly is not correctly interpolated, it produces artiﬁcial features at the corresponding wavenumber. Table 2.2.8 lists the possible wave numbers which are aﬀected by these
artiﬁcial features.
Table 2.2.8: The list of the possible wave numbers aﬀected by artiﬁcial features caused by reset
anomaly.
Reset interval [sec]Wave number aﬀected by the reset structure [cm−1 ]

0.1
0.25
0.5
1.0

SED mode
96.9, 193.8(48.4), 96.9, 145.4, 193.8(24.2, 48.4,) 72.7, 96.9, 121.1, 145.4, 169.6, 193.8(12.1, 24.2, 36.3, 48.4,) 60.6, 72.7, 84.8, 96.9, 109.0, 121.1, 133.2, 145.4,
157.5, 169.9, 181.7, 193.8

0.1
0.25
0.5
1.0

Full-Resolution mode
93.1, 186.2
46.5, 93.1, 139.6, 186.2(23.3, 46.5,) 69.8, 93.1, 116.4, 139.6, 162.9, 186.2(11.6, 23.3, 34.9, 46.5,) 58.2, 69.8, 81.5, 93.1, 104.7, 116.4, 128.0, 139.6,
151.3, 162.9, 174.6, 186.2

Chapter 3

Instrumental characteristics
In this chapter the diﬀerent eﬀects (due to the properties of the detector, instrumental eﬀects
caused by the electronics and optics and other external inﬂuences) to be corrected during theprocessing of the FIS observations are described.

3.1

Imaging Quality

3.1.1

Point Spread Function (PSF)

It was impossible for the AKARI team to carry out a ground based end-to-end performance test
for the telescope and instruments in the cryogenic environment. After the launch, the telescopefocus was adjusted using the IRC data. During the PV phase and the calibration time in Phase 1& 2 various measurements of the PSF were carried out. As expected, the actual image from the
FIS observations is inﬂuenced by many complicated instrumental characteristics. Investigationof the most reliable PSF information is still ongoing. Only preliminary data is shown at the
moment.
Figure 3.1.1 and Figure 3.1.2 compare the measured PSF with a Gaussian and theoretical PSF model (Airy function of the telescope + FIS instrument optics). The central part(±2 arcmin for SW, ±3 arcmin for LW) of the observed PSFs can be well ﬁtted by a Gaussianfunction of FWHM=37 ± 1, 39 ± 1, 58 ± 3, & 61 ± 4 arcsec for N60, WIDE-S, WIDE-L, and N160
respectively. The measured PSFs are based on the observations of asteroids. Theoretically, thePSF of broad band imaging observations depends on the colour of the target. However, wehave not detected any signiﬁcant diﬀerence between the PSF measured from the observationsof asteroids and luminous infrared galaxies. The evaluation is continuing.
1

N60

Model
Gaussian
Data

0.8

Normalized Intensity

Normalized Intensity

1

0.6
0.4
0.2
0
-180

Wide-S

Model
Gausian
Data

0.8
0.6
0.4
0.2
0

-120

-60
0
60
Distance [arcsec]

120

180

-180

-120

-60
0
60
Distance [arcsec]

120

180

Figure 3.1.1: Comparison of the measured and modeled PSF for the SW detector.

21

AKARI FIS Data Users Manual221

Wide-L

Model
Gaussian
Data

0.8

Normalized Intensity

Normalized Intensity

1

0.6
0.4
0.2
0
-180

Model
Gausian
Data

0.8
0.6
0.4
0.2
0

-120

-60
0
60
Distance [arcsec]

120

180

-180

-120

-60
0
60
Distance [arcsec]

120

180

Figure 3.1.2: Comparison of the measured and modeled PSF for the LW detectors
There are large deviation in the ‘skirt’ part of the measured PSF from the model. The reason
for this deviation has not yet been well understood.

3.1.2

Cross-talk and ghosts

Cross-talk between the detector pixels will broaden the eﬀective FWHM of a point source image.
Moreover, it will increase the eﬀective number of glitches, and complicate the transient response.
This eﬀect is more serious in the SW detector, since it is a monolithic structure.
N60

WIDE-S

WIDE-L

N160

Figure 3.1.3: Cross-talk and ghost eﬀects as seen in a FIS01 observation of a bright point source.
See text for details.
A FIS01 observation of a bright point source is shown in Figure 3.1.3 (the colour scale isadjusted to emphasise the faint features). Points to note are:
• The target on the SW images does not look like a point source but shows long extensions along the detector array dimensions. It is more signiﬁcant in the direction of 20
pixels (cross-scan). These extensions are considered to be due to cross-talk between the
pixels. Both optical cross-talk and electrical cross-talk possibly contribute to this. The
phenomenon has not yet been completely understood. therefore, at present, users should
administer special care when they discuss the detailed morphology of targets.
• We also see spider-like features around the target that are probably due to the telescope
optics.

Version 1.3 (September 14, 2007)23• We observe ghost images in all detectors. The ghost image on the N60 detector appears
when the target object is passing through the WIDE-S detector. The same relation holds
for ghosts on WIDE-S, as well as the WIDE-L, N160 pair (though it is not clearly recognized in the WIDE-L band in Figure 3.1.3). The cause of these ghost images is not yet
clear. The strength of the ghost image is less than 1% (probably ∼ 0.3% of the target
in the same image for the SW, and around 10% in the LW detectors). Users should takespecial care when looking at images of bright targets.
Figure 3.1.4 and 3.1.5 show time proﬁles of the detector signals from wide and narrow banddetectors. These plots clearly demonstrate that the ghost signal appears in one detector when
the other detector observes a bright object.

Output Siganl [V/s]

Calibration
Sequence

0.012

SW

ON-Source@Wide-S
ON-Source@N60

Calibration
Sequence

Channel-49 (Wide-S)
Channel-69 (N60)

0.008

0.004

0.000

Ghost
@Wide-S

600

Ghost
@N60

700

800

900

1000

Time [s]Figure 3.1.4: Time proﬁle of signals from two SW detector pixels during one round trip scanof a Slow-Scan observation of a very bright source. The target passes from N60 to WIDE-S,
then back again after the calibration sequence in the middle of the operation. A ghost signal isobserved when the other detector was looking at the target.
Calibration
Sequence

Output Siganl [V/s]

0.030

LW

ON-Source@Wide-L

Calibration
Sequence

Channel-38 (Wide-L)
Channel-53 (N160)

0.025
ON-Source
@N160
0.020
Ghost
@N160
0.015

0.010
600

700

800

900

1000

Time [s]Figure 3.1.5: Same as Figure 3.1.4 but for the LW detectors. Only the ghost in the N160 isshown. Ghosts in the WIDE-L band appear in diﬀerent detector pairs and are not shown in thisplot.
The relative displacement of the ghost from the source object are approximately evaluatedAKARI FIS Data Users Manual24

and summarized in Table 3.1.1. The direction of the ghost is most easily understood when the
NARROW and WIDE images are aligned in time domain such that the edges in the in-scandirection of the two images are overlaid to each other. The ghost appears where the source isobserved in the other bands. See Figure 3.1.3.
Table 3.1.1: Relative displacement of the ghost from the source object. The uncertainty of thevalues is about 0.2 arcmin.
Detector
In-scan (arcmin)
Cross-scan (arcmin)
Distance (arcmin)

3.1.3

SW
5.4
0.2
5.4

LW
3.6
2.0
4.1

Leakage

A combination of two blocking ﬁlters at the entrance aperture are expected to reduce the shortwavelength light leakage to factors of 10−5 at 10 μm and 10−9 at 0.5 μm optimally. However,no end-to-end measurement has been made and the leakage could be as worse as 10−5 –10−6 at
0.5 μm. Note that we have not detected any signature of blue light leakage from the blocking
ﬁlter so far.

3.1.4

Distortion

Figure 3.1.6 shows the detector shape projected onto the sky. Due to the fact that the FIS
aperture entrance is about 20 arcmin away from the center of the focal plane (boresight), andthat the FIS optics are an oﬀ-axis design, signiﬁcant distortion of the detector shape is observed(red lines). The shape without optical distortion is also plotted for comparison (blue). Thisdistortion data is provided as part of the data reduction pipeline package. The distortion will befurther evaluated with the in-ﬂight data and the information will be correspondingly updated
in the future.
FIS-SW Distortion Pattern [2005-June]
CORNER
1
2
4
3

InScan [arcmin]

3
4
5
1 2

3 4

5 6

7 8

2
1

15

1 2

3 4

5 6

7 8

9 10
1112
1314
1516
1718
1920

9 10
1112
1314
1516
1718
1920

CORNER
4

25

3

InScan [arcmin]

25

20

FIS-LW Distortion Pattern [2005-June]

WIDE-S

3
4
5
1

20

1
2
2

3

1

15

2

3

4

5

6

N60

10

10
0

5

10
CrossScan [arcmin]

15

20

4

5

6

2
1

0

5

7

8

9

7

10

8

11

9

12

10

13

11

14

12

13

WIDE-L
14

15

N160
15

10
CrossScan [arcmin]

15

20

Figure 3.1.6: The FIS detectors projected onto the sky, with (red) and without (blue) optical
distortion.

Version 1.3 (September 14, 2007)

3.1.5

25

Polarization

The dichroic beam splitter used in the FIS is known to have polarization. In Figure 3.1.7 thetransmittance and reﬂectivity of the ﬁlter is given. The reﬂection was measured at two diﬀerent
angles perpendicular to each other. A diﬀerence in performance at a few speciﬁc wavelengths at
approximately 160 and 180 cm−1 (corresponding to 63 and 55 μm, respectively) was observed.
Unfortunately the angle of the ﬁlter in the ﬂight module instruments is not accurately known,
and this diﬀerence will result in an uncertainty in the measurement of the polarized light.

Figure 3.1.7: The reﬂectivity and transmittance of the dichroic beam splitter. The blue andgreen curves are the reﬂectivity in the SW channel side. The two curves deviate around 160 and
180 cm−1 .

3.1.6

Stray Light

It is found that the light from the Earth can enter the cryostat under certain conﬁgurations of
the current orbit. Ideally, such light should be reﬂected back outside the telescope into space,however, a small amount of scattered light may in fact enter the telescope and be detected in
the IRC and FIS instruments.
The stray light is most strongly observed over about 100 days with a maximum at the Summer
solstice, when the telescope is observing the northern sky. The eﬀect was most signiﬁcant duringobservations of the NEP region in Phase 1. Most of the Mission programmes and Open Timeproposals were carried out after September, when the stray light should be much smaller.
The intensity of the stray light seems to be 20–100% of the dark sky. Also, it is larger at the
beginning and end of a pointed observation.
Two methods are suggested to remove stray light eﬀects in the standard Slow-Scan data
reduction tool (Section 4.2.5). For point source observations, median ﬁltering methods canremove all large scale radiation components.

26

AKARI FIS Data Users Manual

Figure 3.1.8: Time proﬁle of a WIDE-S detector signal during a Slow-Scan observation overabout 15 minutes. the stray light component is seen as a global variation of the backgroundlevel. Brightness scale is based on an old calibration and may not be accurate.

Version 1.3 (September 14, 2007)

27

3.2

Detector and instrument responsivity

3.2.1

Dead pixels

The SW- and LW-arrays are known to have 4 and 7 dead pixels, respectively. In addition one
SW pixel has a large noise level and an unstable signal output. Figure 3.2.9 shows the positionsof the dead and more badly performing pixels. In addition, one SW pixel (98) is very noisy andone LW pixel (42) shows strong non-linearity in the integration ramps. They are masked by thedata reduction toolkit and not used in the data reduction.

Figure 3.2.9: The positions of dead (blue cross) and badly performing (green cross) pixels in the
FIS detector arrays. Deﬁnitions of the pixel numbering indices are also presented.

3.2.2

Responsivity and Uniformity

Figure 3.2.10 shows the in-ﬂight and pre-ﬂight responsivities of all the SW detector pixels.
The values are calculated for a νIν = constant spectrum at the central wavelength deﬁned inTable 2.1.1. The same information is shown for the LW detectors in Figure 3.2.11.

-3

SW Responsivity [(V/s)/(MJy/sr)]10

In-flight
Pre-flight

-4

10

-5

10

0

20

40

60

80

100

Pixel number

Figure 3.2.10: In-ﬂight and pre-ﬂight responsivities of all pixels in the SW detector arrays. See
ﬁgure 3.2.9 for the deﬁnition of the pixel number.

AKARI FIS Data Users Manual28-3

LW Responsivity [(V/s)/(MJy/sr)]10

In-flight
Pre-flight

-4

10

-5

10

0

10

20

30

40

50

60

70

Pixel number

Figure 3.2.11: In-ﬂight and pre-ﬂight responsivities of all pixels in the LW detector arrays. See
ﬁgure 3.2.9 for the deﬁnition of the pixel number.

3.2.3

Excess Noise

After the results of the early FM performance tests, the FIS introduced two noise ﬁlters, theﬁrst in the cryogenic part near the detector and the other outside the cryostat. These ﬁlters
successfully reduced the noise signiﬁcantly.
The typical frequency of the excess noise is close to the data sampling rate. Therefore,
the noise component especially aﬀects the All-Sky Survey observations which uses every data
sampling. A software module to reduce this noise component has been implemented into thesurvey data reduction pipeline and is working successfully. On the other hand, the Slow-Scandata reduction toolkit calculates the slope of every integration ramp to derive the sky signal andis relatively robust to the high-frequency noise.

Figure 3.2.12: Noise spectral proﬁle of a SW detector pixel. Time variation of the noise spectra
taken during the laboratory test under dark conditions. White stripes are excess noise components. We observe a number of components at speciﬁc frequencies. Some of them move withtime.

Version 1.3 (September 14, 2007)

3.2.4

29

Ramp curve

Ideally the output signal of the FIS detector should be proportional to the number of incoming
photons, but this is not the case in reality. The actual ramp curves show deviations from theoptimal linear line even when the data is taken under constant incoming radiation. The SW
detector shows this eﬀect more prominently (Figure 3.2.13). As the slope of the ramp curve
gives the instantaneous ﬂux, any deviation from linearity means that the responsivity changes
along a ramp. Currently, the SW ramp curve becomes ﬂatter as an integration progresses. The
non-linearity of the LW detector is 5–10%.
If the shape of the ramp curve is reproducible, and if the curve is monotonic with incoming
ﬂux, it will be possible to correct this eﬀect.
Analysis of the ﬂight data has shown that the ramp shape is relatively stable over a period ofseveral months. The ramp curve correction module works eﬀectively, except in the case of a few
pixels that require further investigation. in addition, the ramp curve correction for observations
in extremely dark or bright sky conditions can be further improved.

Figure 3.2.13: Comparison of integration ramps under constant irradiation levels. The LW
detector (right) shows an almost linear response while the SW signal (left) deviates from linearity
as the integration progresses.

3.2.5

Saturation After Eﬀect

It is found that the saturation of the detector is followed by a decrease in the signal level(Figure 3.2.14). This eﬀect is more obvious in the LW channel. The eﬀect appears moresigniﬁcantly when the saturation is heavier. It seems that the eﬀect is already present when the
integrated signal has reached about half of the full saturation level. The recovery time scale is of
the order of a minute. This eﬀect is still under quantitative analysis and is not yet implemented
in the data reduction software.

