Re: Re: [APML] IC1396 CCD with Schmidt-like FOV

["Mike Cook, AF9Y" <mwcook@concentric.net>]

Loke,

    Maybe.  But, the CCD response on the blue end is not that much better
than commercial film.  And Nikon's performance for a single mid band
focus is so good that it is hard for me to believe that a blue center focus
would not be sufficient.  An easy test would be to check a blue filter
focused image with and without the IDAS filter which cuts off everything
below 410nm.  If there is a significant difference in FWHM between
the two images, then we would have the answer.

Mike



From:           	lkt <tan@cox.net>
To:             	astro-photo@seds.org
Subject:        	Re: Re: [APML] IC1396 CCD with Schmidt-like FOV
Date sent:      	Sun, 4 Aug 2002 0:59:25 -0400
Send reply to:  	astro-photo@seds.org

> Mike,
> 
>  Like I said in another response, chromatic abberation is continously changing from in
>  focus to more and more out of focus. Splitting focus to RGB just ensures that you are in
>  focus within the central wavelengths of the 3 filters. Most APOs are well corrected for
>  visual wavelengths but the CCD is has a much wider spectral sensitivity. Remember the IR
>  focus problems of the past. Well, to a lesser degree, this is happening at the low end of
>  the blue spectrum as well. I believe the camera lens makers mostly optimize their color
>  correction for color film. Both Tech Pan and CCDs have extended blue and IR response. For
>  the blue filter especially, you might be way out of focus for the lower wavelengths even
>  if you focused for the center region of the blue filter. I believe the blue end of the
>  spectrum goes out of focus exponentially. Some of the ATM guys here can coonfirm or
>  correct me on this.
> 
> Loke
> 
> 
> 
> > 
> > From: "Mike Cook, AF9Y" <mwcook@concentric.net>
> > Date: 2002/08/03 Sat PM 07:38:35 EDT
> > To: astro-photo@seds.org
> > Subject: Re: [APML] IC1396 CCD with Schmidt-like FOV
> > 
> > Loke,
> > 
> >       Thanks for the insightful comparison.  I would like to further understand
> > your comments about the chromatic aberration of Nikon ED lens.  I have the
> > Nikon 180 F/2.8 ED lens and have not seen this problem.  Michael Covington
> > has also commented about the excellent chromatic aberration performance of
> > the Nikon ED lens here:
> > 
> > http://www.mir.com.my/rb/photography/hardwares/speciallenses/180nikkor.htm
> > 
> >       Besides, why should chromatic aberration of a lens be much a problem
> > anyway?  If you refocus for each of the RGB filters and register the images
> > with a program like RegiStar, it would appear to be a non issue.  What am
> > I missing?
> > 
> > Mike Cook
> > AF9Y
> > 
> > 
> > >From:           	lkt <tan@cox.net>
> > 
> > >............ The Nikon ED lens has noticeable blue haloing on the bright blue
> > > stars. Imagine how much nicer your image could have been w/o chromatic
> > > abberation. The blue custom scientific filter allows light into the CCD
> > > detector almost all the way down to 400nm. .................
> > 
> > Mike Cook, AF9Y
> > http://www.af9y.com
> > mwcook@cris.com
> > 
> > 
> > 
> > 
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> 
> 
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Mike Cook, AF9Y
http://www.af9y.com
mwcook@cris.com




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