Re: Re: [APML] OT: CCD sensitivity [was: first CCD images]

[lkt <tan@cox.net>]

Philip,

  My thoughts are similar to the ones you have just expressed. Also, unfortunately, I cannot prove this. Interestingly the CCDs linear response can also be a disadvantage when bright stars are in the field and cause blooming. These can be a real nuisance in widefield CCD shots. Two widefield taboo CCD objects are the Pleiades and Rho Ophiuchus. I think Malin's Rho Ophiuchus film shot is still unbeatable.

Loke



> 
> From: Philip Perkins <philip@astrocruise.com>
> Date: 2002/03/25 Mon PM 08:11:25 EST
> To: astro-photo@seds.org
> Subject: Re: [APML] OT: CCD sensitivity [was: first CCD images]
> 
> Loke,
> 
> >  The main question is, what makes CCDs seem almost immune
> >to light pollution vs. film.
> 
> I think 'more immune' - CCDs will tolerate a higher level of light 
> pollution because the linear response means that there is more "separation" 
> between unwanted and wanted light.  The unwanted light needs to be 
> significantly stronger before it gets far enough up the graph to start 
> interfering with the wanted light.
> 
> 
> >There must be something about
> >film which makes is much more susceptible to light pollution. Maybe its
> >the toe of the response or the logarithmic sensitivity (your 2nd point).
> 
> Yes, I think it's exactly the non-linear response of film - the compressed 
> dynamic range at lower light levels means that the unwanted light is much 
> "closer" to the wanted light on the response curve and it takes much less 
> of it before it starts interfering with the wanted light.  The DDP process, 
> which subsequently makes the CCD response also non-linear, happens after 
> the event - in many cases the light pollution is already separated 
> sufficently from the wanted light that it gets chopped off by the DDP 
> process - i.e. becomes non existent.  Whereas with film the non-linearity 
> is being imposed during the actual recording process and there is no way to 
> separate the wanted from the unwanted light.
> 
> Well those are my thoughts but I am not sure if anyone has verified this 
> empirically.  That great quality of film, of recording images that are 
> intrinsically analagous to the display media, can be a disadvantage in the 
> case of light pollution.
> 
> --Philip
> 
> Philip Perkins - philip@astrocruise.com
> Wiltshire UK & Luberon France
> Astrocruise - http://www.astrocruise.com
> 
> 
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