Re: Re: [APML] Equivalent ISO of an ST10E CCD

[Jerry Lodriguss <jml@astropix.com>]

Hi Dave,

Thank you for all of the work you did on this, it is fascinating.


>Here's my guess -- and you're welcome to challenge it.  At 0.1D, the 
>signal-to-noise ratio is about 3 (edge of detectability for 6.8 micron 
>pixel scans).  I don't have any justification for this value except from 
>experience with TP.


I'm not about to try to challenge you!  I don't totally understand this 
though (my fault, not yours).

You say "at 0.1D the S/N is about 3 for the edge of detectability".  Kodak 
says in "Scientific Imaging with Kodak Films and Plates" (p. 56) that a S/N 
of 5 is the minimum needed for signal detection.




>  One could find a region on TP that has a 0.1D density above base+fog and 
> measure the S/N in photoshop to get a better answer, or better yet, look 
> at many regions and draw a graph, etc.  Lots of work to get it close.


How would you measure this in PS?  I'd be glad to do it.



>For the CCD the answer is easy.  One thousand recorded electrons has a S/N 
>of about 30, dominated by shot noise.  So, its S/N is very roughly 10 
>times higher than TP.  A factor of 10 change in S/N is a factor of 100 
>change in "speed" in the shot noise limit, so I'd say that for the case 
>analyzed (low light levels) the "equivalent ASA" of the ST-10E is 
>something like 20,000 plus or minus a factor of two or more.


Is this comparison for a film with high reciprocity failure? Or for a film 
like techpan assuming that it is hypered with no reciprocity failure?

I could accept the 20,000 figure if we were comparing a CCD with a QE of 
55% with a film with high reciprocity failure, but my gut just tells me 
something doesn't jive here, because I see the long exposures that people 
like Rob are using for their images.

I am guessing that these long exposures are necessary to get the signal up 
out of the light-polluted sky background noise? So perhaps we could guess 
at some kind of "effective ISO for city dwellers"?  Maybe I'm babbling 
here, quite possibly.

By what factor would the exposures change if an ST10E was used under a 
really dark sky and the same level of quality was required in the final images?

The other reason I'm having a hard time accepting the 20,000 figure is, I 
use the Nikon D1H digital camera every day for work, and the equivalent ISO 
is 200 to 6400, but at 6400 the image is so noisy as to be not usable. 
Cooling it for long exposures must make all the difference.



>If you told me I was all wet, I wouldn't argue too hard.  It's all Jerry's 
>fault, anyway.  <g>

I'd never say that Dave, I look to you, Chuck, and Thor for the answers, I 
merely ask the questions.  I accept full responsibility for the mess 
though. <G>

Jerry















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