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The OBJECT file denotes a science frame. The software performs sky subtraction on a fram-by-frame basis, NOT with A-B-B-A sets. Only one file at a time is handled although we will loop on these in the next release.
The ARC file is a wavelength calibration image. Our default is to use a NeNeAr combination, which has produced stable wavelength solutions. An arc line identification plot for this lamp combination may be found here.