Source Frames Multiple exposures on a single source are the rule. Thus, in the absence of a bright source, readnoise and the detector's linearity limit will combine to constrain the exposure to between about 1,000 and 15,000 DN, that is, well above the readnoise limit, and well below non-linearity. Sky Frames Infrared photometric measurements usually require a nodding or chopping technique which can result in half the observing time being spent on sky measurements. These sky frames not only measure the sky brightness, but, in some cases, may also be used for flat-fielding . Thus the sky brightness should be measured frequently, and should be recorded at the same bandpass as the source image to which it will be applied.
Most observers choose to make skies at intervals of a few minutes.
It should be possible to find a reasonably blank patch of sky relatively near most sources. In some cases, the source frames can themselves serve as sky frames, provided the field is sufficiently sparse. Dithering is the prefered method when the source frames can also be used as sky frames. Nodding, Dithering, and Mosaicing Nodding refers to the technique of moving the telescope between the target and an adjacent position in the sky, making exposures at each position to create a matched set of source and sky frames. If the field is dense, or the target large compared to the size of the array, the nod may be relatively large.