Re: [APML] Winter Milky Way

["Bobby Middleton" <bobm@koyote.com>]

Great mosaic Wei-Hao! And the details for using Registar with a wide-field
image to help defeat the distortion is very good. I have used wide-field
images in some of my mosaics (up to ten frames) but I have never had to skew
the widefield image in Photoshop as you describe. Perhaps I will be able to
use this technique sometime.
Bobby Middleton

----- Original Message -----
From: "Wei-Hao Wang" <whwang@asiaa.sinica.edu.tw>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Monday, January 12, 2004 10:14 PM
Subject: Re: [APML] Winter Milky Way


> On Tue, 13 Jan 2004, Axel Mellinger wrote:
>
> > Wei-Hao,
> >
> > this is a stunning image! I did a similar mosaic one year ago
> > (
http://canopus.physik.uni-potsdam.de/~axm/images/winter_hexagon.html ),
>
> Yes, Axel.  I say your very nice picture several months ago.
> When I first time saw it, I thought "shit! somebody did it already."
> It's is a really nice job.
>
> > but yours goes a lot deeper. Isn't it nice when the zodiacal light
counts
> > as 'light pollution'? <g>
>
> hmmm... Perhaps.  Last summer, I tried to take a picture of M45.
> I had more than one hour before the twilight.  I gave up because
> M45 was soaked in the zodiacal light.  The zodiacal light is so
> bright that I start to hate it.
>
> > Just curious: did you notice any residual distortions after running
> > Registar? As far as I know, Registar only allows you to register two
> > images at a time. So, with three mutually overlapping images A, B and C,
> > you can register A/B and B/C, but this may not guarantee that B and C
are
> > registered outside their common area with A.
>
> Yes, you are correct.  Usually what I did is to find an old
> wide-field picture which covers the whole mosaic.  Then I
> registered each frame to that wide-field picture.  In other
> words, I registered all of A,B,C to D and combine A,B,C only.
> As long as D covers all of A,B,C, this solves the problem you
> mentioned.
>
> For this particular mosaic, I couldn't find a picture that
> covers the whole field.  I used a star chart generated in
> The Sky and inversed the white background into black to make
> a "picture."  Then I register each frame to the star chart.
>
> However, because the mosaic covers a significant portion of
> the celestial sphere, the distortion at the edges of the
> star chart is so large that Registar couldn't properly
> register the images initially.  I warp the chart in PhotoShop
> a little by a little until Registar works.
>
> I believe the method you use is the proper one to handle this
> problem.  What I did is too dirty and astrometrically incorrect.
>
> Thank you for the information about Sh2-245.  I will look into it.
>
> Wei-Hao
>
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