mranch1 I might add that I was doing 600s exposures in a bortle 1 Sky with a 678mm focal length.
Bob,
(1) keep in mind that imaging in a Bortle 1 does not make an object any brighter than imaging from a Bortle 9 backyard. The same number of photons from the target hits your sensor. Bortle 1 is only better than Bortle 9 because the target has a chance of being brighter than the sky background. So, you still need to expose for long enough for the target to get above your sensor noise; the sky noise will just not contaminate the image.
(2) an exposure time of 600 seconds at gain of 0 (0 dB for ZWO camera) has the same exposure value as an exposure time of 60 seconds at a gain of 200 (20 dB for ZWO camera). So, adjust gain accordingly, but keeping in mind the dark current noise of the camera too since it also increases linearly with exposure time. I.e., longer exposures will not help if the dark current noise is stronger than your target. Read noise is less important for very long exposures. If you have a good mount and good polar alignment, just expose for longer.
(3) a narrow band filter does not increase the signal from a target, it simply reduces anything that is not inside the passband of the filter (like background sky). Indeed, even good narrowband filters will reduce the transmission by 5% or so (cheaper narrowband filters may reduce it by even more than 15%). As such, add 15% or so exposure value when you use the narrowband filter.
In a Bortle 1 area, you will not even need such a filter. A 4 nm wide filter will roughly reduce sky contamination (assuming sky background is between 400 nm and 800 nm) by a factor of 100, while only reducing the actual target by the 5% to 15%. (Think of it as expossing your target for 600 seconds while only exposing the sky background for 6 seconds. but only if your target is completly insde the passband of the filter.)
A filter with 8 nm bandwidth will knock only half of the amount of sky background compared to a filter with 4 nm bandwidth. In general, you get what you pay for, but you also need high quailty filters (manufacturers who know what they are doing) to avoid garbage like halos.
(4) for an extended source like a nebula, you want to be close to correct sampling. Oversampling will cause each pixel to be "dimmer." For extended nebulas, the f-ratio is important. A 50mm aperture at 678mm focal length will produce an image that is 100 times dimmer than a 500mm aperture at the same focal length.
Chen
