aXedrizzle - Spectral 2D Resampling using Drizzle

ASTRONOMICAL DATA ANALYSIS SOFTWARE AND SYSTEMS XIV ASP Conference Series, Vol. 347, 2005 P. L. Shopbell, M. C. Britton, and R. Ebert, eds.

aXedrizzle - Spectral 2D Resampling using Drizzle Martin W. K¨ ummel, J. R. Walsh, S. S. Larsen, and R. N. Hook Space Telescope – European Coordinating Facility, Karl-Schwarzschild-Str. 2, D-85748 Garching b. M¨ unchen, Germany Abstract. The aXe spectral extraction software was designed to extract spectra from slitless grism images such as those taken with the Advanced Camera for Surveys on HST. In aXe version 1.4 we implemented aXedrizzle, a new technique to resample 2D spectra which uses the “traditional” drizzle software. aXedrizzle forms deep 2D spectra by coadding the 2D spectra from the individual, dithered images. This method has several advantages compared to the approach of making the 1D extraction on each image and then coadding the 1D spectra. aXedrizzle is the first implementation of the drizzle code to combine spectral data, and the technique can also be applied in other reduction packages for spectral data.

1.

Slitless Spectroscopy on the Hubble Space Telescope (HST)

After the demise of STIS (Space Telescope Imaging Spectrograph), the Advanced Camera for Surveys (ACS) is currently the only instrument on the HST which is capable of doing optical and UV spectroscopy. The ACS has three channels (WFC – Wide Field Channel, HRC – High Resolution Channel, and SBC – Solar Blind Channel), and each channel is capable of delivering slitless spectroscopic images by inserting a grism (G800L) or prisms (PR110L, PR130L, and PR200L) into the optical beam. In total, the five possible combinations of ACS channel and dispersing element offer low resolution (R ∼ 100) spectroscopy from the UV to the far-red wavelength regime. Table 1 gives an overview of the spectroscopic modes of ACS and lists important parameters such as wavelength range, spectral resolution, pixel size, and field of view (FOV). 2.

The aXe Spectral Extraction Software

The Space Telescope – European Coordinating Facility (ST-ECF) is responsible for the support of the spectroscopic modes of ACS. A key component of this support is the development of the aXe software package. aXe is a spectroscopic data extraction software package that was designed to handle large format spectroscopic slitless images such as from the ACS. It is distributed as part of the IRAF/STSDAS software package in the subpackage “stsdas.hst calib.acs.axe”. An earlier version (aXe-1.1) was presented at ADASS XII (Pirzkal et al. 2002). As data input, aXe needs a grism/prism image, a corresponding direct image, and a catalogue which lists the objects detected on the direct image. Driven by the object catalogue, the various aXe tasks extract wavelength and flux calibrated 1D spectra for each object from the grism image. 138

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Datasets, such as those obtained with ACS, often consist of several images with small position shifts (dithers) between them. The direct approach of coadding the 1D spectra extracted from each slitless image to form a combined, deep spectrum has several disadvantages: • The data must be rebinned twice, once when extracting the spectrum from the image and again when combining the individual 1D spectra. • A complex weighting scheme is required to transport the number and contribution of cosmic ray affected pixels through the summation in the crossdispersion direction and the combination of the individual 1D spectra. • Low level information on the cross dispersion profile is lost when many 1D extracted spectra are combined to a deep spectrum. Also problem detection and error tracking is more difficult on a 1D than a 2D spectrum. Channel

Disperser

WFC HRC HRC SBC HRC

G800L G800L PR200L PR130L PR110L

Table 1.

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Wav. Range [˚ A] 5500 − 10500 5500 − 10500 1600 − 3900 1250 − 1800 1150 − 1800

Resolution [˚ A/pixel] 38.5 23.5 20[@2500˚ A] 7[@1500˚ A] 10[@1500˚ A]

Pixel Size [mas/pixel] 50 × 50 28 × 24 28 × 24 34 × 30 34 × 30

FOV [arcsecond] 202 × 202 29 × 26 29 × 26 35 × 31 35 × 31

The spectroscopic modes of ACS.

aXedrizzle

Extracting the 1D spectra from MultiDrizzle-resampled images shown in Figure 1 does not deliver optimal results. The description of the spectra, e.g., the trace angle α in Figure 1, as well as the spectral wavelength solution vary with the object position on the chip. The MultiDrizzle procedure corrects the spatial geometrical distortion of the ACS images but does not correctly handle the spatial variation of the dispersion for slitless spectral images. Applying the corrections for ACS geometric distortion leads to distortion of the spectra but without beneficial effects on the dispersion solution. It is possible, however, to correct the field dependence of the slitless spectra in local solutions, where only a small part of the 2D spectral image is resampled. With our new aXedrizzle technique a deep, co-added 2D spectral image is formed for each object by resampling all spectra in the individual images. For the resampling and co-addition of the individual spectra we use the “traditional” drizzle software (Fruchter & Hook 2002) The advantages of aXedrizzle as applied to slitless spectra can be summarized: • Regridding to a uniform wavelength scale and a cross-dispersion direction orthogonal to the dispersion direction is achieved in a single step. • Weighting of different exposure times per pixel and cosmic-ray affected pixels are correctly handled through the drizzle weights. • The combined 2D spectra can be viewed to detect problems and the deep spectra can reveal fainter features which were subsumed in the 1D spectra.

