aXeTwoZero: The next
generation of aXe
M. Kümmel1(mkuemmel@eso.org), H. Kuntschner1, J.R. Walsh1, H. Bushouse2(1) Space Telescope - European Coordinating Facility,Karl-Schwarzschild-Str. 2, D-85748 Garching b. München, Germany(2) Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218, USA

Abstract
The aXe spectral extraction software was designed to extract spectra from slitless datasuch as taken with the Advanced Camera for Surveys (ACS) on board of HST. Wide FieldCamera 3 (WFC3), which was installed on HST during Service Mission 4 this May, also
contains slitless spectroscopic modes on both, the IR and the UVIS arm, and aXe is therecommended tool for reducing its slitless data. Rather than adjusting the previous version
aXe-1.71 to WFC3,we have chosen to re-factorize the current aXe to build aXeTwoZero(aXeTZ) ,the next generation of the aXe software. We explain the structure and theproperties of aXeTZ. The reduction of WFC3 slitless data with aXeTZ is demonstratedusing early in-orbit calibration data. aXeTZ is scheduled to be released as part of theSTSDAS in November 2009. A brief outlook on further developments is given.
1. Slitless spectroscopy with aXe
Slitless spectroscopy has some special features andproperties which require a dedicated extraction package suchas aXe [1,2,5]. A slitless data set as shown in Figure 1 usuallyconsists of a slitless image (lower panel) and a corresponding
direct image (upper panel, here F850LP) taken at the same
position in order to derive the zeropoint of the wavelength
calibration. Due to the low spectral resolving power, there are
multiple spectral orders of the same object visible in a singleimage. The absence of slits makes contamination, which is themutual overlap of spectra, a ubiquitous phenomenon in boththe spatial direction and in dispersion direction, even acrossdifferent spectral orders and at distances of many hundred
pixels. The blue circles in Fig.1 mark two object pairs withoverlapping spectra in the slitless image. Table 1 lists all HST
slitless spectroscopic modes to which aXe has been applied.
Instrument/ Disperser Wavelength Dispersion
Camera[Å/pixel]Range [Å]WFC3/IR
WFC3/IR
WFC3/UVISACS/WFC
ACS/HRC1
ACS/HRC1ACS/SBCACS/SBCNICMOS/NIC3

G102
G141
G280
G800L
G800L
PR200L
PR130L
PR110L
G141

FOV
[“]

7800-10700
24.7
123x137
10500-17000
47.0
123x137
2000-4000
13.0
160x160
5500 – 10500
38.5
202x2025500 – 10500
23.5
29x26
1600 – 3900 20 [@2500Å] 29x26
1250 – 1800 7 [@1500Å] 35x31
1150 – 1800 10 [@1500Å] 35x31
11000 – 19000
80
51x51

Figure 1: The basic data set of one slitless spectroscopic
image (bottom) and the corresponding direct image (top)Figure 2: The structure of aXeTZ

2. aXeTZ (aXeTwoZero)
The structure of aXeTZ is depicted in Figure 2. As
compared to previous versions, aXeTZ contains apython interface layer which is capable to fully replace
the PyRAF layer and would allow an easytransformation of aXeTZ into a pure python module.
Further features of aXeTZ and its development include:• backward compatibility to previous aXe versions. All
instrument modes listed in Table 1 are supported;• a clear, 'pythonic' software design;• usage of object oriented techniques and patterns;• reduced use of external libraries;• complete 64bit compatibility;
In addition to the usual suite of test programs, stresstests with aXeTZ were done using HLA/ACS grism data
sets [6], which offer the largest challenges in terms ofnumber of objects and data volume. Moreover aXeTZ
was extensively tested on data from the 'new‘
WFC3/IR G102 and G141 grisms (see Sect. 3) toprovide users of these modes with a flawless data
reduction software.

1: operation discontinued
Table 1: HST slitless spectroscopic modes to which aXeTZ is to be applied3. aXeTZ applied to WFC3 data
The reduction of WFC3 data with aXeTZ is illustrated in Figure 3. Slitless data of thePlanetary Nebula Vy2-2 has been taken as part of the Cycle 17 calibration program 11937(PI: Bushouse) and reduced with a release-ready version of aXeTZ. The left panels of Fig.3
show an example of a G102 image (top) together with its associated direct image (bottom).
The right panels show the combined, aXedrizzled stamp image from the entire dataset (top)and the extracted spectra (bottom) with the identification of all discernible spectral lines.
The assessment of the WFC3/IR grism properties as determined with aXeTZ will be
published in a series of Instrument Science Reports (ISR) in mid-October [4].
4. Release and future developments
aXeTZ is currently in the final testing stages and is going to be released as part of the next
STSDAS release 3.11 in November 2009. An identical version of aXe to be installed as alocal PyRAF package will be distributed via the ST-ECF homepage [5]. On the basis ofaXeTZ, the future developments of aXe will focus on the following areas:• a new version of aXedrizzle which allows the rejection of deviating pixel values solely
based on spectroscopic criteria. This method promises advantages especially for datataken with the WFC3/IR detector;• getting more pythonic by e.g. replacing all the IRAF/PyRAF tasks;• combining the aXe software and the aXeSIM [6] software package in the python layer andbelow;• adjustments that may arise from the exploration of WFC3 slitless data.

Figure 3: Example of Vy2-2 observations reduced with aXeTZ

References
1. Kümmel, M., et al., 2009, PASP 121, p.59
2. Freudling, W., et al., 2008, A&A 490, p.1165
3. Kümmel, M., et al., 2009, ST-ECF Newsletter 46, p.6
4. Kuntschner , H., et al., 2009, WFC3 ISR, in preparation
5. aXe/aXeSIM webpage: www.stecf.org/software/slitless_software
6. Kümmel, M., et al., 2007, ST-ECF Newsletter 43, p. 8