5.3 Working with Imaging Data
5.3.1 Sensitivity Units and Conversions
Your calibrated image (_flt or _crj file) has units of counts. The conversion to flux (erg cm-2 sec-1 Å-1) for a flat spectrum is: 
where PHOTFLAM is the sensitivity for the observing mode in units of
erg cm-2 sec-1 Å-1, and EXPTIME is the exposure time in seconds. Both of these parameters are given in the science header. These fluxes can be converted to magnitudes in the STMAG system by the relation
STMAG = -2.5 log10 f
- 21.1.
5.3.2 Spatial Information
The tasks trxyeq and treqxy can be used to convert between pixel coordinates and Right Ascension and Declination. The pixel coordinates can either be uncorrected for geometric distortion or corrected by the solution in the IDCTAB. To improve the accuracy of the conversion, one can first use the task ucrpix to adjust the values of the reference pixel coordinates (CRPIX1, CRPIX2) based on user-specified pixel and celestial coordinates of one point in the image. The tasks xy2rd and rd2xy can also be used to convert between pixels and celestial coordinates, but only for images that have already been corrected for geometric distortion. This correction has been performed if GEOCORR=COMPLETE in the primary header of the image. All of these tasks are available in both IRAF and PyRAF. Details on their execution are given in the help files.
5.3.3 Combining Images
Multiple imaging exposures made with CR-SPLIT or REPEATOBS can be combined using calstis or its components, described in Table 3.1. By running the component tasks, you can select different options or adjust the values of the task parameters to override the values provided by the reference files. For example, you can change the values of the parameters which control cosmic ray processing in ocrreject to strike a balance between missing cosmic rays and clipping real flux from point sources. You may also want to generate improved reference files and run the component tasks to apply them to your data. See Section 3.5 for detailed discussions of improvements that can be made by recalibrating the data.
The Python code MultiDrizzle in PyRAF can be used to combine dithered images or images made with the same aperture and optical elements but with different target centering or orientation, as well as multiple imset (CR-SPLIT or REPEATOBS) exposures. You may want to use it for improved cosmic ray rejection or to handle exposures that are misaligned because of target drift (Section 5.2.5). See Section 5.1.4 for more on MultiDrizzle.
