Agreed it could have gone longer but it's already fairly bright at 3
hours. I don't think it could have gone 15.
Chuck Norcutt
On 8/31/2011 10:24 AM, Roger Wesson wrote:
> Chuck Norcutt wrote:
>> I've never taken an exposure with my 5D beyond 30 seconds but even at 30
>> seconds it starts to get a bit noisy. Since I don't know the answer I
>> just visited an astrophotography forum where there was discussion of
>> exposure times. Most seem to be using less than 5 minutes but a few
>> were using as long as 10-15 minutes. As exposure time increases the
>> sensor chip heats up and gets noisy. The battery drain on long
>> exposures is also considerable. But the argument there was that signal
>> still builds faster than noise so long exposures are required to reach
>> dimmer objects. However, the noise is still there and lots of it. It's
>> the stacking that can eliminate the noise. Exactly how I don't know.
>
> This works because an exposure is a kind of counting process - counting
> the number of photons that are hitting the film/CCD chip. If you tossed
> a coin 10 times, you probably wouldn't get exactly 5 heads and 5 tails.
> But if you tossed a coin a million times, you'd expect to get much
> closer to 50% heads and 50% tails. The more events you count, the less
> your counting is skewed by chance.
>
> Mathematically, in these kind of processes, the noise is proportional to
> the square root of the number of events. So the signal to noise ratio
> is 1 over square root of N. If you double N (for example by adding two
> exposures together), you improve the signal to noise ratio by 41.4%.
>
>> Therefore, I have to conclude that these digital sky images are built
>> from multiple shorter exposure stacked images. Film would likely be
>> different. I don't know what happens with reciprocity failure beyond
>> the range of minutes. 15 hours on a single film exposure might still
>> lead to a whiteout.
>
> The limiting factor would be sky background emission building up. Even
> on a completely dark moonless night far from any artificial light source
> there is still a faint "airglow" caused by cosmic rays striking the
> upper atmosphere among other things. I think this is probably what the
> purple glow was near the horizon in one of the shots. I've never done a
> 15 hour exposure on film but have done 3 hours a few times. Here's one:
>
> http://www.world-traveller.org/images/CL09-01-S01-004.jpg
>
> I was in a very dark place there and certainly could have left it a lot
> longer without blowing it out.
>
> Roger
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