[Shiraz (Ray)]

Hi

"How long should I expose for?" has been asked many times. As a rather hopeful attempt at an answer, please find attached some luminance calculations of sub length, total exposure time and dynamic range for the ASI1600MM-C. These apply to galaxy imaging under two different sky conditions and for two scope fnumbers. They show what can be expected as the gain varies and were derived using a moderately complex end-to-end astrograph model.

The sub length calculations are based on the Moore/Smith 5% RN criterion used in previous tables. The starting point for the exposure time calculations was a personal assessment that a stack of 84 subs of M104 was at a minimum acceptable SNR in regions where the surface brightness was about 23 mag/arcsec^2 (assessed from published photometry). This scaled conveniently to a starting point of 12.4 SNR after 1 hour at f4 under mag 21.5 sky - at gain100. Of course, this is a personal assessment and others will have other criteria. The (objective) sub length and (subjective) time to the chosen mimumum SNR could be combined to estimate the dynamic range (DR) after stack (max possible signal/sky noise). The model provided DR estimates that matched very well with the measured DR from a number of real stacks - to some extent validating the procedure.

Only a limited range of parameters have been investigated and the tables should be taken as showing general trends rather than as a definitive guide on how long to expose for. Nonetheless, they provides a way to illustrate some of the characteristics of imaging with the camera.

How to read the tables:
Choose the sky condition nearest your own and then look for the fnumber nearest yours. That section shows the sub length needed for sky limited imaging, the total time needed to get to a SNR of 12.4 on a galaxy region with surface brightness of 23 mag/arcsec^2 and the dynamic range of resulting stack - all for different values of gain.

Some observations:
1. dark sky is of extreme importance. going from mag 21.5 to mag 19 increases the imaging time to the same SNR by about 8x. This doesn't mean that galaxy imaging cannot be done under bright sky - just that dimmer regions may be difficult to reach in a reasonable time.
2. Under dark sky (mag 21.5), provided that the sub length is chosen to give sky-limited imaging, the total time and dynamic range is pretty much independent of gain for values below about 200. At gain 300, the optimum sub length is less practical and the dynamic range has fallen off considerably. ie - it doesn't matter much what gain you use up to about 200 - provided the number of subs is manageable. The widely held assumption that the 1600 has best dynamic range at low gain doesn't really hold after stacking and in dark skies. The subs will individually have more dynamic range at low gain, but the stack will be about the same for gains up to about 200.
3. Under brighter sky (mag 19), the total imaging times are so long that optimum (short) subs will be fairly impractical. The table is based on the assumption that 1 minute subs will be used throughout under bright sky (even that is a bit short for 30+ hours of imaging). Clearly the dynamic range can be much greater than under mag19 sky, but at the cost of ~8x the total integration time. The dynamic range falls off as the gain increases, so it is probably best to use low gain under bright sky and at least get some advantage from the conditions.
4. Using a slow scope at native resolution for galaxy imaging does not seem to be all that practical with the 1600 under brighter skies - long exposures are required to get to reasonable depth.

Please treat these tables as general guides only - the sub length calculations can be traced back to objective measures, but the total exposure and dynamic range are based on a subjective assessment. However, the general trends might be useful.

Thanks for looking. Cheers Ray