Re: [APML] Winter Milky Way

[Wei-Hao Wang <whwang@asiaa.sinica.edu.tw>]

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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