In many ways, IR array data is similar to that taken with optical CCDs; however, there are a number of important differences that are mainly due to the large, variable sky and instrumental backgrounds that arise at these longer wavelengths. In extreme cases, the objects of interest can be several thousand times fainter than the sky background. This, together with the variable sky background and the unusual bias patterns of some readout methods, implies that objects of interest are generally not visible in a single image.
For the above mentioned reasons, fully automated reduction is difficult to achieve, and is unlikely to produce optimally reduced data even if achievable. It is thus inevitable that a more ``hands on'' approach be adopted for the data reduction, with the astronomer monitoring the quality of such things as flat fielding and sky subtraction much more carefully than is usually the case for optical CCD data.
In addition, if observers are to get the most out of their observations, some online processing of the data at the telescope is required. The IRAC2 context in MIDAS includes a number of commands that are most useful at the telescope.
The first part of this appendix concentrates on the data reduction that can be done at the telescope. The second part describes what is required for off-line reduction of IRAC2 data (and IR data in general), with a specific view towards use of standard MIDAS routines.
The IRAC2 context can be activated by the command SET/CONTEXT IRAC2. Together with the IRAC2 context, the CCDRED context is also activated. The CCDRED context contains many useful commands for image combining, mosaicing, etc.