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"  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 , .
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.