3.2.6

Transient response

It is known that the Ge:Ga type detectors show a strong transient response; the detector output
signal does not respond instantaneously to a change of incoming ﬂux but rather has a delay of
a few – hundreds seconds time scale.
Figure 3.2.15 demonstrates how the transient response of the FIS detectors changes witha step change of incoming ﬂux. In order to simulate the real ﬂight conditions, a calibration
lamp is turned on weakly to provide a background level for the SW detector, and a Bias Light(Section 2.1.3) is used for the LW array. In both these cases, the time constants are 10–30 sec.
Since AKARI/FIS almost always observes the sky in scanning mode, an understanding of
the eﬀect of the transient response to the observed data is essential. The most extreme case is

AKARI FIS Data Users Manual

30

Figure 3.2.14: An example of Saturation after eﬀect for a LW detector pixel. The diﬀerential
signal for a constant irradiation is plotted against time after the saturation. In this ﬁgure an
attempt is made to ﬁt the data by a two component exponential function. More detailed analysis
is ongoing.

the All-Sky Survey, in which a point source is scanned in only 0.2–0.4 sec, much smaller than
the transient time constant, resulting in a lower output signal than expected by a factor of a few(Figure 3.2.16). This also aﬀects the detection limits. The eﬀect is less severe for the Slow-Scanmode for the pointed observations.
Ideally, the transient response should be understood and reproduced by modeling the chargetransfer in the detector element. However, strong non-linear physics prevent us from making aperfect model. The analysis is continuing. For the All-Sky Survey data, the point source ﬂux isplanned to be corrected empirically by tables comparing the signals for the instantaneous andcontinuous ﬂux levels.
0.003

0.006
SW 17ch

LW 11ch0.005
differential output (V)differential output (V)0.00250.002

0.00150.001

0.0005
0

0.004
0.003
0.002
0.001

0

200

400
600
800
sampling @24Hz

1000

1200

0

0

100

200

300
400
500
sampling @16Hz

600

700

800

Figure 3.2.15: Examples of transient response for the FIS detectors for a step-wise change ofthe incident ﬂux.

Version 1.3 (September 14, 2007)

31

Figure 3.2.16: Transient response to pulse irradiation. On the left side of the each ﬁgure thecalibration lamps are ﬂashed at very short periods simulating the passage of a point source in
the All-Sky Survey. While on the right, the lamps are turned on continuously for 5 sec. The
intensity of the calibration lamps was the same for both cases. Although 5 sec is not enoughtime for the detectors to reach a constant output level, it is obvious that the output signal for
ﬂashes is smaller than that of the 5 sec case. This ratio will be used for the ﬂux calibration ofpoint sources.

3.2.7

Spectral Responsivity

The relative spectral responsivity of the FIS system is presented in Figure 3.2.17. The responses
are normalized at their central wavelengths. The ﬁlter transmittance as well as the eﬃciency ofthe optical elements, and the detector spectral responsivity are taken into account. The ﬁltersand optics were measured in the end-to-end system conﬁguration at room temperature. The
narrow band ﬁlters for N60 and N160 were measured individually at the cryogenic temperature,since it was known that these ﬁlters showed a temperature dependent variation.

Figure 3.2.17: The spectral response curves of the four FIS photometric bands normalized at thecentral wavelength of each band. These proﬁles should be regarded as ‘typical’ for the bands.

32

AKARI FIS Data Users Manual

The detector spectral responsivity was evaluated using the FTS of the FIS itself, with ablackbody source at diﬀerent temperatures. Each pixel of the detector shows a diﬀerent spectral
response. This diﬀerence is larger in the LW detector, which is stressed Ge:Ga type. The
diﬀerence arises from the non-uniform eﬀective stress on the detector tips. The stronger the
stress (pressure) on the detector element, the longer the wavelength limit can be extended. It isobserved that the cut-oﬀ wavelength (the longest wavelength at which the detector is sensitive)
varies from one pixel to another, probably because of inhomogeneous stress among the pixels.
Figure 3.2.18 demonstrates the pixel by pixel variation of the spectral responsivity of the LW
channel. It is seen that the cut-oﬀ wavelength ranges from 165–185 μm. The N160 band extends
longer than the WIDE-L band as the detectors are under even higher pressure.
The plots in Figure 3.2.17 are regarded as the ‘typical’ proﬁles. Revision of the overall RSRF
with the in-ﬂight data has not yet been completed.

Figure 3.2.18: Pixel by pixel variation of the spectral response of the LW detector.
The diﬀerence in wavelength proﬁle directly inﬂuences the photometric accuracy, especiallyfor cool targets such as molecular clouds. The error in photometry is less than a few per centfor sources warmer than 100 K, but is more than 10% for a target of 30 K.

Version 1.3 (September 14, 2007)

3.3

33

Charged Particle Hits (Glitches)

The preliminary in-orbit measured average glitch rate on the FIS detectors is ∼ 1 / pixel / minfor both the SW and LW arrays. There are also observed after eﬀects of the glitches. The LW
detectors show the strongest eﬀects, with after eﬀects of time scales of ∼1 sec. At present, suchdata are recommended to be discarded in the data reduction process.
Observations carried out immediately after SAA passage have also been carefully checked but
seem to show little diﬀerence in quality compared to the data taken in more quiet regions. Someobservations that were carried out while the spacecraft passes through the polar-cap region,
where many electrons hit the detectors show signiﬁcant enhancement of glitches, although theeﬀect diﬀers from observation to observation and carefully inspection of the data may be requiredin these circumstances.

3.4

Diﬀuse and Point Source Confusion

For a realistic evaluation of the FIS sensitivity, external sources of noise in addition to theintrinsic Instrumental noise have to be considered. The sky confusion noise is observational
dependent and can arise from both the superposition of sources in crowded ﬁelds and fromextended structures which vary in surface brightness on scales of the telescope and instrument
resolution. For the FIS instrument, the major components are the sky confusion due to thedust emission from irregular interstellar clouds at high galactic latitudes known as the “infraredgalactic cirrus” and the confusion due to the ﬂuctuation of the extragalactic background built
up by the superposition of individual faint sources below the resolution of the telescope beam.
Confusion can cause centroid position shifts and ﬂux uncertainties leading to positional
errors and spurious sources, therefore careful consideration must be given to the treatment ofconfusion noise and the confusion limit, as in reality, the confusion noise is a convolution of theobservational phenomenon and the observing instrument.
At present there are no in-ﬂight measurements for the confusion noise for AKARI observations, therefore users are directed to the pre-launch estimates of confusion for the mission.

3.4.1

Confusion Due to Diﬀuse Background Emission

The galactic cirrus is a function of galactic latitude and is serious for wavelengths longer than
60 μm. The formalisation of Helou & Beichman (1990) can be used to provide an estimate of
the confusion noise (σ/mJy) due to infrared Cirrus as a function of Cirrus brightness < Bλ >for the AKARI telescope;
λD −2.5 < Bλ > 1.5σ(λ)= 0.732()2.5 (
)
(
) ,
1mJy
100μm
0.7m
1MJysr−1(3.4.1)

Table 3.4.2 shows the estimated confusion limit over a range of average Cirrus brightnesses,< Bλ >.

3.4.2

Confusion Due to Point sources

The point source (galaxy) confusion limit is deﬁned as the threshold of the ﬂuctuations in the
background sky brightness below which sources cannot be discretely detected in the telescope
beam ∼ λ/D. Point source confusion will aﬀect AKARI observations in the faint ﬂux regime
and in crowded ﬁelds.
As a benchmark for observations, a useful, practical benchmark for the confusion limit isadopted by assuming a limiting the number sources per beam before the beam becomes confusedAKARI FIS Data Users Manual

34

Table 3.4.2: Estimated confusion limits due to galactic cirrus in AKARI FIS bands assuming
the model of Helou & Beichman (1990). The tabulated FIS sensitivity is the 5σ limit for a single
Slow-Scan with a 2 second integration and scan speed of 15 arcsec/s.
Band
N60
WIDE-S
WIDE-L
N160

Sensitivity
5σ (mJy)
78
16
85
198

Confusion Limit (5σ, mJy) for < Bλ >0.5 MJy/sr 3 MJy/sr
10 MJy/sr
0.37
5.49
33
0.77
11
68
3.6
54
330
4.3
63
388

of 1/20 sources per beam (20 beams per source). Table 3.4.3 tabulates the source confusion limit
assuming the above criteria for the FIS bands. From the table it can be seen that source confusion
will only become problematical in the WIDE FIS bands.
Table 3.4.3: Estimated confusion limits (in mJy) due to point sources for AKARI FIS bands.
The tabulated FIS sensitivity is the 5σ limit for a single Slow-Scan with a 2 second integrationand scan speed of 15 arcsec/s.
Band
N60
WIDE-S
WIDE-LN160

5 σ Sensitivity (mJy)
Slow-Scan Mode
78
16
85
198

Source Confusion Limit (mJy)
20 beam/source
3
7
45
50

Version 1.3 (September 14, 2007)

35

3.5

FTS mode

3.5.1

Transient Eﬀects on the FTS data

In the FTS mode, the detectors have to observe a rapidly changing signal from the interferogram.
Naturally, strong transient eﬀects appear in the output signal. Figure 3.5.19 plots the interferograms corresponding to the forward and backward motion of the mirror. Large deviations are
seen.
Correction of transient eﬀects in the FTS data is being investigated.
0.1[V/sec]SW#23

forwardbackward

0.05
0
-0.05
-0.1
-0.15
-0.1

-0.05

0

0.05

0.1

Optical Path Difference [cm]0.1

[V/sec]0.05

forwardbackward

LW#24

0
-0.05
-0.1
-0.15
-0.2
-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Optical Path Difference [cm]Figure 3.5.19: Comparison of forward (blue) and backward (red) interferograms. The diﬀerence
between the two interferograms is thought to be due to detector transients.

3.5.2

Detector Response Inhomogeneity

As described above, pixels in the detector arrays do not have a uniform responsivity. Figure 3.5.20 indicates the relative responsivity variation of the detector pixels in the FTS mode
integrated over an eﬀective wave number range of 85-160 cm−1 for the SW array and 65-85 cm−1for the LW array. The relative responsivity of most of the pixels are within 40% for the SW
array however the pixels of the LW array have larger variation. There are several pixels withextremely high responsivity, e.g. SW pixel 23 and LW pixel 31.
The wavenumber dependence of the responsivity also diﬀers from pixel to pixel, which should
be calibrated as the spectral correction factor in the analysis tools (see section 4.3). In the LW
detectors, there is also a variation in the wavelength proﬁle (long wavelength cut-oﬀ).

AKARI FIS Data Users Manual

2.0

8

1.5

6
Normalized responsivity

Normalized responsivity

36

1.0

0.5

4

2

0.0

0
1

11

21

31
Channel

41

51

1

11

21
Channel31

41

Figure 3.5.20: The relative responsivity variation of the detector pixels in the FTS mode integrated over eﬀective wave numbers of 85-160 cm−1 for the SW array (left) and 65-85 cm−1 forthe LW array (right). These data are obtained from the internal calibration source.

3.5.3

Spectrum Reproductivity

Normalized output

In order to evaluate the stability of the spectroscopic data, three spectra were taken during thelaboratory tests. The ﬁrst two were taken with a one hour interval, then the third one wasobtained after three hours. The derived three spectra are almost equivalent to each other within
about 10%.

0.04
0.03
0.02
0.01
0.00
50

60

70
80
90
Wavenumber [cm-1]

100

Figure 3.5.21: The internal calibration source spectra of LW pixel 7 obtained over the ﬁrst yearof the AKARI mission. They are normalized by the integration over the eﬀective wavenumberof 65-85 cm−1 .
In orbit, the reproductivity of spectra from the internal calibration lamp was examined.
Figure 3.5.21 shows the spectra over the ﬁrst year of the mission. The spectra are consistent
with each other within 10% after scaling with the integration over the eﬀective wavenumber.
For most LW pixels, it has been conﬁrmed that the spectra can be reproduced to within 10%.
Although the spectra of the SW channel show large amplitude fringes, the spectra of most pixels
can also be reproduced to within 10% in wavenumber for those pixels less aﬀected by fringes.

Version 1.3 (September 14, 2007)

3.5.4

37

Fringes

The SW channel suﬀers from strong fringes in the spectra. Figure 3.5.22 shows an enlargementof the fringe patterns in the Full-Resolution mode and the SED mode respectively. The causeof the fringes is thought to be a reﬂection within the detector element, by the parallel surface ofthe chip (therefore it should also present in the photometric mode). Throughout the laboratory
measurements, the fringe pattern remains at the same position, implying that it can be easily
removed. The fringe pattern looks quite similar for diﬀerent in-ﬂight observations. The largest
fringe component can be signiﬁcantly reduced by applying a predeﬁned model pattern. Further
investigation is ongoing to remove the residual fringes. However, observations of sharp lines on
the fringe shoulders will require special care.
0.0015

0.001

0.001

0.001
2.53 cm -1

2.53 cm -1
0.00080.00080.00080.00060.00060.00040.00040.00020.0002[V/sec/cm-1]

0.0004[V/sec/cm-1]

0.0006[V/sec/cm-1][V/sec/cm-1]

0.001

0.00050.0002

0
90

92

94

96

98

0
100

Wavenumber [cm -1]

0
110

112

114

116

118

0
120

Wavenumber [cm -1]

Figure 3.5.22: Fringe pattern of the FTS spectra in Full-Resolution mode (left) and SED mode(right). In Full-Resolution mode, the fringe pattern is resolved out and an “Airy pattern” ofthe Fabry-Perot interference is clearly seen.The blue lines in the bottom panel are the rms noiselevel. The three lines in the right panel are diﬀerent measurements, and match each other well,indicating that the fringe pattern is reproducible.

3.5.5Interference from the Cryocooler

It is known that the vibration from the cryocooler interferes with the FTS mirror drive. The
vibration appears as a change of the mirror scan speed of up to 30%, periodically with a frequencyof 15 Hz. As the detector readout is at a constant frequency, this means that the sampling of theinterferogram is at inhomogeneous intervals of the optical path. As a result, the FFT techniquecannot be applied. The frequency of 15 Hz corresponds to 196 cm−1 = 51 μm, which is outside
the wavelength coverage of the FTS. Therefore it is expected that the eﬀects of interference
will not appear in the observed spectra. No clear inﬂuence on the resultant spectra has been
observed at this stage of the data analysis.

3.5.6

Detector position

In-ﬂight measurements of the relative positions of the SW and LW detectors indicate that thetwo detectors are not aligned as expected from the pre-ﬂight design. Three reference positions
are newly introduced as an AOT parameter (See section 2.2.5 and Figure 2.1.6). Note that thepositions for the SW detector (Position 1) is not centered on a SW pixel for historical reasons,although the absolute pointing accuracy may be is as large as the pixel size.

Chapter 4

Data processing
FIS observations taken in (All-Sky) Survey Mode are processed by means of the AKARI/FIS
Survey Data Reduction Pipeline. For the Pointed Observations (Slow-Scan mode) a dedicatedtoolkit is provided to the users. The toolkit is still evolving and users are recommended to checkthe updates.
Currently the distributed ﬁles contain the raw data and example reduction results (jpeg
image) to give users a ﬁrst impression of the data. Although they are fully reduced data, it
must be emphasised that the processing is done in an automatic way (using the default optionsof the Slow-Scan tools) which does not involve any scientiﬁc judgment.
Instructions of how to use the FIS Slow-Scan tool (Cookbook) are given in Appendix A.

38

Version 1.3 (September 14, 2007)

4.1

39

AKARI All-Sky Survey (ASS) Pipeline

The AKARI All-Sky Survey Pipeline can be divided into three steps; (1) construction of FITS
datasets from telemetry data, (2) the scan processing and calibration (Green Box), and (3) conﬁrmation, source extraction, and catalog production (ASTRO-F/FIS Data Reduction Team 2001
Sept. 4, “The ASTRO-F/FIS survey data reduction – The Pipeline Design and the Calibration
Strategy –”).

4.1.1

Out-line of the data ﬂow

The pipeline Green Box modules process and calibrate the time series data (TSD; see Section 6.1). The procedures included are (see Figure 4.1.1); 1. ﬂagging of the bad data, 2. correction of non-linearity arising from cryogenic readout electronics (CRE), 3. diﬀerentiation of thesignal ramps, 4. detection of cosmic-ray glitches, 5. correction of transient and cross-talk eﬀects,
6. subtraction of dark current level, 7. correction of DC-responsivity, and 8. ﬂux calibration.

