Re: [APML]: Broadband filters and city astrophotography

[kodama@alumni.caltech.edu]

I'm not an expert on this, but I think that dynamic range
is not the only problem.  Don't you have to have linearity
(like the CCD) in order to properly subtract the 
background?  So you would probably need more film dynamic
range so you could first linearize the film response, then
subtract the background?

Dave

---------------Original Message---------------


> Tom Krajci wrote:
> 
> > For city astro imaging I strongly recommend a CCD.  
> > http://www.ghgcorp.com/egrafton
> >
> > If that's not enough, look at the recent (sometime in fall/winter
> > of 97?) examples in Sky & Telescope from Dennis Di C's Boston home. .
 
> I wonder about that. I don't see why a CCD should be any less sensitive
> to light pollution than film. Film and CCDs both have broad-spectrum
> sensitivity, so it doesn't seem to follow that one should be very
> sensitive to light pollution and the other one not. Some CCDs (such as
> the ST-6) have a spectrum-response curve almost identical to a typical
> color film.
 
> It gets me thinking if the reason CCD imaging can be done in more
> light-polluted skies is not because of anything in the CCD, but because
> the field of view is so tiny that light-pollution build-up is minimized....

No that's not it.  Both film and CCD chips "see" the same thing - 
lotsa light pollution and a little bit of light from the astronomical 
object.  Both the CCD and film will "fog" from the light pollution.

The big difference is in the resolution of the light levels in a CCD. 
A CCD for light polluted skys should be at least a 12 bit (4096 
grey levels) and probably a 16 bit (64 thousand grey levels!) system. 
Even if the light pollution is 90% of the image in a 12 bit CCD, 
and you subtract that crap out, you still have several hundred grey 
levels to play with of remaining astro image.  Not too bad.

*If* you could get film (and mostly the scanner!) to resolve that 
many grey scales (you can't as far as I know, not within our budgets) 
you would find that film and CCD imaging in the city would be on more 
equal footing.  

Hmmmm, I wonder how much a good 12 or 16 bit negative scanner costs?  
Maybe I should be afraid to ask!  ;-)

Tom Krajci

Capt Tom Krajci
B-52 Intelligence Officer
"In God we trust, all others we monitor!"
http://spur.barksdale.af.mil


----------End of Original Message----------

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E-mail: kodama@alumni.caltech.edu
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