R: [APML]: Image Registration

["Marco Lorenzi" <lorenzi@mail.dex-net.com>]

The Astro-Photography Mailing List
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> -----Messaggio originale-----
> Da: owner-astro-photo@nightsky.com
> [mailto:owner-astro-photo@nightsky.com]Per conto di Jerry Lodriguss
> Inviato: giovedì 24 settembre 1998 13.01
> A: astro-photo@nightsky.com
> Oggetto: Re: [APML]: Image Registration
>
>
> The Astro-Photography Mailing List
> ------------------------------------
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>
> >I've had the same problem with Picture Window, and that was with
> virtually
> >identical photos, taken back to back with the center and
> orientation exactly
> >the same. (The autoguider continued uninterrupted during the two
> exposures,
> >so these two photos were as exactly aligned as is humanly possible.)
> >Nevertheless, a 4-point alignment near the corners still left
> very visible
> >misalignments over most of the photo. I can't explain it, but I'm glad to
> >hear I'm not the only one struggling with it. Any ideas?
>
>
> Several things come to mind.
>
> 1. Someone said that Jonathan Sacks discovered a problem with the
> registration engine in the current release version of PW.  I believe he
> correted it in one of the later "astro" non-public releases, maybe the one
> with the ADD function as an addition to the composite methods. Philip may
> know the answer to this.
>
> 2. Film creep during the exposure.  I see this all the time on my
> negs shot
> under same conditions you mention above.
>
> 3. Slight buckeling/warping of the film, different from frame to frame, in
> the film holder during scanning.
>
> 4. Scanning irregularities, especially in scanners that move the film to
> scan. Try scanning the same neg twice in a row without touching anything
> and then see if you can perfectly align those two scans. Won't gain you
> anything s/n wise, but might reveal if it's a scanning problem.
>
> 5. You're behind on your payments to the devil. <G>
>
> Jerry
> mailto:jml@astropix.com
> Web page: http://www.astrosurf.com/astropix/index.html
> Astrophotography,Photoshop enhancement tips and techniques
>
There are also other two possibilities:
1) field rotation during the exposures (greater with longer exposures or
when a long gap intercourses between them)
2) atmospherical refraction (for low declination object) while the guide
star is not close to the object and it tracked with a separate guide scope:
it happened to me last year when I took some pictures of a very low Hale
Bopp.
Personally I think the main problem is the scanning process because I
believe it is quite impossible hold both the films *exactly* in the same
position during the process

Clear skies
Marco Lorenzi