The "traditional" aXe reduction scheme used through version 1.3 produces a spectrum for each object in each science image. WFC3 and 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 again when combining the individual 1D spectra.
- The weighting scheme to flag bad pixels and exclude CRs is very complex.
- Low-level information about the cross-dispersion profile is lost when only the 1D spectra are combined into a deep spectrum.
The aXedrizzle reduction scheme is available in aXe version 1.4 and later to circumvent these drawbacks. In this scheme all the individual 2D spectra of an object are coadded into a single, deep 2D spectrum. Then a deep 1D spectrum is extracted from this combined 2D spectral image. The combination of the individual 2D spectral images is done with the "drizzle"  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, CRs 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.
The extended aXedrizzle
aXe versions 2.1 and later contain a new version of aXedrizzle, which can detect and reject deviant pixels from e.g. cosmic-ray hits just 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 spurious emission feature at ~15800 Å. When processed with the new aXedrizzle, these pixels are rejected, and the artificial emission feature no longer exists.
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