[Shiraz (Ray)]

The attached image shows a table of "optimum" broadband sub exposures for the ASI1600MM-cool for a range of gain settings. The optimum is that exposure where read noise adds only 5% to the total noise - ie read noise is effectively submerged under the sky noise. A few assumptions were made in these calculations (for example the QE was guessed at), so the results may not exactly match with reality. However, these suggestions should provide a useful starting point for deciding on sub exposures for broadband imaging.

To use the table, select the column that applies to the Fno of the system and the type of sky - 21.5 is very dark and 20 is moderately dark (the sky values have units of Vmag/arcsec2 or ~SQM units). The column values show the suggested sub exposure lengths (in seconds) for the gains shown in the left hand column.

The graph shows how the dynamic range (indicated by the maximum electron count in a pixel after stacking), varies with gain setting, assuming that the suggested optimum sub exposures are used. The DR values shown are specific to a chosen system, but the shape of the underlying curve will apply to any system - and the message is that if there is a need for as much dynamic range as possible, avoid high gain (ie anything above about 100). If high gain is used to enable short exposures, you can do so if the loss of dynamic range is acceptable.

For narrowband imaging, there are too many variables to produce a useful summary table. The best advice may be to use an intermediate gain as a starting point (eg maybe 70) and then choose the longest sub exposures that fit within the mount's capability and that keep star saturation at acceptable levels. Short subs may be possible on very bright nebulae, but in general, it might be a reasonable approach to start at 5 minute subs and increase from there as much as possible. If looking for very dim NB structures, minimum read noise will help and it may be useful to run at high gain (eg 200 or more). You will then need to choose subs that are long enough to retain the advantage of the low read noise, but short enough to stop too many stars from saturating.