Quote:
Originally Posted by BlueAstra
Thanks for the feedback. I've read the link, which I'm not entirely sure I understand. Is he saying that if the calculation indicates a TargetADU of 2150 I should choose an exposure time so that the background (area 'between' the stars) should have an ADU > 2150 ?
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Hope you don't mine me dropping in Rick.
Hi Graham. If you get 2150 for the targetADU calculation, then that is what you should aim for in the sky background. You could use longer, but then you lose dynamic range and run the risk of saturating the sensor. You could use less, but then the read noise will become a more significant fraction of the total noise and the SNR decreases. The rule of thumb accounts for varying sky brightness and applies equally well when using filters.
As Peter has stated, you can get around the problem by increasing the total exposure time, but that will not be just a little bit extra. To put some numbers on it, if you have sky which is 10x as bright as true dark sky (eg bright suburban), you will have to expose dim targets for about 10x as long (in total, not sub length) to get the same result as you would from a dark sky - this is what Rick was getting at - you won't be imaging dim galaxies in any reasonable time unless you have a very big aperture. Apart from pollution reducing filters, there is no way around it - software can't help, since the noise is part of the background light and it cannot be separated from dim signals.