INPUT: Time Series Data (TSD)
Object Reference
Flagging bad data : GB_Flag_Bad_Data
GB_Flag_RstAnon

Used for Slow-Scandata reduction
CCF, Status

Conversion from ADU to Volt: GB_Conv_to_Volt

CCF

CRE Ramp Curve Correction: GB_Ramp_Curve_Corr

CCF

Differentiation: GB_Differentiation

CCF

Glitch detection: GB_Set_Glitch_Status_GP
GB_Set_Glitch_Status_MMT

CCF

Making DC Cal.-lamp table: GB_Make_DC_Cal_Lamp_Table

CCF

Transient/Cross-Talk correction: GB_Transient_Corr

CCF

Making Dark Table: GB_Make_Dark_Table

CCF

Responsivity Table
Dark Table
Dark Subtraction: GB_Dark_Subtraction
DC Responsivity Correction: GB_DC_ResponsivityCorr
Absolute flux calibration: GB_Flux_Calib

CCF

OUTPUT: Calibrated TSD

Figure 4.1.1: Outline of the ﬂow of the time series data through the Green Box pipeline modules.
TSD refers to Time Series Data, and CCF for Calibration Constants Files. The input andoutput of each procedure are TSD object reference. Blue boxes (Transient/Cross-talk correction,
absolute Flux Calibration are not yet implemented in the pipeline structure)
The input and output of each procedure are TSD object references on memory within

AKARI FIS Data Users Manual40

IDL. Physical TSD ﬁles are not created by default until the end of the pipeline. Parameters areprovided in an appropriate calibration constants ﬁle (CCF; see Section 6.2).
Four of these GB modules (ﬂag bad data, ﬂag anomaly after reset, conversion to voltage units
and ramp curve correction) are also part of the Slow-Scan pointed observation data reduction
tools and are described in the next section.

4.1.2

Green Box Modules used in the Slow-Scan data reduction process

These GB modules are automatically called from the Slow-Scan toolkit. At present, there arelittle or no options available,therefore users do not need to pay special attention to them. They
are described here only for information and clarity.
Setting the ﬂags to bad data (GB Flag Bad Data)
This function sets ﬂags in the data with the following conditions:
Frame condition

Each pixel condition; blank frame; in SAA; near moon; dead pixel; saturated pixel; reset position

These ﬂags are applied to each frame (sampling) and each pixel. The output is stored in the
branch FIS OBS (under Flags) of the TSD ﬁle (see Section 6.1)
Besides this speciﬁc module for ﬂagging data, all the Green Box modules of the All-Sky
Survey pipeline set frame/pixel ﬂags in the case where the particular module fails to processdata properly or when any data have an anomaly. In addition, the BAD ﬂag is an OR operationof the other ﬂags (see Section 6.1). Therefore, the BAD ﬂag can be used for a quick check ofdata quality. For more detailed analysis the individual ﬂags should be inspected.
Digits to Volt conversion (GB Conv to Volt)
This procedure converts from ADU to a signal in Voltage units. The output is stored in the
branch FIS OBS (under Detector data: Flux) of the TSD ﬁle (see Section 6.1)
CRE non-linearity correction (GB Ramp Curve Corr)
The output of the cryogenic readout electronics (CRE) has non-linearity characteristics depending on the output voltage. This function corrects this non-linearity, assuming that the degreeof deviation from an ideal integration ramp curve is a function of output voltage.

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4.2

Slow-Scan processing and calibration: FIS01 and FIS02

4.2.1

Detecting glitches of cosmic ray-hits

Glitches are caused by the eﬀects of cosmic ray particles on the detectors. A radiation hit or glitch
shows up in the Slow-Scan data as an abrupt change of signal (∼0.1s) while celestial sources
exhibit a slower time proﬁle (∼1s). Thus, in principle, it is straight-forward to discriminate
glitches from a source signal in the time domain.

Figure 4.2.2: Glitch example
Glitch events are detected by taking the 1st and 2nd diﬀerential of the data and comparing
the diﬀerential signal with the RMS noise. Then a glitch event is ﬂagged for each pixel. There
are more complex glitch detection algorithms prepared for the All-Sky Survey data. However,for Slow-Scan data this algorithm is suﬃcient and faster.

4.2.2

Calculation of signal current

The slope of an integration ramp (or signal in V/s) is proportional to the photo-current which
in turn is a measure of the number of photons falling on the detector per unit time. The signal
current is calculated by a linear ﬁt to the ramps. In the case that glitches are detected in asingle ramp, the ramp is divided into (number of glitches + 1) and a linear ﬁt is made for eachsegment of the divided ramp. The weighted average ﬂux of each segment is calculated as themean ﬂux for the ramp concerned. The uncertainty or ﬂuctuation of the slope is the RMS of
the ﬁt residuals. The RMS noise of the LW detectors is very similar between shutter-close (darksignal) and shutter-open (sky signal). For the SW detectors, the shutter-open noise is slightly
higher than that with shutter-close (see Fig 4.2.3).
The noise spectrum is under investigation. Preliminary results shows no clear diﬀerence inthe noise spectra between shutter open/close signals, as shown in Fig 4.2.4.

4.2.3

Dark signal subtraction and ﬂat-ﬁelding

Shutter-close data is used for dark current subtraction. Currently, the Slow-Scan tool uses anaverage value of the data before the calibration lamp (CalA; see Section 2.1.2) is turned on for
2 minutes during shutter close.
For the responsivity correction and ﬂat-ﬁelding the Slow-Scan tool uses by default a ﬂat-ﬁeld
built from the PV observations of zodiacal light and cirrus emission. The average ﬂux of the 2

AKARI FIS Data Users Manual42

Figure 4.2.3: Comparison of the RMS noise for LW and SW detectors when the shutter is open(ﬂat) and closed (dark)

Figure 4.2.4: FFT noise spectra for a 2s integration data of shutter-open (left) and shutter-close(right)minutes calibration lamp in that observations are used.
In Figure 4.2.5 the data intervals used for the dark subtraction and ﬂat-ﬁelding are shown.
Examples of dark and ﬂat-ﬁeld tables are shown in Figure 4.2.6.
Flat-ﬁelding with the observation (sky) data is attempted in the Slow-Scan tool (option
LOCAL FLAT) and often works successfully for ﬂat (quiet) skies. An average responsivity for eachwavelength band is corrected with the 2 minutes calibration lamp and the ﬂat-ﬁeld is built fromthe observed sky.

4.2.4

Conversion to surface brightness

The conversion from the slope of the integration ramp (V/s) to surface brightness (MJy/sr) is
made by using the responsivity correction table.
The responsivity correction table is calculated as the ratio between the calibration lamp
signal and the signal of celestial “ﬂat” sources comprised of the zodiacal light and cirrus emission
observed during the PV phase (see Section 4.2.8 for details).

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Figure 4.2.5: Sequence of FIS01 and FIS02 observations with the corresponding ﬂags for shutterclose (red) and CalA on (blue). In bold red numbers, the average value during shutter-close usedas the dark measurement is shown. In bold blue numbers the average value used for responsivity
correction when the calibration lamp is on (CAL). Note that CalASOn status turned on every
minute (pulses) but there is no signal on the detectors, as the current of the calibration lamp is
set to 0.

Figure 4.2.6: Examples of dark and responsivity tables.

4.2.5

Background oﬀset subtraction

Only the instrumental (thermal) background inside the FIS entrance shutter is subtracted during
the default processing. Stray light due to Earth shine reﬂected at the telescope baﬄe is thenthe major internal contribution to the observational background (see Section 3.1.6).
For point sources, the background subtraction can be performed by median ﬁltering (option/MEDIAN FILTER) in the time domain with a default window size of 10 data points (= 10 ramps).
The stray light is also removed by this DC-rejection ﬁlter (see Figure 4.2.7).
However, diﬀuse sources with a size comparable to the median ﬁlter width will be also ﬁlteredout by this procedure. For this case, a diﬀerent routine to model and subtract the stray light(/SL RMV) is under construction (see Figure 4.2.8).
In the model, the baﬄe function is assumed to follow an exponential law:
I = B + A × exp[−(Θ/C)2 ]where I is the signal, B is the sky background, A is a ﬁtting constant, Θ is the Earthavoidance angle and C is a scale angle (∼ 0.4 rad).
In addition, the option /SKY SUB is also provided in the Slow-scan toolkit to subtract themean sky level determined from the data before the Slow-Scan (during the stabilization).

44

AKARI FIS Data Users Manual

Figure 4.2.7: Background subtraction for point sources and small scale structures.

Figure 4.2.8: Baseline ﬁtting with a stray light model.

4.2.6

Making images by co-adding multi-pixel data

Co-add images are created from the scan data. The positions of every pixel at every data pointin the scan data are calculated from the boresight position given in the data ﬁle and the oﬀset
between the boresight and the detector pixels. Optical distortion of the detector arrays fromthe pre-ﬂight simulation is taking into account in the pixel position table. Fine tuning of thealignment between the boresight to the arrays are carried out from the observation data.
An image plane of appropriate grid size is prepared and signals from the data points are
summed up and then averaged over the number of data points per pixel. By default a pixel sizeof 15 arcsec and 30 arcsec are adopted for the SW and LW data in the FIS01 mode. The pixel
size is doubled for the FIS02 mode.
The image grid positions are of simple equal pitch in local sky. Currently no projection to
the world coordinate system (WCS) is considered. Therefore the FITS format output of theresultant image has a deviation from the real position towards the periphery of the images. It is
recommended to use the image and coordinate information packed into the provided IDL .sav
ﬁle for further analysis. Proper projection will be considered in a near future version of thetoolkit.
By default all available data are accumulated on the image plane. An option /SCUT produces

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images per individual (one-way) scan. See the Cookbook (Appendix A) for more details.

4.2.7

Image processing for multiple pointings

A tool is provided within the Slow-Scan data reduction toolkit to co-add/mosaic multiple pointings together. The tool may be used for both the co-addition of multiple pointings to improve
redundancy or to make small maps from multiple FIS AOTs. See Cookbook (Appendix A) for
details.

4.2.8

Photometric calibration

Calibration of scan data
The absolute calibration of the FIS Slow-Scan data is based on the measurements of the diﬀuse
sky emission from zodiacal light and interstellar cirrus averaged over areas of several Slow-Scanobservations. The absolute brightness is derived from COBE/DIRBE measurements.
The conversion factor from the detector signal I (V/s) to the sky brightness B (MJy/sr) for
each pixel is derived as the ratio between the averaged sky brightness level and the averaged
detector signal while the calibration lamp is continuously turned on (in the calibration sequence
data at the beginning and end of each pointed observation). The conversion factor is given fordiﬀerent signal levels of the calibration lamp (corresponding to selected reset interval).
R[(V/s)/(MJy/sr)] =(Isky − Idark )flat
Bflat(4.2.1)In the data reduction, observation data is ﬁrst ﬂat-ﬁelded by the signal of the calibration
lamp, assuming that the variation of the detector responsivity is slower than the duration of theobservation, then the conversion factor is adopted.
CT OF =

Ical − Idark
Isky − Idark(4.2.2)(Ical − Idark )obs
(4.2.3)CT OF · R
So far for the limited number of regions tested, this calibration is consistent with COBE/DIRBE
even in very dark sky areas.
Bobs [MJy/sr] =Point source photometry (based on preliminary initial results)
Images generated by the standard Slow-Scan data reduction tool are in brightness scale units of
MJy/sr. Aperture photometry on these images should give the ﬂux of the sources in Jy. Resultsof aperture photometry often depend on the size of the aperture and the sky area. This eﬀect
is signiﬁcant in the FIS observations, especially in the LW channels. In the following we give an
example photometry analysis using the calibration standard stars. Preliminary conclusions are:
• There are systematic diﬀerences between the measured ﬂux and the expected ﬂux fromthe model.
• The ratio of two ﬂuxes seems to be constant within ∼20% (1σ) for the examined sourcesbrighter than about 2 Jy.
The procedures are explained below. Photometry following this method should in essence
provide consistent results. Note that this is not the deﬁnitive solution that will eventually beprovided.

AKARI FIS Data Users Manual46

1. All data were taken in FIS01 with 8 arcsec/sec scan speed and 70 arcsec shift length. Datataken with other scan speeds or with FIS02 need further consideration (Section 5.3.1).
2. Images are generated using a local ﬂat-ﬁeld, with a sigma clipping threshold of 1.5 anda grid size of 7.5 arcsec for SW and 15 arcsec for LW. Note that the grid size is half ofthe nominal value used by the Slow-Scan tool for AOT FIS01 (see Appendix A). Such asmall grid size produces many void pixels and therefore, it is not recommended for general
purposes.
3. Following aperture and sky region are adopted:
Channel Signal [arcmin] Sky [arcmin]SW
2.25
2.25–3.25
LW
3.0
3.0–4.0
Sky level is determined by taking the average of all the available pixels.
4. For faint sources these aperture sizes are too large and may introduce contamination fromthe background. Smaller aperture sizes improve the S/N of the photometry. The aperture
correction factor is derived from the ‘curve of growth’ analysis of the objects as follows.
Band
Aperture [arcmin] Correction factor
N60
0.625
1.58
WIDE-S
0.625
1.74
WIDE-L
0.750
1.71
N160
0.750
2.03
5. The measured ﬂux is compared with the expected ﬂux of the sources. Asteroids and a star(α Boo) is used as the primary calibrators. Expected ﬂuxes are derived from the TPM
model (Müller and Lagerros 2002, A&A 381, 324) or stellar SED templates by M. Cohenfor the Spectral Response of the FIS instrument. A colour correction is adopted to convert
them to a ﬂat spectrum (νFν = const.). In addition, several infrared luminous galaxies areincluded in the sample. Their ﬂuxes are estimated from ISO observations or their IRAS
ﬂux.
6. The ratio of the measured ﬂux to the expected ﬂux is fairly constant among the samples andis tabulated below. These numbers are regarded as ‘correction factor’ from the measured
ﬂux to the real ﬂux of the sources.
Band
FactorN60
1.7
WIDE-S
1.7
WIDE-L
1.9
N160
3.8
For the moment, there is no clear explanation of the discrepancy between the expected
ﬂux and measured ﬂux. The uncertainty of the photometry using the above method is crudely
estimated as 20% in SW and 30% in LW. Especially the number for the N160 band needs special
care to use, as only three objects were available for the analysis.
Colour correction(Based on internal report by Y. Hibi 2006 Dec. 25)
The FIS photometric ﬂux is deﬁned for a ﬂat spectrum (νFν = const.) at the deﬁned Central
Wavelength (Frequency) of each band (Table 2.1.1). Colour correction factor K is given by;K=(Fν (ν)flat /Fn u(νc)flat )R(ν)dν,(4.2.4)

Version 1.3 (September 14, 2007)47

where Fνobs is the ﬂux of the object, Fνflat is the ﬂux of the ﬂat spectrum, R(ν) is the Relative
Spectral Responsivity Function (RSRF; Figure 3.2.17) normalized at the Central Frequency.
Colour correction is applied as:
Fobs(4.2.5)
Freal =
K
Correction factors for typical cases are tabulated in Table 4.2.1 and 4.2.2.
Table 4.2.1: Colour correction factor K for a spectrum F (ν) ∝ ν α
Band
N60
WIDE-S
WIDE-L
N160

-4
1.08
0.97
1.23
1.06

-2
1.01
0.96
1.05
1.01

α
0
1.00
1.08
0.96
0.99

2
1.05
1.41
0.94
0.99

4
1.17
2.20
0.97
1.01

Table 4.2.2: Colour correction factor K for a black body
Band
N60
WIDE-S
WIDE-L
N160

10
3.38
1.84
1.55
1.10

30
1.05
0.89
0.96
0.99

Temperature [K]100 300 1000
0.99 1.03 1.04
1.15 1.32 1.38
0.93 0.94 0.94
0.99 0.99 0.99

3000
1.05
1.40
0.94
0.99

10000
1.05
1.40
0.94
0.99

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4.3

FTS spectroscopy processing: FIS03

FTS analysis toolkit is expected to be available very soon. Reduction of the FTS mode observations will consist of many iterative procedures and trial processing. There are various instrument
anomalies that have to be settled and corrected, for example, distortion of the interferogram
due to the transient response of detectors, Fabry-Perot like fringes in the SW spectra due tointerferences in the detector, etc...
The current version of the FTS analysis toolkit can not yet be applied to all the FTS
observations. There remain instrument anomalies which are not considered in the current toolkit,
and the present calibration is only applicable to a limited subset of the observations. In addition,
the absolute ﬂux is also yet to be calibrated. Therefore, users are urged to contact
the FTS team in Japan via the Helpdesk in order to use the FTS analysis tools.
Note that, without following this route, the output of the toolkit may result inincorrect scientiﬁc results. Joining the FTS team for establishing the analysis toolkit ishighly welcomed.

