2007/9/17

FIS Slow Scan Tool, advanced

ISAS/JAXA
Shuji Matsuura

I. Need for correction of various detector effects
Output [V/s]Time domain:
1. Dark subtraction
2. Responsivity correction
3. Flat fielding
4. Correction of slow transients
5. Stray light removal
6. Glitches

LW data

CAL lamp

Dark sky signal

Slow transient
after CAL lamp

Dark current
Pixel #II. Optional functions of “ss_run_ss.pro”
1. /LOCAL_FLAT (,T_FLAT_START=value, T_FLAT_END=value)
Produce a flat field by integrating the data for a given time range.
2. /TRANS_COR
Correction of the drift after shutter close/open.
3. /SL_RMVStray light removal with a very slow filter.
4. /SMOOTH_FILTER or /MEDIAN_FILTER (,WIDTH_FILTER=value)
High-pass filter with smooth or median function for a given time width.
5. BAD_THRSHLD=value
Threshold for the bad-pixel rejection by checking the responsivity.
6. SIGMA=value
Threshold for the bad-pixel rejection in the co-add process.
7. N_RAMP_DIV=value
Each ramp is divided into this number.
8. GRID_SW=value, GRID_LW=value
Grid size specified for making co-added image.
9. /PIX_MAPPING
To produce a smooth image with finer pixel by the pixel convolution.
10. /SCUT
To produce individual co-added images for all 4(2) scans of FIS01(02).

II-1. Flat field:
Pre-measured flat (default)

Cal-Dark Zodi / Cirrus observations in PV Dark Cal - Dark
 Flat_sky - Dark

Dark

 CTOF= (Cal - Dark) / (Flat_sky - Dark)

SW

Sky-Dark Flat = (Cal - Dark) / CTOF,by default
LWSWII-1. Absolute calibration
Responsivity
Conversion factor
from: detector current
to: absolute sky brightnessAbsolute scale for pix-averaged
responsvity was scaled to
DIRBE/COBE data.

Responsivity for diffuse sourcein orbit was consistent with preflight measurement in lab.

Calibration for point sources wasseparately done.

II-1. Flat field option: /LOCAL_FLAT

A flat field is produced by integrating the data during the attitudesettling time (after CAL sequence and before starting the scan),instead of the pre-measured flat.
Time range used for the flat field can be specified as :T_FLAT_START = value and T_FLAT_END = value.
If a smooth sky is chosen, better result than default is obtained.

630s
Flat data: 90s
T_FLAT_START

T_FLAT_END

II-1. /LOCAL_FLAT/LOCAL option is effective to clean up stripes.
WL w/o /local

WL w /local

WS w/o /local

WS w /local

II-2. Treatment for transient effects
After effect induced by cosmic-ray events
 Current version removes the glitches in the linear fittingprocess.
 Tails due to responsivity drift are partly rejected in the coaddition process by the redundancy.
– correction for the tails, to be implemented soonAfter effect induced by CAL & Dark measurements– correction process is implemented as /TRAS_COR option

Slow response to astronomical signals– future workII-2. Rejection of glitchesFor slow-scan observation, glitch signal is much faster than the source signal.
Data affected by glitch can be easily flagged out. (set to zero)

Including glitches

after deglitchingflagsII-2. /TRANS_CORSlow transient correction for the data after shutter close/open.
Applicable in any observations, but over/under correction may
happen in some rare cases.
“ FIS_[SW|LW]_*_ar.sav “

Shutter close
corrected

uncorrected

WL data at dark skyopencloseopen

II-2. /TRANS_COR
Useful for mosaic imaging which requires smooth connection
between different observations at the edge.

w/o /TRANS

w /TRANSII-3. Need for stray light removalTime series data plotted as a function of ecliptic coordinate.
Co-addition of the data including the stray light may smear out
faint sources.
End of scan

FIS02
: Single round-trip

sourceBrightness [MJy/sr]sourceStart of scan

TurnDEC [deg]II-3. /SL_RMVStray light intensity depends on the earth avoidance angle (EAA).
High-pass filtering with a very long time constant (fixed to 90s).
Minimum brightness during the observation at the maximum EAA isassumed to be “real” sky brightness.
 If monotonic increase/decrease of signal exists, mean brightness aroundthe maximum EAA is used as “real” sky brightness.

