[Shiraz (Ray)]

There is a clear trend around here to take longer exposure images and some that have been recently posted demonstrate how effective long exposures can be. But then the question is, how long is "long enough".

Well, it seems that it depends ....

To demonstrate some of the issues, take 2 hypothetical but quite practical broadband systems that may be used by amateurs:
system 1: 12 inch f3.8 with a KAF3200ME based camera used under dark sky
system 2: 4 inch f7 with a small pixel (4.5micron) DSLR used under suburban sky (assume sky is about 2 mags brighter)

- system 1 has at least 3x the broadband quantum efficiency of system 2 by virtue of the Bayer filters,
- system 1 has ~8x the aperture area of system 2, assuming some central obstruction in system 1,
- both have about the same pixel scale, so there is no major sensitivity or resolution difference due to sampling,
- system 1 sky produces <1/2 the noise of system 2 sky

Combining all of these factors and assuming that system 1 will require additional time to gather colour data, yields the conclusion that system 1 will have more than 10x the signal level and less than 1/2 the noise of system 2 for a given exposure time. If system 1 needed 2 hours to image NGC 253 in colour, system 2 would need almost a week spent under the stars to produce a result with the same SNR - and this is for two quite practical systems.

two messages:
1. design your system very carefully if you want to minimise imaging time - system choices can greatly affect sensitivity and imaging time
2. different systems may require vastly different imaging times to reach the same signal to noise ratio on a given object, so there is no universally applicable yardstick as to how long is "long enough".

thanks for reading - appreciate any discussion. Regards ray