4.3.1

Overview

The FTS analysis toolkit consist of two pipelines, named f ts part1.pro and f ts part2.pro. The
former reduces the Time Series Data (TSD) and provides raw spectra, and the latter applies
ﬂux & spectral calibrations and reduces the fringes. The processes contained within these twopipelines are listed below.
[raw data (TSD)]
1. Flag bad data : gb ﬂag bad data, gb set ﬂag rstanom
2. Convert to voltage units : gb conv to volt
3. Ramp curve correction : gb ramp curve corr
4. Diﬀerentiate data : gb diﬀerentiation[Interferogram + DC component]
5. Separate into individual scans
6. Remove reset anomaly and glitches
7. Subtract DC component
8. Interpolate bad data
9. Determine the position of the center burst
10. Discrete Fourier transform
11. Average the spectra[Raw spectra](end of fts part1)
12. Apply spectral correction factor and ﬂux calibration
13. Remove fringes[Spectra](end of fts part2)

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49

Brief description of each process

Ramp curve correction
The standard Green Box module is used to correct for any non-linearity of the ramp curve(see section 4.1.2). The correction table is made from the interferogram far from the centerburst using observations of targets of various brightnesses. (N.B. For very bright targets whosebrightness exceeds the range of the correction table, you have to make a correction table fromyour data itself. In the present toolkit, the calibration will be incorrect in this case.)
Determination of the position of the center burst
Since the value of the position sensor is correct only relatively, the center of the optical path must
be determined. In fact, the center depends on the wave number so a wavenumber-dependent
phase correction is needed. However, the present toolkit determines the position of the center
that minimizes the imaginary part integrated over the eﬀective wavenumber. The determination
is a two step process. First, the deviation of the center position between diﬀerent scans isestimated. For this purpose, the most sensitive pixel (SW pixel 23 and LW pixel 31, see Fig.
3.5.20) is used, or alternatively, a pixel detecting a point source. It is then assumed that therelative deviation of the center position between diﬀerent scans is the same for all pixels. Next,
the deviation of the center position between diﬀerent pixels is estimated.
Interpolation of bad data
We replace any bad data by the median of other scans. Since the ﬁrst scan in the SED mode
is aﬀected by an irregular drive pattern at the starting point, we ignore the ﬁrst scan whencalculating the median in the SED mode.
Fourier Transform
After applying Fourier transforms to the interferogram of each and every scan, a correction forthe slight variation of the responsivity within one pointed observation is made by scaling with
the integration of the eﬀective wavenumber, 85-160 cm−1 for SW and 65-85 cm−1 for LW (except
for the ﬁrst scan in the SED mode). The average spectrum is then taken.
Spectral correction factor and ﬂux calibration
The spectral correction factors are primarily made from the spectra of the internal calibration
source. These include the correction for the wavenumber-dependent response and the relative
sensitivity of the pixels. At present, celestial calibration sources have not been taken into
account, thus the diﬀerence between the spectra of the internal sources and external sources is
not taken into account and thus absolute ﬂux calibration is not applied.
Removal of fringes
In the present toolkit, only fringes in the SW full-resolution mode are reduced using model
calculations of an airy pattern with a period of 2.53 cm−1 . Since the spectral correction factors
are made using data from the internal calibration source, which are in the SED mode, fringes
in SED mode observations should be removed when the spectral correction factors have been
applied. For Full-Resolution mode observations, the fringes are removed from both the observed
data and the spectral correction factor using an airly pattern of the corresponding wavenumber
resolution. Residual fringes and LW fringes are not reduced in the present version of the toolkit.

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4.3.3

Further limitations in the functionality of the current version of the
FTS toolkit• Transient correction.
• Wavenumber calibration.
• Apodization.
• Wavenumber-dependent phase correction.
• Diﬀerence between the internal source and the external source.
• Absolute ﬂux calibration.

Chapter 5

Caveats in the Data Processing
The reduction software and calibration for the FIS Slow-Scan observation is still at a rather
preliminary stage. As the analysis is progressing new complicated problems of the data will
inevitably become apparent. We try to describe the current limitation of the calibration to
avoid user over-interpretation of the data. Up-to-date information about the calibration anddata reduction will be included in this document and displayed on the observer support web
page. You are also welcome to contact the Helpdesk for any queries regarding the data reduction.

5.1

Treatment for transient eﬀects

At present, there is no formal treatment of transients in the Slow-Scan data reduction tool. Only
after-eﬀects induced by calibration light pulses are considered, and this only aﬀects observations
taken during the PV phase as the periodic pulse ﬂashes were stopped for pointed observations
in Phase-1 (see Section 2.1.2).

5.2

Straylight correction

For point sources, the stray light caused by the Earth shine reﬂected at the telescope baﬄe
can be removed by the median ﬁltering used for the background subtraction. However, diﬀuse
sources with a size comparable to the median ﬁlter width are also ﬁltered out by this medianﬁltering. The modelling and correction by stray light is still being analysed (see Section 4.2.5).

5.3

Photometric calibration

5.3.1

Scan speed

Most observations of the ﬂux calibration targets are carried out in FIS01 with a scan speedof 8 arcsec/sec. If your observation uses a higher scan speed, 15 arcsec/sec or 30 arcsec/sec,
the observed signal would be reduced due to detector transient eﬀects and will result in anunderestimation of the source ﬂux. An investigation into these eﬀects has recently begun. The
initial report implies that the measured ﬂux from the aperture photometry for the 15 and 30
arcsec/sec data are ∼10-20% fainter than that from the 8 arcsec/sec case. Reliable calibration
information for all scan speeds will be given as soon as this analysis has been concluded.

5.3.2

Calibrators

Correction factors are derived for all detectors from the diﬀerent calibrators (see Section 4.2.8).
The photometry uncertainty is estimated to be ∼20% (SW) and ∼30% (LW). However, for the51

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N160 band, special care is needed as only three objects were available for calibration measurements.

5.4

Attitude Determination During the Pointed Observations

The attitude information for the FIS pointed observations is provided from two sources. The
on board computer AOCU (Attitude and Orbit Control Unit) calculates the satellite attitude andthese results are downlinked to the ground as telemetry data together with the data from theon board sensors. The Ground-based Attitude Determination System (G-ADS) re-calculates theattitude with a greater degree of care. Both data are recorded in the observation data ﬁle.
Both systems use the same input data from the on board Attitude Control System sensors
and there should only be minor diﬀerences in the absolute positional accuracy. Nevertheless, the
G-ADS is expected to provide more reliable information than the AOCU since this data includes ﬁnetuning of the algorithm and software system to the input data. The Slow-Scan data reduction
tool uses G-ADS information by default. However, as of February 2007, the tuning of the G-ADS
system has not been completed and the results are not accurate enough under some conditions.
Such inaccuracies may appear in the reduced images as an unexpected elongation of the images.
It is recommended to run the software with the /AOCU ADS option and to compare the resultswith the default (G-ADS) processing.
The G-ADS is expected to be improved in around a months time. Note that the Pointing
Reconstruction processing is carried out with the data from the Focal-Plane sensors for the
All-Sky Survey by ESAC/ESA, which will provide better than a few arcsec accuracy.
Positional error has been crudely evaluated using around 40 (SW) and 20 (LW) point-likesources reduced by the Slow-Scan tool in the standard manner. The deviation of the centroid
position on the image maps from the catalogued positions scatters with a 1σ dispersion of ∼6and ∼11 arcsec in the in-scan and cross-scan directions respectively. SW positions and LW
positions are consistent to within ∼11 arcsec (in-scan) and ∼23 arcsec(cross-scan) respectively
for bright sources.

5.5

Pixel position table

The current pixel position table is based on simulations. Corrections based on in-ﬂight data arebeing analysed.

5.6

Projection

Currently, the FIS images in FITS format have a known problem with projection and thecoordinates of the image may deviate from the real position; from the image center to the edge.
This deviation is typically a few arcmin at the corner. It is recommended to use the data in the
IDL .sav ﬁle for further analysis of the image data.

Chapter 6

Instrument Related Data Products
In this Chapter we describe the internal structure of the AKARI/FIS observation data. A setof IDL data interface routines is provided as a part of the data reduction toolkit. Those whoare interested in modifying or developing data reduction tools are welcome to contact the data
reduction team in Japan via Helpdesk for more detailed information.

6.1

Time Series Data (TSD): Overview and structure(H. Baba, I. Yamamura, A. Kawamura, S. Makiuti, H. Kaneda, T. Nakagawa and C. Yamauchi
2006, “ASTRO-F FIS/Survey Data Structure”)

6.1.1

Overview

The data consists of header parts and arrays of data record. A record (corresponds to a samplingof the detector) consists of the instrument data and necessary information from other house
keeping (HK) instruments, as well as positional information given by the ground station. Usually
the sampling rate of HK-data is much slower than that of instrument data. These information
are regridded by a suitable method (e.g. interpolation) to synchronize with the clock of thetarget instrument.
The data for individual instruments (or source of information) are each maintained in a
separate unit known as a ‘branch’. TSD data are examples of these branches. In the TSD
data structure, all data are related to each other by a time stamp. The reference time stamp
throughout the data is called the “Key Time”, and is given by the main instrument (FIS detector
readout timing). The Key Time is copied to other branches and data from the instruments are
regridded to it.
All information from the main instrument is included in the TSD dataset, while only thesubset of any information that is needed for the data reduction is provided from other instruments.

6.1.2

Physical ﬁle format of TSD

The physical ﬁle format of the TSD to exchange the data is the FITS Binary Table Extension.
In the table, each row corresponds to a sampling which is identiﬁed by PIM-TI, the clock counter
provided by the DHU (Data Handling Unit) of the spacecraft.
For the detailed information about FITS Binary Table Extension, see “section 8.3 Binary
Table Extension” in “Deﬁnition of Current FITS Standard - NOST 100-2.0” 1 .
Size of one TSD ﬁle for a single pointed observation is 100–200 MB.
1

http://fits.gsfc.nasa.gov/

53

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TSD
BranchTime

Status(boolean)Telemetry(analog)

Detector Data
(analog)

QualityCounter

Editable

Non-Editable
FIS_OBS

Flags(boolean)

det

flux
ferr

frame flags
pixel flags

FIS_HK
IRC_HK
HK_2
AOCU
GADS
PR
SE

Figure 6.1.1: An overview of Time Series Data (TSD) structure.

6.1.3

Nomenclature of Data Type

Data type are expressed in C-like notation in the following descriptions.
bit
byte
short
long
longlong
float
double: 1 bit0 or 1: 1 byte unsigned byte ( >= 0): 2 bytes integer: 4 bytes integer: 8 bytes integer: 4 bytes single precision floating point: 8 bytes double precision floating point

We express the elements of status column ‘n:’ where n is the element index deﬁned in the
FITS.

6.1.4

Time stamp• The main purpose of time stamp in the data reduction system is to synchronize the information taken with diﬀerent instruments and other information sources (e.g. for pointing
reconstruction).
• For this purpose, time information maintained in TSD is that already corrected various
delays. It is the time when the data is sampled by the instruments.
• Time in the on board clock is 36 bits with LSB of 1/512 sec. Practically 32 bits data
either with 1/32 sec or 1/512 sec are stored in the packet depending on the instruments.
Internally we use a 64 bits integer (long64 in IDL) for time ﬁeld.
• The original point of the time for the AKARI project is 2000 January 1st, 00:00:00 UTC.

Version 1.3 (September 14, 2007)

6.1.5

55

StatusThe ‘Status’, ‘Flags’ and ‘Quality’ of TSD columns are composed of multiple elements of the
TBIT type (i.e., the TFORM deﬁnitions of FITS standard are ’8X’, ’16X’, etc. ).
Figure 6.1.2 shows an example of the status section of this document. The left hand number
shows the element index. The ‘fv’ displays this index when we click the ‘expand’ button. The
ﬁrst element (index=1) corresponds to the MSB (bit31) in the binary. The next is the element
name, which cannot be deﬁned in the FITS standard. However, we deﬁne the element name andwrite them to each header of the HDU. For example, the element names of status in FIS OBS
extension are written in the header of its HDU as follows:
TELEM6 = ’CREON,SHTOP,FWPOSON,FWPOS_B1,FWPOS_B0,MPOSON,MPOS_B1,MPOS_B0, ...
The user should use the element name to access the status bits, rather than element index.

MSB(bit31)

element index
element nameFigure 6.1.2: An example of status section.

6.1.6

TSD: detailed description

Primary Header
string FMTTYPE
integer FTYPEVER
string CNTTYPE
string DATE
string CREATOR; Type of Format in FITS file
; Version of FMTTYPE; Type of data content; File Creation Date
; Data generator program nameAKARI FIS Data Users Manual56
string CRTRVER
string PIPELINE
string DATASTAT
string ORIGIN
string TELESCOP
string INSTRUME
string DETECTOR
string OBSERVER
string PROPOSAL
string OBS-CAT
integer PNTNG-ID
integer TARGETID
integer SUBID
string OBJECT
double OBJ-RA
double OBJ-DEC
string AOT
string AOTPARAM
string INSTMODE
string TIMESYS
string DATE-OBS
string DATE-END
string DATE-REF
double AFTM-OBS
double AFTM-END
double AFTM-REF
string PIMTIOBS
string PIMTIEND
string PIMTIREF
double EQUINOX
double RA
double DEC
double ROLL
double AA-SOL
double AA-EAR
double AA-LUN
double TM-SAA
double SAT-POSX
double SAT-POSY
double SAT-POSZ
string DAYNIGHT
integer STTA-NUM
integer STTB-NUM
string STTA-MOD
string STTB-MOD
string HISTORY; Version of CREATOR; Data Processing Pipeline name; Data status; Organization creating FITS file
; AKARI mission; Identifier of the instrument; Detector name
; PI Name (Observer’s ID); Proposal ID; Observation Category; Pointing ID; Target ID; Sub ID
; Object name
; [degree] Target position; [degree] Target position; Observation AOT; AOT Parameter; Instrument operation mode
; Explicit time scale specification; Observation start date+time; Observation end date+time; Reference time in the Observation; DATE-OBS in ASTRO-F Time; DATE-END in ASTRO-F Time; DATE-REF in ASTRO-F Time; DATE-OBS in PIM-TI (36bits DHUTI); DATE-END in PIM-TI (36bits DHUTI); DATE-REF in PIM-TI (36bits DHUTI); Epoch of Coordinate; [degree] Target position at DATE-REF; [degree] Target position at DATE-REF; [degree] Roll Angle at DATE-REF; [degree] Solar avoidance angle at DATE-REF; [degree] Earth avoidance angle at DATE-REF; [degree] Lunar avoidance angle at DATE-REF; [sec] Duration since last SAA passage at DATE-REF; [km] Satellite position at DATE-REF; [km] Satellite position at DATE-REF; [km] Satellite position at DATE-REF; day/night status at DATE-REF; number of stars in STT-A at DATE-REF; number of stars in STT-B at DATE-REF; STT-A Mode status at DATE-REF; STT-B Mode status at DATE-REF; Processing History. Any length text.

Version 1.3 (September 14, 2007)57

Description##### FITS basic information, data size information
SIMPLE =T / Standard FITS format# Fixed
BITPIX =# FIS (Binary table) :16 / number of bits per data pixel
8 (Fixed)

NAXIS=# FIS : 0 (Fixed)

0 / Number of axes

EXTEND =# ’T’ Fixed

T / Extension may be present##### Data type, Data creation / processing program information
FMTTYPE = ’ASTRO-F TSD FIS_SW’ / Type of File Format in FITS file
# Unique name of the data format# Keep ’ASTRO-F’ by historical reason# One of: ’FIS_SW TSD’, ’FIS_LW TSD’FTYPEVER=
4 / Version of FMTTYPE# Version of the file format described in FMTTYPE
CNTTYPE = ’FIS_SW ’# Data content# One of: ’FIS_SW’, ’FIS_LW’/ Type of data content

DATE= ’2006-09-25T09:45:24’/ File Creation Date
# File Creation Date
CREATOR = ’tsd_bin2dat’/ Data generator program name
CRTRVER = ’1.0’
/ Version of CREATOR# Name and version of the program that creates this file
# FIS: tsd_bin2dat
PIPELINE= ’fispl ver. 1.0 ’/ Data Processing Pipeline name# name and version of the pipeline program
DATASTAT= ’GOOD’/ Data status# Describes data status mainly from completeness of telemetry data.
# This does not tell detailed scientific quality of the data
# All appropriate error status are listed, otherwise GOOD is given# GOOD: No problem# INCOMPLETE: (Scientific data) incomplete due to telemetry loss etc.
# NOHK: HK Status not available# NOADS: Attitude information not available# STTINI: STT did not work properly# More status may be added as analysis progresses# Data other than GOOD may not be in the archive at the first stage.

