The terms ISO and Gain basically mean the same thing. ISO is used with DSLRs (digital single-lense reflex cameras) and gain is used with astro cameras. Both are numerical values that give a measure of the gain applied during the conversion of electrons produced by the pixel into a digital signal. This is useful when there are very few electrons in the pixel memory. As a result, the image is brightened. One advantage of a high ISO/Gain is that as the value increases, the readout noise decreases. The higher the gain, the less noticeable the readout noise. However, the gain reduces the full well capacity and thus the dynamic range, since the evaluation unit reaches an ADU value for the maximum possible number of electrons more quickly, even though the pixel memory is not yet full. Bright image areas such as stars are then burned out more quickly (pixels are saturated and completely white). (Source:

The goal, then, is to set an ISO/Gain at which the pixels do not become saturated for a given exposure time and the readout noise is as low as possible.

Since readout noise no longer plays a role in background-limited capturing (see section on exposure time), the lowest possible ISO/Gain value should therefore be selected in order to keep the image dynamics as large as possible and avoid saturation.

With CMOS cameras, it is best to use at highest the value at which the electronics jump from one mode to another and the readout noise drops. A higher gain is not useful for background-limited capturing. Instead, the gain should be reduced in order to increase the image dynamics.
(Source: Presentation by Robin Glover:



Even with short exposure times, a low ISO/Gain value need not be a disadvantage. Short exposure times again result in more single exposure shots, which improves the signal-to-noise ratio, and the higher readout noise loses its dominance.

Only with very short exposure times, the use of narrow band filters or with a very dark sky where background limiting is difficult to achieve, a higher ISO/Gain value should be used to keep the readout noise low.