Quote:
Originally Posted by Crushellon
Thanks for the replies, there's some interesting points to consider there.
I don't understand the short exposures though, if the 1600 can do long exposures at neutral gain settings why are they commonly used for short subs and lots of them? What's the benefit to it? Surely longer exposures are better for NB imaging, or does it not matter so much because the background has a low noise level with the 1600?
Also, if used at low gain settings, are you just holding back the capabilities of the camera unnecessarily? As in, if I did 20x600subs at low gain and 20x90sec subs at a higher gain, would the resulting amount of signal captured be the same? Therefore making the longer subs redundant? Or would you have to do 130x90sec subs to match the amount of signal as the 20x600sec subs?
I'm just trying to get a better idea of why they are "generally" used for sort subs and lots of them to make up integration time rather than fewer long subs. The main reason I'm trying to figure this out specifically is because the idea of capturing and processing 100's of subs does not sound desirable to me at all. Measuring weights, registration, local normalisation, integration, then at the end something didn't work out right and you have to go back and find the issue within 100's of subs... uggh makes me shudder, lol.
But in saying that, if lots of short subs is the best way to get results with the camera (even if it is capable of longer subs), then that's just how it is.
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there are some advantages to short subs - eg:
- better resolution and star shape due to ability to remove transient bursts of poor seeing by rejecting a small percentage of the subs
- easier guiding/better resolution by keeping exposures well within mount PE periods .. normal PA errors and guide scope drift are immaterial
- less data lost to transient events such as clouds
- more effective non-linear rejection of transient noise such as cosmic rays
however, if you don't want to take advantage of the above, then there is no major penalty to running the 1600 camera at zero gain and using "normal" long subs for broadband imaging. Provided you use long enough subs to overcome read noise, the total exposure time is what matters - 10x5 minutes = 50x1 minute. As far as I can tell, the CCD equivalent mode of the chip is at zero gain even though it seems to be widely assumed that "unity gain" has some inherent significance.
Narrowband imaging is a different kettle of fish altogether. The 8300 will be severely limited by read noise for all practical sub lengths under dark sky, whereas the 1600 at high gain can give genuine shot noise limited imaging - ie, the 1600 final result will not include ~any read noise, whereas the 8300 result will still have a significant read noise component. Thus, the 1600 will go deeper (possibly by quite a bit). Of course the 1600 will require quite long narrowband subs to get to sky-limited performance under dark sky, but at least it can do it. There are quantisation effects from the 12/16 bit conversion (not hidden by read noise), but FWIW, I have not found this to be an issue. Suggest that you ignore any advice that says that you should
always use short subs for narrowband 1600 - sometimes worthwhile, but not always so.
http://www.iceinspace.com.au/forum/s...d.php?t=155908 may possibly be useful if you haven't already read it.
The other thing that might be worth considering is the very fast download rate of the 1600. A full res download takes ~ 1 second or less and once you have done a 9 step full frame focus run in under 1 minute, it is very painful to go back to a CCD camera (I tried).
I think every serious imager would benefit from an understanding of the basic concepts. It would save a lot of pointless arguments 