AKARI FIS Data Users Manual58##### Instrument information
ORIGIN = ’ISAS/JAXA’# FixedTELESCOP= ’AKARI# Fixed’/ Organization creating FITS file

/ AKARI mission [Satellite Name]

INSTRUME= ’FIS’
/ Identifier of the instrument# One of ’FIS’, ’IRC’, ’FSTS’, ’G-ADS’
DETECTOR= ’SW’# Detector name# One of ’SW’, ’LW’

/ Detector name##### Observation details# following information is taken from Observation Database
OBSERVER= ’PI Name ’/ PI Name (Observer’s ID)
PROPOSAL= ’PRPID’/ Proposal ID# information about observation programme# e.g., LSNEP, LSLMC, AGBGA, ...
OBS-CAT = ’MP’/ Observation Category# Observation category# One of ’LS’, ’MP’, ’OT’, ’DT’, ’CAL’, ’ENG’PNTNG-ID=1234567 / Pointing ID# Identification of the pointing observation. Usually it is identical# with the Target ID, but is different for ’parallel mode’ observations.
TARGETID=
1234567 / Target ID
SUBID=
1 / Sub ID# Unique number to specify the observation
OBJECT = ’target ’/ Object name# Object name# ’SURVEY’ for survey mode observations
OBJ-RA=
320.5533 / [degree] Target position
OBJ-DEC =-23.3325 / [degree] Target position# Target position recorded in observation database# Only for pointed observations.
# Values are in double precision
AOT= ’FIS01’/ Observation AOT# AOT Used for the information# ’FIS01’, ’FIS02’, ...., ’IRC04’, ’IRC11’, ’SURVEY’, ...

Version 1.3 (September 14, 2007)
AOTPARAM= ’8;0.5;70’# Parameters for each AOT

59/ AOT Parameter

INSTMODE=/ Instrument operation mode
# Detailed instrument setup not described in AOT / AOTPAR# FIS: readout mode (Telemetry name: PACKETID)TIMESYS = ’UTC’
/ Explicit time scale specification# Time system used in this file
DATE-OBS= YYYY-MM-DDTHH:MM:SS / Observation start date+time
DATE-END= YYYY-MM-DDTHH:MM:SS / Observation end date+time
DATE-REF= YYYY-MM-DDTHH:MM:SS / Reference time in the Observation
AFTM-OBS= [double]/ DATE-OBS in ASTRO-F Time
AFTM-END= [double]/ DATE-END in ASTRO-F Time
AFTM-REF= [double]/ DATE-REF in ASTRO-F TimePIMTIOBS= ’0xXXXXX ’/ DATE-OBS in PIM-TI (40 bits)
PIMTIEND= ’0xXXXXX ’/ DATE-END in PIM-TI (40 bits)
PIMTIREF= ’0xXXXXX ’/ DATE-REF in PIM-TI (40 bits)## [FIS]# ????_OBS: Time stamp/PIMTI of the first frame in the file
# ????_END: Time stamp/PIMTI of the last frame in the file
# ????_REF: On-target observation start: 690 sec from the#observation start (file top)#
# Note: PIMTI is the primary information directly from telemetry.
#
AFTI- and DATE- are from the timing correction based on PIMTI###### Attitude Information (at DATE-OBS)
EQUINOX =2000.0000 / Epoch of CoordinateRA=
320.5533 / [degree] Target position at DATE-OBS
DEC=-23.3325 / [degree] Target position at DATE-OBS
ROLL=-30.553 / [degree] Roll Angle at DATE-OBS
AA-SOL =90.0021 / [degree] Solar avoidance Angle at DATE-REF
AA-EAR =180.2083 / [degree] Earth avoidance Angle at DATE-REF
AA-LUN =210.6821 / [degree] Lunar avoidance Angle at DATE-REF
TM-SAA =1829. / [sec] Duration since last SAA at DATE-REF# Definition of SAA is different for different detectors. SAA region# is defined by the glitch rate map observed by the Star Tracker with# arbitrary threshold level. IRC follows this threshold. Shifts of 30# and 60 seconds are applied to FIS SW and LW, respectively.

AKARI FIS Data Users Manual

60

SAT-POSX=
2903.5528 / [km] Satellite position at DATE-OBS
SAT-POSY=
1704.3092 / [km] Satellite position at DATE-OBS
SAT-POSZ=
1968.4286 / [km] Satellite position at DATE-OBSDAYNIGHT= ’DAY’
/ Day/night status at DATE-REF# These files are updated as pointing analysis goes on from# On-board AOCS -> G-ADS (-> Pointing reconstruction for Survey mode)
STTA-NUM=
4 / number of tracked stars in STT-A at DATE-REF
STTB-NUM=
4 / number of tracked stars in STT-B at DATE-REF# number of tracked stars in the two STT field at DATE-OBS
STTA-MOD= ’TRK’
/ STT-A Mode status at DATE-REF
STTB-MOD= ’TRK’
/ STT-B Mode status at DATE-REF# Status of the STT# one of ’TRK’, ’ACQ’, ’STB’, ’INI_R’, ’INI_N’
COMMENTHISTORY
END/ Any strings/ Any strings

Version 1.3 (September 14, 2007)

61

FIS OBS extension
Here we show the data structure for FIS data.
(1) Time
double aftime
longlong pimtiorg
longlong fistiorg
longlong pimti
longlong fisti; ASTRO-F (AKARI) Satellite Time (calibrated); which is the seconds since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet); FISTI counter (from original packet); PIMTI counter (interpolated); FISTI counter (interpolated)(2) Status
Status information is provided in the telemetry packet. Each status is copied from the corresponding bit in FIS OBS packet. All four band’s status are included considering possible
interference.
bit status[32]1: creon
2: shtop
3: fwposon
4: fwpos_b1
5: fwpos_b0
6: mposon
7: mpos_b1
8: mpos_b0; FIS instrument status; CRE on:1 / off:0% 0x09:1; Shutter open:1 / close:0% 0x09:1; FW Position sensor on:1 / off:0% 0x09:1; Filter Wheel Position bit #1% 0x09:1; Filter Wheel Position bit #0% 0x09:1; Mirror Position sensor on:1 / off:0% 0x09:1; Mirror Position bit #1% 0x09:1; Mirror Position bit #0% 0x09:1

9: rstwidelon
10: rstwideson
11: rstn170on
12: rstn60on
13: lwbooston
14: swbooston
15: lwbiason
16: swbiason; Reset Wide-L on:1 / off:0; Reset Wide-S on:1 / off:0; Reset N160
on:1 / off:0; Reset N60
on:1 / off:0; LW BIAS Boost on:1 / off:0; SW BIAS Boost on:1 / off:0; LW BIAS on:1 / off:0; SW BIAS on:1 / off:0% 0x0C:1% 0x0C:1% 0x0C:1% 0x0C:1% 0x0E:1% 0x0E:1% 0x0E:1% 0x0E:1

17: calalon
18: calason
19: calbon
20: sinalon
21: sinason(22-32: reserved); CAL AL (LW) on:1 / off:0; CAL AS (SW) on:1 / off:0; CAL B (BG) on:1 / off:0; CAL sin-conv. AL on:1 / off:0; CAL sin-conv. AS on:1 / off:0% 0x0F:1% 0x0F:1% 0x0F:1% 0x0F:1% 0x0F:1

The ’rstn170on’ should be renamed to ’rstn160on’. However, we do not change it forhistorical reasons.
(3) Analog Telemetry
byte packetid; PacketID (FIS obsmode)% 0x00:8

AKARI FIS Data Users Manual62

byte rstcntwidel
byte rstcntwides
byte rstcntn170
byte rstcntn60; Reset counter Wide-L; Reset counter Wide-S; Reset counter N160
; Reset counter N60% 0x0A:4% 0x0A:4% 0x0B:4% 0x0B:4

byte calplscntal
byte calplscntas
byte calplscntb
byte aderrcntsw
byte aderrcntlw
long mposcnt; CAL pulse counter AL; CAL pulse counter AS; CAL pulse counter B; SW A/D error counter; LW A/D error counter; Assigned mirror position; Assigned/interpolated frame; counter in the survey mode

% 0x0C:4% 0x0D:4% 0x0D:4% 0x19:4% 0x19:4% 0x1A-0x2F

byte dacbiaswides
byte dacbiasn60
byte dacbiaswidel
byte dacbiasn170; DAC BIAS Wide-S level; DAC BIAS N60
level; DAC BIAS Wide-L level; DAC BIAS N160
level

% 0x10:8% 0x11:8% 0x12:8% 0x13:8

byte daccalasbyte daccalalbyte daccalb
byte dacsinas
byte dacsinal; DAC CAL AS level; DAC CAL AL level; DAC CAL B level; DAC CAL sin-conv. AS level; DAC CAL sin-conv. AL level% 0x14:8% 0x15:8% 0x16:8% 0x17:8% 0x18:8

short tpbody
short tpsw
short tplw
short tpcalft; FIS body temperature; SW Detector temperature; LW Detector temperature; CAL FT temperature% 0x3E-3F% 0x40-41% 0x42-43% 0x44-45

The ’rstcntn170’ and ’dacbiasn170’ should be renamed to ’rstcntn160’ and ’dacbiasn160’
respectively. For historical reasons, we keep the original name of the narrow LW band (N160)
as ’rstcntn170’ and ’dacbiasn170’ in the data structure.
(4) Detector data
Either SW(NDET=100) or LW (NDET=75) data are included in one data set.
long det[NDET]
float flux[NDET]
float ferr[NDET]; Detector signal [ADU]; Detector data in physical unit
; Flux uncertaintity(5) Flags
Flags are those set in the reduction software. These ﬂags are set for each frame (sampling).
bit flag[8]1: bad_frame; Flag for detector condition; set at all the unrecoverable frames

Version 1.3 (September 14, 2007)

2: undef_anom_frame
3: blank
4: in_saa
5: near_moon

6: untrusted_frame(7-8: reserved)

63; described by the other flags:1; unusable data but the anomaly cannot; be described by the existing flags.
; blank:1; in SAA:1; near moon:1; "aa_lun" is used for lunar avoidance; angle in GADS extension.
; incomplete restitution of higher-order bits of DET.

These ﬂags are set for each frame (sampling) and each pixel.
bit pix_flag[32*NDET]
1: bad2: undef_anom
3: arith_err
4: dead
5: saturate
6: reset
7: rstanom
8: no_diff
9: no_rp_corr
10: no_dk_corr
11: no_dccal_corr
12: no_tr_corr
13: no_flat_field
14: no_gpgl
15: no_mtgl
16: no_abscal; Flag for each pixel condition; bad pixel:1; unusable data but the anomaly is not; described by any other flags.
; special values for undefined results;; NaN and INFINITY; dead pixel:1; saturated pixel:1; data taken just after reset:1; anomaly seen in a few certain; samplings just after reset.
; No differentiation enable:1; impossible to carry out ramp curve correction; dark has not been subtracted successfully.
; DC responsivity correction has not been
; corrected successfully.
; transient effect has not been corrected successfully.
; flat fielding has not been carried out successfully.
; unable to search/correct for glitch; unable to search/correct for glitch; abs. flux calibration has not been done successfully.

; For glitch detection using Gaussian Processing method(Kester)
17: gpgl_type1; Glitch Type 1
18: gpgl_type2; Glitch Type 2
19: gpgl_type3; Glitch Type 3
20: gpgl_type4; Glitch Type 4
21: gpgl_tail; Glitch Tail
; For glitch detection using Median Transformation method (Serjeant)
22: mtgl_type1; Glitch Type 1
23: mtgl_type2; Glitch Type 2
24: mtgl_type3; Glitch Type 3
25: mtgl_type4; Glitch Type 4
26: mtgl_tail; Glitch Bad
27: no_peri_corr; No periodic noise correction available

AKARI FIS Data Users Manual64;; For SUSSEXtractor
28: sx_peak
29: sx_source
30: sx_obj(31-32: reserved); threshold detection; source detected:1; anomalous detected(6) Quality

bit quality[40*NDET]; Quality Flag for each pixel condition;; gb_conv_to_volt ;;1-2: qual_cv_param; conversion quality from ADU to Volts.
;; gb_ramp_curve_corr ;;3-4: qual_rc_param; accuracy of correction parameters
5-6: qual_rc_cf; correction factor (difference between; corrected and the original data);; gb_differentiation ;;7-8: qual_df_eq; order of polynomials/number of points used.
;; gb_dc_responsivity_corr / make_DC_cal_lamp_table ;;9-10: qual_rp_data; accuracy of the derived cal. lamp intensity; from data in a "cal. lamp on."
11-12: qual_rp_param ; availability of "cal. lamp on" to derive; a correction factor
13-14: qual_rp_table ; quality of the correction table (fluctuation etc.);; "correction for flat fielding" ;;15-16: qual_ff_param ; quality of the parameters for flat fielding
17-18: qual_ff_cf; correction factor (difference between; corrected and the original data);; gb_set_glitch_status_gp & gb_set_glitch_status_gp ;;;; -> correction after glitch detection
19-20: qual_gpgl_corr; correction of glitch tails for Gaussian; processing method
21-22: qual_mtgl_corr; correction of glitch tails for Median; transformation method;; gb_transient_corr ;;23-24: quql_tr_hist ; history information availability
25-26: qual_tr_param ; correction parameter availability;; gb_dark_subtraction/ make_dark_table ;;27-28: qual_dk_data ; quality of the dark data in one "shutter-close".
29-30: qual_dk_param ; availability of the data of a "shutter-close"; to derive a dark value.
31-32: qual_dk_table ; quality of the dark values of the correction

Version 1.3 (September 14, 2007);65
table (fluctuation etc.);; gb_flux_calib ;;33-34: qual_fx_corr ; availability of calibration parameters35-36: qual_fx_param ; error in calibration factors(37-40: reserved)(7) Counter
short cnt_saa[NDET]; time in seconds since the last SAA passage; # "tm_saa" is used in SE extension
short cnt_glitch_gp[NDET] ; duration since the last corresponding glitch(GP).
short cnt_glitch_mt[NDET] ; duration since the last corresponding glitch(MMT).

