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Re: [OM] long night time exposures

Subject: Re: [OM] long night time exposures
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 31 Aug 2011 09:01:36 -0400
An interesting question.  I don't know the answer but I suspect these 
are built from multiple shorter exposures and then stacked in image 
stacking software such as PhotoShop or CombineZ.

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.

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.

Chuck Norcutt


On 8/31/2011 7:38 AM, John Hudson wrote:
> I came across these images in one of the UK's on-line newspapers:
>
> http://tinyurl.com/3mlkuh2
>
> Question: how much of what one sees on screen is the result of skilled
> photoshopping and other image enhancement techniques?
>
> jh
>
>
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