aXedrizzle

The "traditional" aXe redcution scheme until version 1.3 produces a spectrum for each object in each science image. ACS data sets, however, usually consist of several images with small dithers between them. The approach of co-adding the 1D spectra from each image to get a deep, combined spectrum has disadvantages:

• The spectral data is rebinned twice, once during the individual spectral extraction and once when combining the individual 1D spectra
• The weighting scheme to flag bad pixels and exclude cosmics is very complex
• Low level information on the cross dispersion profile is lost when only the 1D spectra are combined to a deep spectrum

With aXedrizzle we implemented in aXe-1.4 a new reductions scheme to circumwent these drawbacks. In this scheme all the individual 2D spectra of an object are coadded to a single, deep 2D spectrum. Then the deep 1D spectrum is extracted from this combined 2D spectral image. The combination of the individual 2D spectrum is done with the "drizzle" [1] software within PyRAF/IRAF. The advantages of this technique are:

• Regridding to a constant wavelength scale and cross-dispersion scale is achieved in a single step
• Bad pixels, cosmics and different exposure times are correctly handled
• Only one linear rebinning step is required to produce a 2D spectrum
• The combined 2D spectra can be viewed to detect any problems

Figure 1 gives a schematic overview on aXedrizzle. More information and discussion on aXedrizzle is in the aXe User Manual and in [2], [3].

Figure 1: An overview on aXedrizzle

The extended aXedrizzle

aXe version 2.1 (part of STSDAS 3.12) contains an new version of aXedrizzle, that can detect and reject deviant pixels from e.g. cosmic ray hits such as MultiDrizzle does in direct imaging. In Figure 2, the comparison of a combined 2D grism stamp image reduced with the "basic" aXedrizzle (left) and the new aXedrizzle (right) reveals a cluster of pixels with quite high values in the data, which are just co-added in the "basic" aXedrizzle procedure and result in a fake emission feature at ~15800 Å. Reduced with the new aXedrizzle, these pixels are rejected, and the artificial emission feature no longer exists.

Figure 2:Comparison of a data reduction with the basic (left) and the new (right) aXedrizzle.

References

[1]Fruchter, A. S., Hook, R. N., 2002, PASP, 114, p.144
[2] Kümmel, M., Walsh, J. R., Larsen, S. S., Hook, R., 2004, ``Drizzling and the aXe Slitless Spectra Extraction Software'', ST-ECF Newsletter 36, p.10-12
[3]Kümmel, M., Walsh, J. R., Larsen, S. S., 2004, The HUDF HRC Parallels: From the Pixels to the World Wide Web, ADA III Meeting 2004

 

Maintained by Martin Kümmel <mkuemmel@eso.org>