Before correction

After correction

II-3. /SL_RMV

Average intensity of the stray light as functions of time and EAA.
This information is saved as “ FIS_[SW|LW]_*_sl.sav ”.

WSN60

WStimeEarth Avoidance Angle

II-3. /SL_RMV

WS data at dark sky (~5 MJy/sr)
Stray light (max~3 MJy/sr) has non-negligible contribution.
Co-addition of data including the stray light is awful.
Applicable only for dark sky (<10 MJy) in general, or for compact
source with a size smaller than (90s x scan-speed).
w/o /SL_RMVw /SL_RMVII-3. Another method of stray light removal

Note that the use of /SL_RMV option is one of practical methods.
Development of more advanced method, modeling the stray light
profiles as functions of EAA and satellite orbit, is in progress.

Baffle function is assumed
to follow exp-law

I: total signal
B: sky background
A: fitting constant
θ: earth avoidance angle
C: scale angleθBrightness [MJy/sr]

I = B + A*exp[-(θ/C)^2]

Before subtraction

After subtraction

Time index

II-4. /SMOOTH_FILTER or /MEDIAN_FILTER

High-pass filtering with a default/user-specified time constant as
WIDTH_FILTER = value in [s].
WIDTH_FLITER = 40 by default.
Sky background is subtracted and its mean is set to zero.
(WIDTH x scan-speed) has to be larger than size of the source of interest.
The filtered data and related information, “ FIS_[SW|LW]_*_sl.sav ”.
Width=20

Width=40 (default)

Width=90 (same as SL_RMV)Contour: 0-100 (every 5), peak ~550

II-5. BAD_THRSHLD

Threshold for the bad-pixel rejection in a process of responsivity correction, asBAD_THRSHLD=value.
After strong radiation events, the detector responsivity changes with a very
long decaying time.
Detector responsivity is checked by the CAL light measurements in thesequence.
If the CAL signal of a certain pixel is stronger/weaker than the pixel-averaged
CAL signal by a factor of BAD_THRSHLD, this pixel is removed from the as abad pixel.
BAD_THRSHLD may take any positive value.
Default setting is a large value of 10, and this function is not effective.

II-6. SIGMAThreshold for the sigma clipping in the co-addition process, asSIGMA=value in [sigma].
Default setting is SIGMA=2.
User can specify smaller value, e.g. SIGMA=1.5, as far as number of nonrejected data points is sufficiently large.

II-7. N_RAMP_DIV=value

A ramp is divided into the specified number.
Linear fitting is done for each sub-ramp.
Useful to obtain finer grid sampled image.
Some artifacts
may appear due
to incompletenessof non-linear rampcorrection.

Please check“ FIS_[SW|LW]_*_ar.sav “(processed time series data)

II-8. GRID_SW=value, GRID_LW=value
Finer image is obtained by taking smaller GRID_SW/LW.
The grid size has to be large enough to keep the redundancy,
corresponding to N_RAMP_DIV.
Default: N_RAMP=1, GRID_SW=15

N_RAMP=2, GRID_SW=7.5

II-9. /PIX_MAPPING

To produce a smooth image with finer grid by the pixel convolution.
Be careful that co-added image with too small grid size is not reliablebecause of low redundancy.

OFFONII-10. /SCUT

It produces individual co-added images for all 4(2) scans of FIS01(02).
Useful to check the multiple viewing of a source.
Outputs “FIS_*_[1-4].sav” and “FIS_*_[1-4]_[w|n].fits”
1234

III. Recommended combination of options
Point source:
Bright (>10 Jy)
ss_run_ss, ‘dir’ (, /local, /smooth, width_filter = 90)
Isolated, medium (0.2 ~ 10 Jy)
ss_run_ss, ‘dir’, /local, /smooth, width_filter = 60 ~ 90
Faint source(s) (<0.2 Jy)
ss_run_ss, ‘dir’, /local, /trans, /sl_rmv, /smooth
Diffuse source:
Bright (>10 MJy/sr)
ss_run_ss, ‘dir’, /local, /trans* T_flat_start/end should be specified for better flat
Faint (<10 MJy/sr)
ss_run_ss, ‘dir’, /local, /trans, /sl_rmv

IV. Future plan for the revision of SS-Tools


Calibration

Correction of after effectsConsistency between diffuse and point sources

Correction for the tails after cosmic-ray events
Slow response to astronomical signals

Stray light removal

Physical modeling using many observation samples