Re: [APML] Equivalent ISO of an ST10E CCD

[lkt <tan@cox.net>]

Jerry,

	Covington's latest version of "Astrophotography for the Amateur"
answers this well on page 248. Basically, the speed of a CCD imager
depends on how much of the CCD well you would like to fill. If for
example you filled the lower bits of the well, then the CCD imager is
liken to a very high speed film such as 1600 ISO. If you are willing to
expose longer to further fill the well, the speed might be effectively
100ASA. Filling the well further results in better SNR, remember the
well depth of the KAF3200 has 12-12.8 bits of dynamic range. If our
signal has 6 bits of dynamic range, we could fill the lower 6 bits of
the well and be reasonably fine. But in this case the faintest parts of
the image would be sitting at the noise level. However, if we were to
expose longer, the 6 bits of dynamic range of the image might sit at the
upper 12 bits of the well. In this case, the faintest parts of the image
is 6 bits above the noise. So in summary, a CCD is both like a very fast
film AND like a very slow film. Interestingly the noise is so low that
if you were to treat the CCD as fast film, i.e. short exposures, the
noise level is usually already much better than 100ASA film grain noise.
I say this w/o proof. 

	The explanation above is a bit simplistic but mostly correct. Usually
noise is also a factor of exposure length etc.

Loke



Jerry Lodriguss wrote:
> 
> Ok, here we go again.
> 
> It seems like a simple question:
> 
> What is the equivalent ISO of an ST10E CCD camera.
> 
> If it is not, in fact, a simple question, please formulate a simple answer
> to explain it. <G>
> 
> If I hook up an ST10E to a camera lens and take an exposure of a gray card
> in the sunshine, what exposure would I need in terms of exposure length at
> a given f/stop or correctly record the gray card?
> 
> Simple, see? <G>
> 
> Jerry
> Photoshop for Astrophotography Book:
> http://www.astropix.com/APBOOK/0_PROMO/PROMO.HTM
> 
> Astrophotography, Tips and Techniques
> for Digital Enhancement in Photoshop:
> http://www.astropix.com
> 
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