Interleave and Coadd

In order to fully sample a spectrum and reduce the effect of bad pixels, observations are often taken at different detector positions. The detector is stepped along the spectral axis by a fractional number of pixels; for CGS4this is typically in half-pixel or third of a pixel steps. Each spectral element can be sampled more than one time, which helps increase the signal-to-noise and decrease the impact of hot or bad pixels. This sampling method is often referred to as 2x2 or 3x2 sampling. The first number refers to the number of data points taken per resolution (or the inverse of the fractional pixel step size) and the second refers to the number of times each pixel has been observed. These observations need to be interleaved to create a higher-resolution spectral image.

The interleaving is done by expanding the input frames by the reciprocal of the fractional pixel step size, then blanking out the extra columns in these expanded frames. The origins are then shifted correspondingly, and the expanded frames are coadded together using the mean to create a final spectral image.
[_INTERLEAVE_COADD_, _FLATFIELD_COADD_INTERLEAVE_]