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Figure 1.

A 2D spectrum before (top) and after (bottom) drizzle resampling.

To illustrate this scheme, Figure. 1 shows the slitless spectrum of a bright, F 850W = 20.6 magAB late type star before and after the resampling, in the upper and lower panels, respectively. In the upper panel the trace angle α, the wavelength solution (expressed as wavelength increment ∆λ), and the pixel scale in the cross-dispersion direction (∆d) all vary with the object position on the chip. For all the 2D spectral images of this object on the various grism images a set of individual drizzle coefficients is derived to drizzle each individual 2D spectrum onto a single, common drizzled image as seen in the lower panel. The drizzle coefficients are computed such that the combined 2D grism image has a constant wavelength dispersion and a constant pixel scale in the cross-dispersion direction. After the resampling the trace angle is zero, and the combined drizzle image resembles an ideal (distortion-free) long slit spectrum with the dispersion direction parallel to the x-axis and cross-dispersion direction parallel to the yaxis. The final extraction of the 1D spectra from the deep, coadded 2D grism images is done using aXe tools or standard long slit extraction tasks. aXedrizzle is the first implementation of the drizzle code to combine spectral data. In principle, it is also possible to apply the aXedrizzle resampling scheme and the aXe software to ground based multi-object spectroscopy (MOS). In a pilot study we are currently working on establishing a pipeline to reduce MOS data taken with the FORS2 instrument at the VLT.

4.

aXe Version 1.4

The aXedrizzle technique is one of the major extensions of the new aXe version 1.4 (K¨ ummel et al. 2004a), which was released on October 11, 2004 on the aXe webpage (http://www.stecf.org/software/aXe). aXe-1.4 is also part of the new STSDAS 3.3 which was released in November 2004. In a pilot study, the aXedrizzle reduction scheme was just applied to reduce the data from the Hubble Ultra Deep Field (HUDF) HRC parallels (see K¨ ummel et al. 2004b and the preview pages http://www.stecf.org/UDF/HRCpreview.html).

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Figure 2. Comparison of aXe-1.3 and aXe-1.4 extractions for the same observed spectrum.

To give a quantitative comparison, Figure 2 shows the spectrum of the star J033242.42-274758.8 (F 850W = 19.1 magAB ), taken in the Grism ACS Program for Extragalactic Science (GRAPES, Pirzkal et al. 2004). While the lower spectrum shows the result of a reduction with aXe-1.3 and subsequent coaddition of the 1D spectra extracted from the individual grism images, the upper line shows the spectrum of the same object as extracted using aXe-1.4. The difference in the flux level and the small wavelength shift are caused by a different width of the extraction box and different wavelength calibration, respectively. The overall agreement of the spectra is very good and the aXe-1.4 reduction provides in addition advantages, such as the combined 2D spectrum for better error detection (see Section 3). 5.

With aXe2web to the World Wide Web

Deep ACS WFC grism images can contain spectra of many hundreds of objects, and visual checking of each spectrum is very tedious. For this reason we developed aXe2web (Walsh & K¨ ummel 20041 ), a tool which produces browsable web pages for fast, yet thorough, examination of spectra. This additional task to the aXe package produces an html summary using the input catalogue and the aXe output. Each object produces a line in an html table which lists impor-

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Figure 3.

The aXe2web view of J033157.83-274846.9.

tant object parameters (e.g., reference number, Right Ascension, Declination, brightness), a cut-out image showing the direct object, an image showing the 2D spectrum, a 1D extracted spectrum in counts and the same in flux units. Figure 3 shows the line for the object J033157.83-274846.9 (also shown in Figure 2). An overview and an index page accompany the object pages which carry all the information. The overview page contains the basic information for each object. The index page includes a table with the ordered reference number of all objects. Direct links from both the overview page and the index page point to the corresponding locations of the objects in the object pages. With aXe2web, aXe spectra can be easily published on the web without further interactive work. References Fruchter, A. S., & Hook, R. N. 2002, PASP, 114, 144 K¨ ummel, M., Walsh, J., Larsen, S., & Hook, R. 2004a, ST-ECF Newsletter, 37, 14 K¨ ummel, M., Walsh, J., Larsen, S., & Hook, R. 2004b, ST-ECF Newsletter, 36, 10 Pirzkal, N., Pasquali A., Hook, R.N., Walsh, R., & Albrecht, R. 2002, in ASP Conf. Ser., Vol. 295, ADASS XII, ed. H. E. Payne, R. I. Jedrzejewski, & R. N. Hook (San Francisco: ASP), 485 Pirzkal, N., Xu, C., Malhotra, S., et al. 2004, ApJS, 154, 501 Walsh, J., & K¨ ummel, M. 2004, ST-ECF Newsletter, 35, 9