Quote:
Originally Posted by Hoges
I'd setup last night to take 6 x 5 minute exposures on Centaurus A - I got one done and then the clouds rolled in. Looking at last night's photo made me wonder - more time = more signal = less noise. So, what practical limits are in place for DSLR (or astrocams) length of exposure. My Centaurus A photo is 330 seconds at iso 1600 and I can see the light pollution is starting to make it's presence felt there, so I feel I'm approaching the practical limit in this part of the sky. But, given a nice dark sky and if the autoguiding is good, why not 10 minutes? 15? 20? The one obvious thing that strikes me is that you wouldn't want to be 18 minutes into a 20 minute exposure and have Elon Musk's Sky Hoons turn up and party all over your exposure!
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Hi Hoges,
If you take a look at the exposure histogram for your image either on your DSLR whilst imaging or reviewing or even in Photoshop, like I did (see below), you will see that the exposure histogram is at about 17% say ~1/5 of full scale. That suggests that there is more than enough exposure latitude to increase the exposure in to the the approx 1/4, 1/3 possibly even up to 1/2 of full scale. Take a test image to verify that you are not clipping any highlights, but something in the 1/4 to 1/3 of full scale is usually a good target exposure, especially (depending on DSLR) at appropriate possibly lower ISO settings to help maximise dynamic range which can help retain star colour.
This is sometimes referred to as ETTR (expose to the right) in general photography to improve noise particularly in the shadows. The exposure even if brighter, can simply be adjusted via the blackpoint to suit. Of course as you already recognised longer exposures have disadvantages as they are challenged by clouds, planes, satellites, etc... and well as challenge the guiding/mount to be more perfect.
I like your image. You must be in a reasonably dark area to get that for 330 seconds.
Best
JA