AKARI FIS Data Users Manual66
FIS HK extension

Outputs of APID=’FIS HK’ (which contains some minor status of FIS) are stored in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) Status
bit status[8]; Satellite attitude status
1: blank; blank:1
2: blank_subcom_data ; blank bit for subcom data (for sw_heater_tmp)(3-8:reserved)
Note that some FIS status are stored in the HK 2 extension.
(3) Analog databyte fis_fw_f_pul
byte fis_fw_r_pul
byte fis_lw_wire_lightbyte fis_run_mode
byte fis_obs_mode
byte fis_asq_patternlong fis_asq_step
byte sw_heater_tmp; Filter wheel pulse counter (Forward); Filter wheel pulse counter (Rewind); LW wire light DAC set value; RUN mode
; Observation mode
; ASQ pattern; ASQ step
; SW heater temperature

Version 1.3 (September 14, 2007)67

IRC HK extension
Outputs of APID=’IRC HK’ (which contains status of temperature of the focal plane instruments) are stored in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) IRC status
bit status[8]1: blank(2-8:reserved)(3) Analog data
TBD

; Status of IRC functions which may cause; interference; Data blank:; Details TBD with IRC team

AKARI FIS Data Users Manual68
HK 2 extension

Outputs of APID=’HK 2’ (which contains basic data of Cryostat, FIS and IRC) should be stored
in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) Cryostat/FIS/IRC status
bit status[16]; CRYO/FIS status
1: blank; blank:1
2: blank_cryo_temp_data; blank bit for Cryo temperature data
(3-4:reserved)
5: cryo_temp_data_en_ds; Cryo temperature data enabled/disabled(6:reserved)
7: fis_cpu_run_rst; FIS CPU run/reset
8: fis_fis_on_off; FIS on/off
9: fis_tmp_pol_pos_neg; Thermometer polarity
10: fis_obs_seq; Observation sequence start/stop
11: fis_rsw_auto_cmnd; Auto (periodic) reset mode on/off
12: fis_fcal_frq_b1 ; CAL overlay sine wave frequency B1 bit #1
13: fis_fcal_frq_b0 ; CAL overlay sine wave frequency B1 bit #0(14:reserved)
15: irc_cpu_ru_rs; IRC CPU run/reset
16: irc_irc_on_of; IRC on/off(3) Cryostat temperature
float cryo_he_tank_1
float cryo_baffle_1
float cryo_baffle_2; He tank 1 temperature[K] (one close to heat strap); Baffle 1 temperature[K] (one close to the tel.?); Baffle 2 temperature[K](4) Cryostat current
float cryo_cold_hd_a_dr_i; Cold head A driving current[A]float cryo_cold_hd_b_dr_i; Cold head B driving current[A]float cryo_comp_a_dr_i ; Compressor A driving current[A]float cryo_comp_b_dr_i ; Compressor B driving current[A](5) IRC Analog data
float irc_mainref_tmp1
float irc_subref_tmp
float irc_fpip_tmp1; Primary mirror temperature[K] (IRC); Secondary mirror temperature[K] (IRC); FPI plate[K] (IRC)Version 1.3 (September 14, 2007)

69

Attitude and Orbit Control Unit (AOCU) extension
Outputs of APID=’AOCU’ (Attitude and Orbit Control Unit) is stored in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) AOCU Status
bit status[8]1: blank
2: ao_rom_ram

3: ads_mode_b1
4: ads_mode_b0(5-8:reserved); AOCS operation status; blank:1; ROM mode:1 / RAM mode:0i.e., the blank bit for the ads_mode_b1,
ads_mode_b0 and aodu_ads_q.
; Attitude determination mode (AOCU_ADS_MODE) bit #1; Attitude determination mode (AOCU_ADS_MODE) bit #0(3) Analog telemetry
double aocu_ads_q[4]; Quaternion at boresightdouble aocu_body_rate[3]; Body rate (Angular velocity[deg/s]) at boresightbyte aocu_saab_mode
byte aocu_sbab_mode
byte aocu_saab_win_num
byte aocu_sbab_win_num; STT-A operation mode
; STT-B operation mode
; Number of stars in STT-A; Number of stars in STT-B

double ra
double dec
double roll
double d_ra
double d_dec
double d_roll; J2000 R.A. (deg) at boresight; J2000 Dec. (deg) at boresight; Roll angle at boresight (deg); Scan rate in J2000 R.A.
; Scan rate in J2000 Dec.
; Scan rate in roll angle

double crs_off; Offset in cross-scan direction from; Nominal scan path(4) Quality
bit quality[8]; Quality Flag for condition

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Ground-based Attitude Determination System (GADS) extension
Results of ground-based attitude determination are stored in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) Status
bit status[8]1: blank(2-8:reserved); Satellite attitude status; blank: 1(3) Analog telemetry
double attitude[4]double attitude_er[3]double angl_vel[3]; Quaternion at boresight; Estimated error of position; Body rate (Angular velocity) at boresightdouble ra
double dec
double roll
double d_ra
double d_dec
double d_roll; J2000 R.A. (deg) at boresight; J2000 Dec. (deg) at boresight; Roll angle at boresight (deg); Scan rate in J2000 R.A.
; Scan rate in J2000 Dec.
; Scan rate in roll angle

double el_sol
double aa_sol
double aa_ear
double aa_lun; Solar elongation; Solar Avoidance Angle; Earth Avoidance Angle; Lunar Avoidance Angle

double crs_off; Offset in cross-scan direction from; Nominal scan path(4) Quality
bit quality[8]; Quality Flag for condition

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71

Pointing Reconstruction (PR) extension
Results of pointing reconstruction is stored in this extension.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) Status
bit status[8]1: blank(2-8:reserved); Satellite attitude status; blank: 1; Details will be discussed with ESA.

(3) Analog telemetry
double quaternion[4]double pos_err[3]double body_rate[3]; Quaternion at boresight; Estimated error of position; Body rate (Angular velocity) at boresightdouble ra
double dec
double roll
double d_ra
double d_dec
double d_roll; J2000 R.A. (deg) at boresight; J2000 Dec. (deg) at boresight; Roll angle at boresight (deg); Scan rate in J2000 R.A.
; Scan rate in J2000 Dec.
; Scan rate in roll angle

double el_sol
double aa_sol
double aa_ear
double aa_lun; Solar elongation; Solar Avoidance Angle; Earth Avoidance Angle; Lunar Avoidance Angle

double crs_off; Offset in cross-scan direction from; Nominal scan path(4) Quality
bit quality[8]; Quality Flag for condition

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Satellite Ephemeris (SE) extension

The following information is calculated from the satellite orbital parameters provided three times
a week.
(1) Time
double aftime
longlong pimtiorg; ASTRO-F (AKARI) Satellite Time (calibrated); which is the time since 2000/January/1 00:00 (UTC); PIMTI counter (from original packet)(2) Status
bit status[8]1: blank
2: type
3: daynight
4: in_saa
5: in_polar(6-8:reserved); Satellite attitude status; blank:1; Data type (Prediction/Determinated); Day / Night; in SAA region; in Polar region(3) Analog value
double tm_saa; seconds since last SAA passage.
; < 0 during the passage; Clock start at end of Bias Boost ?

double sat_posx
double sat_posy
double sat_posz; Satellite position in Earth reference frame; Satellite position in Earth reference frame; Satellite position in Earth reference frame

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6.2

73

Calibration Constant Files (CCF)

Calibration Constant Files (CCF) contain either correction factors or instrumental parameters
necessary to:• steer the processing• to remove or to correct for instrumental eﬀects in the data, and• to carry out the ﬂux calibration
In this section detailed descriptions are given of the structure and contents of the CCF ﬁles.
Note that users do not have to deal with the CCF ﬁles directly unless they explicitly wish tochange the calibration.

6.2.1

CCF Structure

A CCF ﬁle contains one set of parameter(s), i.e. a constant, an array, or a structure variable.
Multiple values are packed in IDL type structures.
A CCF ﬁle consist of a header part, a data deﬁnition part, and one or more data bodypart(s).
Header starts with ”#HEADER” and end at ”#ENDHEADER”.
Data type deﬁnition starts with ”#DEFINE” and end at ”#ENDDEFINE”.
Data body starts with ”#DATA” and end at ”#ENDDATA”.
The data deﬁnition appears before the data body.
The deﬁnition part contains a line starting with one of ’C’ (constant), ’A’ (array), or ’S’(structure) followed by variable type and array dimension. Structure deﬁnition also includes
description of the structure in a similar style. Detailed explanation with examples are given
below.
Data body part contains actual value(s) corresponding to the data type and dimension in
the deﬁnition part. In case of array or structure they are read in the order of the deﬁnition.
Header
The header contains information about the CCF ﬁle which is used to maintain it.
Syntax:#HEADERNAME
CCF name
SET_NOCCF set number
REV_NO
Revision of the data
STATUS
Validated or test version
RANGE
Applicable data range in AFTIMECREATOR Creator of the CCF
CDATE
Create date of the CCF
FMTVERCCF Format version.
COMMENT Comments#ENDHEADERAKARI FIS Data Users Manual

74

Example:#HEADERNAME
C2VT_
SET_NO
001
REV_NO001
STATUSTEST
RANGE
0.00000
0.00000
CREATOR
MAKIUTI
CDATE
2005-07-12T14:16:58
FMTVER
001
COMMENT
Threshold for conversion to Volt from ADU#ENDHEADER
A constant parameter
The data deﬁnition contains only one line starting with ’C’ that is followed by data type.
Syntax:#DEFINE
C
type#ENDDEFINE
#DATA
value
#ENDDATA
Example:#DEFINEC
double#ENDDEFINE#DATA
2.55837499234#ENDDATA; this is an example of a constant.

An array parameter
The data deﬁnition part starts with an ’A’ to indicate that the data is an array. Data typeand array dimensions follow. Up to 8 dimensions (IDL limit) can be speciﬁed. Note that theleft-most index changes ﬁrst (IDL standard) in the multi-dimensional arrays (an array of [dim1,
dim2] is created in the example below). The array values are given in the data body part. Any
blank space, tabs or new lines are recognized as a delimiter, i.e., the values can be split overmultiple lines.

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Syntax:#defineA double dim1 dim2 .....
#enddefine#datavalue1 value2 value3 .... value[dim1xdim2x...]#enddata
Example:#defineA double 2 3#enddefine
#data
1 2
3
5.5 3E-10; comment1; comment2

6.5#enddata
A structure parameter
The deﬁnition part of the structure CCF begins with a ’S’ and the dimension size of the structure.
Components of the structure (’Tag’) are speciﬁed in the following lines. A tag deﬁnition starts
with a ’T’ and a structure tag name, then data deﬁnition (constant or array, see below).
The data body is included for each tag. Therefore there are the same number of pairs of’#DATA’ and ’#ENDDATA’ as the number of tags in the deﬁnition. In order to identify eachdata body with the data tag, the data body starts with a ’T’ and the tag name before the values.
A data body is present for each data tag. In case of an array of structures, the corresponding
number of values are in the data body. For example, if the parameter is a 100 element arrayof structures, and one of the tags in the structure is an array of 3 elements, the data bodywill have 300 values, ﬁrst the tag array for the 1st element of the structure, then for the 2nd
structure...(see example).
Syntax:#defineS dimST tagname {C|A} datatype [dimension]#enddefine#data
T tagnamevalue#enddata

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Example:#defineS 2
T tag_cst C uint
T tag_arr A double 2 3#enddefine#data
T tag_cst
100; for the 1st structure element
200; for the 2nd structure element#enddata#data
T tag_arr
0.1 0.2 ; they are for the 1st structure element3
5.5 3E-10
6.5; followings are for the 2nd structure element 1e2 1e2 1e2 2e3 2e3 2e3#enddata

6.2.2

List of CCFs

Detector properties
BADPL
BADPS
DEADL
DEADS
CRNRL
CRNRS
OFSTL
OFSTS
RSTAL
RSTAS
SATUL
SATUS

; Bad Pixel for the LW band.
; Bad Pixel for the SW band.
; Dead Pixel for the LW band.
; Dead Pixel for the SW band.
; Detector corners for the LW band, wide resp narrow.
; Detector corners for the SW band, wide resp narrow.
; Detector oﬀsets for the LW band, wide resp narrow.
; Detector oﬀsets for the SW band, wide resp narrow.
; Reset anomaly for the LW band.
; Reset anomaly for the SW band.
; Saturation level in Volt for the LW band.
; Saturation level in Volt for the SW band.

Calibration
C2VT
FLATL
FLATS
RCTBL
RCTBS
SIGJL
SIGJS
V2FL

; Threshold for conversion to Volt from ADU.
; Ratio of A-Cal/(ﬂat source) and error for the LW band.
; Ratio of A-Cal/(ﬂat source) and error for the SW band.
; Ramp curve correction table (ﬂight preliminary)for the LW band.
; Ramp curve correction table (ﬂight preliminary)for the SW band.
; sigma j estimated from ﬁs20050219 031354 04 lw.
; sigma j estimated from ﬁs20050219 031354 04 sw.
; Scaling factor from Volt to Flux.

Constants
GLGP; Parameters for various Glitch Processing modules.

Appendix A

FIS Slow-Scan Toolkit Cookbook
In this section we describe how to use the FIS Slow-Scan data reduction toolkit provided fromthe project team. This software tools are often globally referred to as the ﬁsdr system.

A.1

Installation

A.1.1

Prerequisites

The toolkit is currently developed under IDL1 version 6.3/6.4. It may work with IDL versions
as early as 6.0 but it is not guaranteed. The toolkit does not support earlier versions of IDL.
It has been reported that the software works under the Linux (Redhat/Fedora/CentOS/Vine),
MacOS 10.4 and Windows XP environments. At least 512 MB of free memory space is required
to process the Slow-Scan data. A computer with 1 GB or more memory is recommended.
The toolkit uses the IDL Astronomy Library (http://idlastro.gsfc.nasa.gov/). The
routines contained in this library are often updated. Therefore, they are included in the distribution package in order to avoid compatibility problems. It is highly recommended to keep anduse the distributed IDL Astronomy libraries and please be sure that your IDL set up will notcause version conﬂict with the package. The standard set up, as instructed in this cookbook,should not cause such a problem.

A.1.2

Distribution package

The ﬁsdr software package is obtained from the Observation Support web page, as a tar+gzipped
ﬁle. The ﬁle is named as fisdr YYYYMMDD.tar.gz, where YYYYMMDD indicates the version.
The package is composed of several directories containing:• All the Slow-Scan tool programs and calibration parameter ﬁles
• GreenBox modules from the All-Sky Survey Pipeline including the Calibration ConstantFiles (CCF) (see Chapter 4)• The IDL Astronomy Library• The FIS Data Visualizer (FISv; see Section A.3)
The distribution package also includes the startup directory which contains the initializationﬁles of the FIS data reduction environment. They start-up IDL and set up the appropriate
environment variables.
1

IDL is a product of IIT Corporation (http://www.ittvis.com/idl/).

77

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78

A.1.3

Setting and Starting up ﬁsdr under Unix environment

The setup procedure described here is for Unix (including Linux/MacOS).
File extraction
Extracting the FIS reduction package ﬁle will create a directory reduction YYYYMMDD/ and will
put all ﬁles in it. It is recommended to put this directory under the default location:${your home directory}/ASTRO-F/.
% cd ~% mkdir ASTRO-F% cd ASTRO-F% tar xzvf fisdr_YYYYMMDD.tar.gz
or% gunzip -c fisdr_YYYYMMDD.tar.gz | tar xvf Change the directory name
The system assumes that the ﬁles are under the ~/ASTRO-F/reduction/ directory. You may
want to do one of the following under ~/ASTRO-F/:• Make a symbolic link (We recommend this method).
% ln -s reduction_YYYYMMDD reduction• Change the directory name.
% mv reduction_YYYYMMDD reduction
Start up ﬁsdr
% ~/ASTRO-F/reduction/startup/bin/fisdr
This will startup IDL and set up several internal variables/constants used in the system.
You may want to add the above path to your default setup, or deﬁne an alias to the commandin your .cshrc or .bashrc etc.
% set path=( ~/ASTRO-F/reduction/startup/bin/ $path)% alias fisdr ~/ASTRO-F/reduction/startup/bin/fisdr
If you prefer idlde environment, you can call it with the ’-de’ option:% fisdr -deEnvironment variables
Two environment variables are deﬁned for stating up the Slow-Scan tool.
FISDR ROOT speciﬁes the fisdr ROOT directory where all the package ﬁles are stored. The default (the value applied when this environment variable is not set) is /ASTRO-F/reduction.

