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 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" [1] software within PyRAF/IRAF. The advantages of this technique are:

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

An overview of aXedrizzle
Figure 1: An overview of aXedrizzle

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.

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


[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