Re: [APML] Rosette comparison (was New Rosette Mosaic)

[Aplanatic@aol.com]

Hi Loke,

>From personal experience with TP, E200, and PPF, I completely agree with you.  Unfortunately, I don't know how the *effective* resolution of film changes at low light levels but I suspect that it doesn't hold up well compared with CCDs.

It's not that the MTF of film changes at low light levels.  When you get on the toe of the characteristic curve the "output" S/N degrades rapidly.  More rapidly than the "input" S/N is decreasing.  This does not happen with CCDs as long as the background is large w.r.t. readout noise.  

-Dave-

> Dave,
> 
> It's very nice to hear from you after a while. I was wodering what you
> thought about film's MTF under very low light conditions. Tech Pan appears
> to me to handle low light detail much better than any color film.
> 
> Loke
> 
> 
> 
> -----Original Message-----
> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
> Behalf Of Aplanatic@aol.com
> Sent: Thursday, January 16, 2003 12:08 PM
> To: astro-photo@seds.org
> Subject: Re: [APML] Rosette comparison (was New Rosette Mosaic)
> 
> 
> Tony writes:
> 
> > Dave,
> >
> >    How the heck can you tell Rob has a TDE from looking at an image? Maybe
> he has convoluted chromo-spherism in his
> > version of MaxIm DL...   : - )
> >
> >      Tony
> 
> Yes, some sort of strange imaging processing could create or enhance the
> appearance of the bright stars in Rob's image, but I don't think so.
> 
> I've built and tested quite a few telescopes.  There are many different
> types of optical defects that *could* give rise to the fuzzy bright stars in
> Rob's image.  In my experience, the only two that actually show up in
> practice are: TDE and surface roughness.  If the cause was surface roughness
> then blue stars would show much more scatter than orange stars. (Scatter
> goes like 1/lambda^4.)  Since this is not the case from inspection of Rob's
> image, my conclusion is that one of the mirrors has some TDE.  Making a fast
> hyperbolic primary mirror or a hyperbolic secondary mirror *good to the
> edge* is quite a difficult undertaking.  This also supports my conjecture.
> 
> Matt,
> 
> >Great to hear from you...
> 
> You too!  Great to see some of your very fine images.
> 
> >If the two images are taken using different focal length, how >should that
> be taken into account?
> 
> >If the two images have different integration times, how >should that be
> taken into account?
> 
> > Matt
> 
> I'm only considering the effect of the detector, not the telescope or its
> focal length.  For film images the detector is the emulsion scanned at 4000
> dpi.
> 
> Because of the nonlinear nature of film there are some difficulties with my
> statements about MTF over the full range of exposures.  The calculations
> were done using Kodak's contrast transfer function from their published
> information.
> 
> 
> Roland writes:
> 
> >You can't tell the cause but if you analyze the raw image >histogram in
> Maxim, you can tell quite a lot about the optic, >especially if you compare
> to a standard.
> 
> >Roland Christen
> 
> Yes, I agree.  Don't you think with a reasonable degree of certainty that
> Rob's RC has a bit of TDE?  By far the most common problem that would yield
> the bright star images.
> 
> -Dave-
> 
> 
> 
> 
> 
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