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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