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setenv FISDR_ROOT my-reduction-directory/reduction/# for csh, tsch etc. andFISDR_ROOT=my-reduction-directory/reduction/
export FISDR_ROOT# for sh etc.
FISDR USER SETUP Some users have their own IDL setup for example the display colour mode
etc. They are often given in /.idlrc. An IDL batch script ﬁle speciﬁed in this environment variable is executed during the startup process and will set appropriate parameters.
Note that the ﬁsdr standard setup may over ride the user’s setup.
setenv FISDR_USER_SETUP ~/.idlrc# for csh, tsch etc. andFISDR_USER_SETUP=~/.idlrc
export FISDR_USER_SETUP# for sh etc.
Details of the startup procedure
When the fisdr command is executed it checks the FISDR ROOT environment variable. If theenvironment variable is not set it assumes the default location (~/ASTRO-F/reduction/). Thenit sets the IDL STARTUP environment variable to the standard ﬁsdr setup script${FISDR ROOT}/startup/pro/startup fisdr. startup fisdr is a simple IDL batch ﬁle that
calls fisdr.pro, which actually sets the ﬁsdr internal environment variables.
The startup script fisdr over rides the IDL STARTUP variable originally set by the user. If
users want to apply their own IDL setting, the FISDR USER SETUP should be used to specify
a setup script. The ﬁle is actually executed in fisdr.pro. Note that the calling the script
given by FISDR USER SETUP is before the main setup in fisdr.pro, and the users setup may beoverrode. Also, it is not guaranteed that the ﬁsdr system will work properly with the user’s
setup.
IDL 6.2 and later versions run the startup script every time the session is reset by the.reset session command so that users can maintain the proper environment. In the earlier
version you may be able to recover the environment with;
IDL> fisdr
command (though earlier versions are not oﬃcially supported).

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80

A.1.4

Setting and Starting up ﬁsdr under the Windows environment

The fisdr system should also work properly under the Windows environment although it is not
thoroughly tested. Contributions from users are welcome.
File extraction
User should prepare ﬁle extraction tools for tar.gz ﬁle. The usage of such tools is dependent on
the speciﬁc software so it is not explained here. The ﬁle can be located anywhere in the PC,
but may be somewhere like
ASTRO-F
reduction
under the users home directory.
Change the directory name
Similarly to the Unix system, change the directory name from reduction YYMMDD to reduction.
Creating a shortcut is also possible.
Setting the startup script
We provide a Visual Basic Script for starting up the ﬁsdr system on Windows. You need toedit the script for your personal environments.
The script ﬁle is ${HOME}/ASTRO-F/reduction/startup/bin/win fisdr.vbs. You can copy
the ﬁle to Desktop or anywhere you like. Open the ﬁle with editor and set the fisdr root
variable to point the directory where you have the system.
’ ==================================================================’ Please edit the following line to point the directory where you’ locate the toolkit.
’ ==================================================================
fisdr_root = "C:\Documents and Settings\astrof\ASTRO-F\reduction\"==> (for example)
fisdr_root = "C:\Documents and Settings\myhomedir\ASTRO-F\reduction\"Start win fisdr.vbs
Click the icon of win fisdr.vbs. If the set up is correct and IDL is properly installed in thesystem you will see that IDL starts and says “FISDR: Environment setup done.” In case oftrouble, check the above setup or contact to Helpdesk.

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A.2

81

The Slow-Scan tool

The Slow-Scan data reduction tools consist of a series of IDL procedures which are under the
directory reduction/slowscan/. In principle, all processes can be controlled from the main routine
ss run ss.pro. The processing of the FIS Slow-Scan data consists basically of two steps which
are executed by ss run ss making use of two main routines:• ss init proc.pro: Flagging bad data, measurement of sky signal, and production of dark
and responsivity correction tables.
• ss make map.pro: Calibration, ﬂat-ﬁelding, and construction of co-added images.
The following people mainly contribute to the development of the Slow-Scan tool: S. Matsuura, M. Shirahata, S. Makiuti, I.Yamamura, T. Suzuki (ISAS).

A.2.1

Running ss run ss

First Look Processing
Consider an example set of pointed observation data in the directory∼/ASTRO-F/fisdata/AKARI FIS 1234567 001/.
% cd ~/ASTRO-F/fisdata/% ls AKARI_FIS_1234567_001/
FIS_LW_20061225081641_1770.fits.gz FIS_SW_20061225081641_1770.fits.gz
FIS_LW_20061225081641_1770.jpg
FIS_SW_20061225081641_1770.jpg
README(WARNING: You may want to preserve the jpeg ﬁles as they are overwritten during the processing).
Start up ﬁsdr and run ss run ss with the default setup:% fisdr
IDL> ss_run_ss, ’AKARI_FIS_1234567_001’, /AOCU
The program will search for all the FIS data (FIS [SW|LW] *.fits.gz) in the speciﬁed
directory and will attempt to process them.
The option ‘/AOCU’ is needed if you want to compare your results with the sample jpeg ﬁles(see Section 5.4). See below for more details on the diﬀerent running options.
You will see some plots and images in diﬀerent plot windows. If the processing completes
without error, the program will produce several output ﬁles in the directory:[Input data]
FIS_[SW|LW]_20061225081641_1770.fits.gz[Intermediate files]
FIS_[SW|LW]_20061225081641_1770_ar.sav
FIS_[SW|LW]_20061225081641_1770_pr.sav
FIS_[SW|LW]_20061225081641_1770_cal.sav
FIS_[SW|LW]_20061225081641_1770_dark.sav
FIS_[SW|LW]_20061225081641_1770_flat.sav

AKARI FIS Data Users Manual82[Output result files]
FIS_[SW|LW]_20061225081641_1770_img.jpg
FIS_[SW|LW]_20061225081641_1770_img.sav
FIS_[SW|LW]_20061225081641_1770_img_[w|n].fits
FIS_[SW|LW]_20061225081641_1770_err_[w|n].fits
FIS_[SW|LW]_20061225081641_1770_num_[w|n].fits(with /CUBE_FITS option the following files are created instead of*_img, *_err, *_num files).
FIS_[SW|LW]_20061225081641_1770_cube_[w|n].fits
Compare the jpeg ﬁles with those attached in the package. If they look identical, the toolkit
works as it is supposed to.
Note that the results may be diﬀerent if the version of the toolkit is diﬀerent. A ﬁle called”VERSION” included in the data distribution package contains the version of the toolkit used
to generate the jpeg ﬁles.
Looking at Images
Several image data/ﬁles are created by ss run ss.pro:*_img.jpg*_img.sav*_img_[w|n].fits*_err_[w|n].fits*_num_[w|n].fits: jpeg format image files: image data in IDL save set files: image map of wide/narrow band data
: error map of wide/narrow band data
: density map of wide/narrow band data*_cube_[w|n].fits : image/error/density maps are in a cube fits file
The * img.sav ﬁles contain the following data:
IDL> restore, ’AKARI_FIS_1234567_001/FIS_SW_20061225081641_1770_img.sav’
IDL> help
COAD_DET
DOUBLE= Array[2, 144, 170]
: Image of WIDE (coad_det[0, *, *]) and NARROW (coad_det[1, *, *]): detectors in MJy/sr.
COAD_STDERRDOUBLE= Array[2, 144, 170]
: Error map of the WIDE/NARROW detectors.
: (RMS of the data points per pixel).
COAD_NUM
DOUBLE= Array[2, 144, 170]
: Density map of the WIDE/NARROW detectors.
IM_LAT_COORDDOUBLE= Array[144, 170]
IM_LON_COORDDOUBLE= Array[144, 170]
: Position of all pixels of the image array in degrees.
DATA_TYPESTRING= ’FIS_SW’: type of input data. Either ’FIS_SW’ or ’FIS_LW’

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GRID_SIZEDOUBLE=: grid size of the image

83
0.0041666667

MAP_MEANDOUBLE= Array[2, 4]: Information of the image data
: MAP_MEAN[0, *] : WIDE band
: MAP_MEAN[1, *] : NARROW band
: MAP_MEAN[*, 0] : Average sky level (3-sigma clipped): MAP_MEAN[*, 1] : Standard deviation of sky level: MAP_MEAN[*, 2] : Number of sky data points
HDR
STRING: FITS header strings

A.2.2= Array[27]Available options for ss run ss

ss run ss has many options for selecting the data reduction/calibration methods, as well as tocontrol its functions.
[Process ﬂow control]/INIT: Stop after ss_init_proc completed.
/MAP: Run only ss_make_map module. Data should have been processed by: ss_init_proc (with /INIT option). This allows different options of: ss_make_map to be tested without running ss_init_proc every time.
/SW, /LW: Process only SW / LW data.

[Input / Output ﬁle names]
TAG_NAME = tag_name: Add a specified string to the output file names. For example,: tag_name = ’_new’ will make the output file names as,: FIS_[SW|LW]_20061225081641_1770_new_img_[w|n].fits.
/FILE_INPUT: The first argument of ss_run_ss is a file name instead of: directory name.
/CUBE_FITS: Generate image cube fits containing image, error, and density: maps in a FITS file, instead of three independent files. The output: file names are *_cube_[w|n].fits.
[Selection of positional information]

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AKARI FIS Data Users Manual/AOCU_ADS: Use the AOCU determined boresight position instead of: default G-ADS position data.

[Options for signal measurements]
The following options are passed to ss init proc.pro.
N_RAMP_DIV = n_ramp_div: By default a linear fit is applied to each integration ramp to derive: the signal level. If N_RAMP_DIV greater than 1 is given, A ramp
: is divided into n_ramp_div’s, and linear fitting is made for each: segment of the divided ramp. Taking n_ramp_div greater than 1 provides: higher spatial resolution in the in-scan direction at the cost of: noise level, as well as increased calibration uncertainty due to any: remaining non-linearity of the ramps after the correction.
/USE_BAD: In addition to automatic detection and flagging of the bad data by the: program, this option force the program to refer to PIXEL_BAD flag in: the input TSD and discard flagged data. This option is used to reflect: the results of manual flagging of data prior to use of the Slow-Scan tool
: with the FISv data browser. Default is that PIXEL_BAD is not checked.
/LOCAL_FLAT: To use the ‘blank’ sky data in the observation for flat-fielding.
: If T_FLAT_START and T_FLAT_END (as specified below) are not: provided, the data during attitude settling (approximately 420-630 sec: from the beginning of the data) are used for the correction.
: Without this option flat-fielding is applied using ‘the nominal flat’: built from the PV observations of zodiacal light and cirrus emission.
: The usage of /LOCAL_FLAT may provide better flat-fielding results: for flat (quiet) skies.
T_FLAT_START = t_flat_start
T_FLAT_END = t_flat_end: They are only valid with the /LOCAL_FLAT option and specify the: data range used for local flat-fielding. They are in seconds: from the beginning of the TSD data. The defaults are::
T_FLAT_START
T_FLAT_END: PV data
540 sec
750 sec: Phase 1&2
540 sec
630 sec
PLOT_PIXEL = plot_pixel: ’ss_init_proc’ procedure displays several plots of data as: a function of time/index during the process. This option: specifies the detector pixel number(s) to be plotted.
: The default setting is plot_pix=5.
/RAMP_COR_OFF

Version 1.3 (September 14, 2007): Ramp curve correction (non-linearity correction) is not applied: before the linear fitting process. Usually ramp curve correction: is needed for all data sets.
/GLITCH_RMV_OFF: Do not detect + mask glitches.

[Options for calibration and map making]
The following options are passed to ss make map.pro.
/PIX_MAPPING: By default ss_make_map.pro creates so called "foot print map";: each data point in the input time series data is placed on: an image pixel closest to the data position. With this option: convolution by a disk of a radius corresponds to the beamsize: is applied. This fills void pixels often seen in some data.
/TRANS_COR: A simple correction of the detector responsivity time variation: caused by the calibration lamp, by iterpolating the signal: level at before and well after the lamp. Default is OFF.
/MEDIAN_FILTER: Apply a median filter to the time series data for removing: remaining background offsets between the pixels as well as: excess by stray light from the Earth shine. This median: filtering is only applicable for point sources. Diffuse: sources with a spatial scale comparable to the ‘WIDTH\_FILTER’: used (see below) would be also filtered out by this process.
: The default filter width is 40 seconds.
/SMOOTH_FILTER: Similar to MEDIAN_FILTER but use boxcar smoothing to: evaluate the background offsets instead of the median.
: The default filter width is 40 seconds.
WIDTH_FILTER: (previously known as WIDTH_MEDIAN).
: The width of the MEDIAN_FILTER/SMOOTH_FILTER, in units of: seconds. The default is 40 seconds.
SIGMA = sigma: Sigma clipping threshold for the map making. Default is 2.0.
: Smaller values (e.g., 1.5) may improve the output quality: with a risk of less data points used to make the maps.
T_START = t_start
T_END = t_end

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AKARI FIS Data Users Manual: Specify start and end of the data used to make the images.
: Given in units of seconds counted from the beginning of the: TSD data. The defaults are::
T_START
T_END: PV data
750 sec
1350 sec: Phase 1&2
630 sec
1350 sec/GALACTIC/ECLIPTIC: Specify the coordinate system of the output images.
: The default coordinate system is Equatorial (J2000)
LON_CENTER = lon_center
LAT_CENTER = lon_center: Centre position of the image. By default centre position is: estimated from the input data. These options are useful when
: one wants to compare the images from different data sets of nearby sky.
: Units are in degree, in the corrdinate system specified.
LON_SIZE = lon_size
LAT_SIZE = lat_size: Image size in degrees. By default image size is automatically: determined to cover the input data. Useful to make consistent: set of images from different observational data sets.
GRID_SW = grid_sw
GRID_LW = grid_lw: Grid size of the resulting image map in arcsec. Default values are:
:
AOT
FIS01
FIS02
:
SW
15 arcsec
30 arcsec:
LW
30 arcsec
60 arcsec: Smaller grid sizes for low-redundancy data may result in: many void pixels in the image. Note that the grid size
: has the lower limit given by the program automatically,: and too small values for the parameters are truncated to: the lower limits. The values are defined as;:
grid_limit = beam_size/3600D/n_int*(scan_speed/15["/s]): where beam_size is 40 and 60 arcsec for SW and LW detectors,
BAD_THRSHLD = bad_thrshld: Responsivity of each pixel derived from the observational data
: is compared with the ‘nominal’ value in the responsivity table: and if the difference is larger than a factor specified by this: option the data of that pixel is not used throughout the observation.
: The cause of responsivity change could be glitch after effects.
: The default is 10 (practically no flagging).
/SCUT: Produce individual images per one way scan, i.e, four images: are produced for FIS01 observations (two roundtrips) and two

Version 1.3 (September 14, 2007): images for FIS02 observations (one roundtrip). This option is: useful to confirm source detections by comparing independent: scans. Normal coadded image (from all scans) is also created.
: Output image files are labeled as;: *_img_[1,2,..].sav & *_img_[1,2,..]_[w|n].fits.
/SL_RMV: When this keyword is set, the stray light caused by the Earth: shine reflected at the telescope baffle is subtracted. The: stray light component is evaluated by a boxcar filter of 90: seconds width, then subtracted from the data.
/MASK_OFF: Normally, data in the state of Shutter close and CAL-ON are: masked and do not appear in the output data.
: If this keyword is set, the masked data appears in the output: data file and the output image. For experts and debugging.
/RESP_COR_OFF: If this keyword is set, the responsivity correction is not: performed and the output is the slope of the ramp in units: of [V/s]. For experts.

[Image plot style options]
The following options are passed to ss make map.pro.
/COORD_GRID_ON: Display coordinate grid on a JPEG image.
/OUTLINE: Produce jpeg image with outline contour.
CONTOUR_NUM=contour_num: Number of contour lines drawn in the JPEG image map.

[Batch mode process control]
These options are prepared for batch processing at ISAS and are not relevant for the users.
/NO_DISPLAY: Running in batch process mode and do not display: any plots on the screen.
/MK_TXT_FILE: Create text file format intermediate files.
: This option remains for compatibility with the older versions.
: It is not recommended for the observers.

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AKARI FIS Data Users Manual/DEL_FILES: Delete output files (*.txt, *.sav, *.fits) other than image jpeg files.
/VERSION_COPY: Copy the version file into the processed directory.

A.2.3

Tips and suggestions for Slow-Scan data reduction

Here we note some useful recipes for the Slow-Scan tools for several typical cases we haveexperienced. The Slow-Scan toolkit has been used with a relatively limited number of data setsby the developers. Comments and contributions from the users are welcome and will improve
future versions of the pipeline.
/AOCU option
As of September 2007 the default positional information from G-ADS is yet to be conﬁrmed
to be reliable. Therefore we recommend to use this option to apply AOCU position andcompare the results.
Improving ﬂat-ﬁelding
It is reported that the /LOCAL FLAT option often improves the ﬂat-ﬁelding of the image.
It is most eﬀective if the background sky is relatively smooth. /MEDIAN FILTER is anotheroption to correct any remaining oﬀset between the scans/pixels.
Very dark sky
Give T FLAT START and T FLAT END to cover the entire observing region together with
/LOCAL FLAT. If the sources in the region give only minor contribution to the total ﬂux
from the region this could even further improve the ﬂat-ﬁelding. The data range in seconds
from the beginning of the ﬁle can be obtained with FISv (See Section A.3) by selecting
aftime in X-axis.
Measuring the sky brightness
If absolute sky brightness of the diﬀuse radiation is important, try /SL RMV and /TRANS
options together.
Point sources / Small extended sources
Try /SL RMV + /TRANS + /MEDIAN, or /TRANS + /MEDIAN FILTER. Applying /MEDIAN FILTER
option improves the RMS noise of the image and is useful to detect faint point sources.
/MEDIAN FILTER and /SMOOTH FILTER For the bright sources /SMOOTH FILTER results in smaller“holes” next to the sources.
SIGMA option
Try SIGMA=1.5 which may remove more outlier data points resulting in better images.
Comparison between scans in an observation/SCUT option will produce separate images for each one-way scan in the observations. You
can compare them for cross-checking of detections and consistency of the positions, etc...
Removing glitch eﬀects
If bright lines (stripes) are observed, try using a BAD THRSHLD lower than the default (10).
Often such stripes are responsivity jumps due to glitches. By giving a lower BAD THRSHLD,
pixels with a responsivity level higher than nominal can be discarded.

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Behind ss run ss

In the following section we describe the two major steps in the Slow-Scan processing. They
correspond to two separate programs: ss init proc and ss make map. These programs are executed from ss run ss. Note that the contents of the following sections are subject to continuous
update without notiﬁcation.
Dark and responsivity correction tables: ss init proc.pro
ss init proc generates dark and responsivity correction tables in [V/s], which are used byss make map for further calibration and map making. This routine is called from the main
program ss run ss with all available options. Therefore users are not required to apply thisroutine directly.
ss init proc procedure makes use of the following routines:
ss flux ave.pro calculates mean ﬂux of line ﬁt data in a given time range.
ss aftcal ave.pro makes a template of CAL after eﬀect signal and output data table (onlyfor PV-phase observations).
ss q to radec calculates equatorial coordinates of each pixel from quaternion in GADS or AOCU
data.

ss init proc produces several intermediate data ﬁles in the IDL save ﬁle format. Theseﬁles will be used at the following steps in the processing. Users can ignore these ﬁles but thoseinterested in the details and who would like to participate in the data reduction activity to
improve the results, are advised to contact the data reduction team in Japan via the Helpdesk.
The AKARI data reduction support team welcomes the users’ participation.
FIS_[SW|LW]_20061225081641_1770_pr.sav: Time series calibrated detector signal and related information.
: ({\tt "pr"} denotes Pre-Reduction).
FIS_[SW|LW]_20061225081641_1770_cal.sav: Information of calibration lamp signal strength.
FIS_[SW|LW]_20061225081641_1770_dark.sav: Information of dark measurement signal strength.
FIS_[SW|LW]_20061225081641_1770_flat.sav: Information of flat-fielding measurements.

FIS [SW|LW] * pr.sav contains the following information. The data are time sequential
3542 data points of the SW (=100 pixels) detector. * cal.sav, * dark.sav, and * flat.sav
ﬁles are also explained.
FISDR> restore, ’FIS_SW_20070705213613_1770_pr.sav’
FISDR> help
RCOUNT
LONG=
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90

X_AVE
DOUBLE= Array[3542]: Data sampling index averaged over each ramp.
COEFF
DOUBLE= Array[100, 3542, 2]: Results of the linear line fit of each ramp.
: [#pixels, #ramps, coeff]: [*, *, 0] = offset, [*, *, 1] = tilt.
ERROR
DOUBLE= Array[100, 3542, 2]: Uncertainty of the linear fit.
EG_GL_CNT
INT= Array[100, 3542]: Number of glitches detected in each ramp.
CAL_FLG_TOT
INT= Array[3542]: Number of data points with calibration lamp is ON in each ramp.
: If not zero, the ramp contains calibration lamp signal
SHT_FLG_TOT
INT= Array[3542]: Number of data points with shutter open - number of data points: with shutter close.
DRV_MASK_TOT
INT= Array[3542]: Number of data points with calibration lamp is ON or: shutter is closed per ramp. If not zero, the ramp
: does not contain sky observation data.
Q_MEANDOUBLE= Array[4, 3542]: Quarternion averaged over each ramp.
AA_MEANDOUBLE= Array[3542]: Earth Avoidance Angle (EAA) averaged over each ramp.
AOT
STRING= ’P12_FIS01’
AOTPARAMSTRING= ’0.5;8;70’
DATA_TYPESTRING= ’FIS_SW’: Information about the observation.
N_INT
INT
=
1
: Value given by N_RAMP_DIV option. 1: default (data point: corresponds to each ramp).
RST
STRING= ’05N’: Internal status to identify the reset mode.
RST_INTDOUBLE=
0.47468354: The actual reset interval in seconds.
AOCU_ADS

INT

=

0

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GLITCH_RMV_OFF INT=
0
RAMP_COR_OFF
INT=0: Options for ss_init_proc.pro. 1 : ON.
* cal.sav, * dark.sav, * flat.sav contains calibration information.
FISDR> restore, ’FIS_SW_20070705213613_1770_cal.sav’
FISDR> help
FLUX_CALDOUBLE= Array[4, 100, 7]: Signal level of calibration lamp data.
: [#data, #pixel, #cal.lamp_data].
: An FIS01 observaton data file contains seven calibration: lamp data and an FIS02 data file has five.
: [0, *, *] = Pixel index.
: [1, *, *] = Average calibration lamp strength [V/s].
: [2, *, *] = Uncertainty of the averaged value (rms/sqrt(N-1)).
: [3, *, *] = Number of data samples N.
FISDR> restore, ’FIS_SW_20070705213613_1770_dark.sav’
FISDR> help
FLUX_DARKDOUBLE= Array[4, 100, 8]: Signal level of dark data (shutter close signal).
: [#data, #pixel, #dark_data].
: An FIS01 observaton data file contains eight dark
: measurements and an FIS02 data file has six.
: The meaning of the first dimension is the same as FLUX_CAL.
FISDR> restore, ’FIS_SW_20070705213613_1770_flat.sav’
FISDR> help
FLUX_FLATDOUBLE= Array[4, 100]: Signal level of the flat-field data.
: [#data, #pixel].
: The meaning of the first dimension is the same as FLUX_CAL.
/MK TXT FILE option additionally creates the following text format ﬁles. The contents of theﬁles are almost the same as those in the .sav ﬁles. They are only left for compatibility and are
not recommended to use by the observers.
FIS_[SW|LW]_20061225081641_1770_dark_[0,1,2,...].txt
FIS_[SW|LW]_20061225081641_1770_cal_[0,1,2,...].txt
FIS_[SW|LW]_20061225081641_1770_cal_dark.txt
FIS_[SW|LW]_20061225081641_1770_flat.txt
FIS_[SW|LW]_20061225081641_1770_flat_dark.txt
FIS_[SW|LW]_20061225081641_1770_fit_pr.txt
FIS_[SW|LW]_20061225081641_1770_fit_ar.txt
FIS_[SW|LW]_20061225081641_1770_ctof.txt
Co-add image processing: ss make map.pro
ss make map reads the output ﬁles of ss init proc, applies calibration, and creates the co-add
image in various formats. The procedure ss init proc is called from ss run ss with all options.
Therefore users are not required to use this routine directly.

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ss make map procedure makes use of the following routines:
ss resp corr returns the absolute responsivity.
ss q to radec calculates equatorial coordinates for each pixel from the quaternion in the GADS
or AOCU data.
ss trans corr improves transient eﬀects after calibration lamp operation.
ss eg lpf applies median/boxcar-smooth low-pass ﬁltering to extract DC/slow components
Earth shine.
ss eg slrmv extracts slow component and stray light from time-series data by an easy way
with median/boxcar smooth ﬁlter.
ss coad image Constructs co-add images of the entire array.
ss make map produces the following output ﬁles:FIS_[SW|LW]_*_ar.sav: Time series processed data, RA & DEC position, and parameters: ({\tt "ar"} denotes After-Reduction).
FIS_[SW|LW]_*_img.sav: Co-add image data, RA & DEC position, and parameters.
FIS_[SW|LW]_*_img_[w|n].fits: image map of wide/narrow band
FIS_[SW|LW]_*_err_[w|n].fits: error map (statistical uncertainty of each pixel): of wide/narrow band image
FIS_[SW|LW]_*_num_[w|n].fits: density map (number of data points contributed to each pixel): of wide/narrow band data
FIS_[SW|LW]_*_img.jpg: co-add image as jpeg image.
(with /CUBE_FITS option the following files are created instead of*_img, *_err, *_num files).
FIS_[SW|LW]_*_cube_[w|n].fits

Description about the image ﬁles are given in the previous section. FIS [SW|LW] * ar.sav
are the IDL save ﬁle that contains the following information. The following example data aretime sequential 879 data points of the SW (=100 pixels) detector.
IDL> restore, ’FIS_[SW|LW]_*_ar.sav’
IDL> help
RA
DOUBLE= Array[100, 879]DEC
DOUBLE= Array[100, 879]
: Position information of every pixels at every: sampling points.

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DET
DOUBLE= Array[100, 879]
: Calibrated detector signal.
ERR
DOUBLE= Array[100, 879]
: Uncertainty of the detector signal.
EG_GL_CNT
INT= Array[100, 879]
: Number of glitches detected in each ramp.
BAD_FLG
INT= Array[100]: 1 for band pixels (not used in the processing).
X

DOUBLE= Array[879]: Data sampling index averaged over each ramp.

AOT
STRING= ’P12_FIS01’
DATA_TYPESTRING= ’FIS_SW’: Information about the observation.
N_INT
INT
=
1
: Value given by N_RAMP_DIV option. 1: default (data point: corresponds to each ramp).
RST
STRING= ’10N’
RST_INTDOUBLE=
0.94936709: The actual reset interval in seconds.
AOCU_ADS
INT=
1GLITCH_RMV_OFF INT=
0
RAMP_COR_OFF
INT=0: Options for ss_init_proc.pro. 1 : ON.

A.3

The FIS Data Visualizer (FISv)

The FIS Data Visualizer (FISv) is a Graphical User Interface (GUI) tool that has been createdin order to provide a method of easy access to the FIS data (Time Series Data: TSD) and toeasily display time series data such as the “detector signal”, “Instrument status”, “ﬂags”, etc...
FISv is distributed as part of the Slow-Scan toolkit and is written in IDL. To begin the FISv
tool, you simply start a ﬁsdr session as explained at the beginning of this chapter and type ﬁsv
or FISV.
Then you will see a window similar to that shown in Figure A.3.1.

A.3.1

Main Functions: Input/Output

There are four action buttons to open and save TSD ﬁles and to close the FISv application.
When a TSD ﬁle is opened all the diﬀerent options and functions become active as shown in
Figure A.3.1.
The option “Open TSD ﬁle from LDS” is not applicable to the users as it is for accessing
the Local Data Server at ISAS (Japan).

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Figure A.3.1: TSD ﬁle opened with FISv
In the two windows, labeled “X axis” and “Y axis”, appear all the extensions contained in
the TSD ﬁle (that can be visualized using the Header View button) and the items under theextension FIS OBS. Any of these items can be plotted against one another by selecting theappropriate entry in the corresponding “X” and “Y” axis windows. A Preview graph appears
as is shown in Figure A.3.2.

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Figure A.3.2: Detector 10 signal plotted against AKARI satellite time
Output options include:
Plot. The currently displayed graph image can be plotted by clicking on the action button“Plot”. A FISv plot window as the one shown in Figure A.3.3 will appear.

Figure A.3.3: Detector 10 signal plotted against AKARI satellite time

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Table. A Dump of the data into a text format ﬁle is also possible with the action button“Table” (see Figure A.3.4).

Figure A.3.4: Dump of data into text format ﬁlePrint. Print out the currently displayed graph image. Button “Print” will give the option
to print out the currently featured graph or to save it as a postscript ﬁle.

Figure A.3.5: Printer setup dialog
PS. A postscript ﬁle can also be generated from the “PS” button. A window with the
conﬁguration options will appear.

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Figure A.3.6: Postscript output setup dialog
Processed data can also be saved into another TSD ﬁle (FITS binary).

A.3.2

Main Functions: Display

As shown in the previous section it is possible to display detector signals (for each pixel) and thestatus (Shutter open/closed, calibration lamp on/oﬀ, etc...). More than one value can be plottedat once by selecting multiple items using the [Ctrl] or [Shift] key on your computer keyboard.
They will be plotted in the same graph as shown in Figure A.3.7 where the detector signal and
Shutter Open status (ShtOp) are selected.

Figure A.3.7: Detector signal and Shutter Open plot
The button “SFlag” on the plot window allows both keywords to be displayed separately on
a Double Y axis as shown in Figure A.3.8.

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Figure A.3.8: Detector signal and Shutter Open plotted separately using the SFlag option

The plot window also allows one to perform simple statistics (STAT), diﬀerentiation (DIFF)
and Fast Fourier Transforms of the signal in the time domain (FFT).
Other display functions allow the display of all the pixels (see Fig A.3.9) and the display of
a 2D and 3D image of the detector array (Fig A.3.10).

Figure A.3.9: Signal plot of all pixels

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Figure A.3.10: 2D and 3D images of the detector array

A.3.3

Main functions: Processing

FISv also allows some simple calculations and processing besides the mentioned statistics, differentiation and FFT.
• A subset can be made from the TSD interactively. The current TSD will then be replacedby the subset.
• Plots can also be masked by some of the deﬁned ﬂags.
• Signal processing can be carried out by calling the GB pipeline modules (see Chapter 4).
However, FISv is not intended to be at the moment a data reduction tool. It should be
noted that using GB pipeline functions will override the original processing of the data.
A history of the GB modules used is kept in the primary header of the TSD ﬁle.

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A.4

Contribution tools

Contribution tools are extra tools which provide additional functions to the standard processing
package, or tools for data reduction/analysis beyond the scope of the standard tool.

A.4.1

Making mosaic images from multiple observations

Program Name
ss mosaic image.pro
Developers
M. Shirahata and S. Matsuura (ISAS)
PurposeThis tool is prepared to make mosaic images map using multiple scan data. It is useful forobservations of wide area covered by multiple pointings, and deep imaging by repeating
the pointed observations.
Calling Sequence
ss_mosaic_image, targetdir, SIGMA=sigma, $T_START=t_start, T_END=t_end, $GRID_SW = grid_sw, GRID_LW = grid_lw, $LON_CENTER = lon_center, LAT_CENTER = lat_center, $LON_SIZE = lon_size, LAT_SIZE = lat_size, $ECLIPTIC = ecliptic, GALACTIC = galactic, $CUBE_FITS = cube_fits, $TAG_NAME = tag_name, $aot_mix=aot_mix, NO_DISPLAY=nodisp

Input
targetdir : directory that contains * ar.sav ﬁles produced by ss run ss you want to
combine.
OutputImage ﬁles in IDL save ﬁle and FITS format in the directory.
Options
Most of the options for ss mosaic image are the same as ss run ss and already explainedin the previous section. An only unique option for this module is AOT MIX. Note that nolower limit is applied for GRID SW and GRID LW parameters.
/AOT_MIX: Use all available data in the directory. See Explanation below.
Explanation
ss mosaic image searches for all processed scan data (* ar.sav ﬁle) in the target directoryand constructs images using all available data. It assumes that the directory contains a‘uniform’ data set, i.e., same AOT and AOT parameters. If data ﬁles of diﬀerent data

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sampling modes are in the target directory the program takes the mode of the ﬁrst foundﬁle (normal OS list order). /AOT MIX option forces the program to use all available data,
regardless of their sampling modes. The internal algorithm and the results are the same as
those of the standard ss run ss tool. Note that this program attempts to make an image
that covers all available data. If the directory contains scan data that is (spatially) far
from the others, the program tries to allocate a huge amount of memory for the creationof the image plane which could cause many problems.