Re: [APML] SFL and Flats

[Wei-Hao Wang <whwang@gmail.com>]

----- Original Message -----
From: Carlos Milovic F. <cmilovic@yahoo.es>
Date: Wed, 7 Jul 2004 19:01:06 +0200 (CEST)
Subject: Re: [APML] SFL and Flats
To: Discussion of Film Astrophotography <astro-photo@seds.org>

> Just to make a comparison shot... Let's see if I understood which is the Zone   IV: I have a neutral gray card, and I set the diafragm to the "correct" exposure. Now, I have to close it to underexpose 1EV, right? I.E. Is it a gray a bit darker than the neutral one?

  Yes, this is Zone IV.

> The problem I have isn't the second one you mentioned. I see a difference of about 20 (in 8bits) between the stars in the center and in the corners. I'm talking about the star's intensities, or whole flux. They are overcorrecter due the division. That's the reason I always use substraction. I haven't see that problem (or so heavy) substracting.

  That's right.  Sometimes I also found the corner stars overcorrected
  by division.  I found them undercorrected by subtraction.  Indeed, 
  cubtraction doesn't correct the stars' intensities at all.  It just removes
  the background.  I have to admit both subtraction and division are not
  perfect treatment for film images.

> Does it works well, or we have to make further corrections to the sky background?

  It does improve the overcorrection but doesn't totally solve the problem.
  It looks like normal division for the sky background.  For stars, which are
  always brighter than the background, the dividing facotr becomes smaller
  to prevent overcorrection.  

> How about linearizating the film? Becouse we usually shot wide fields, and almost all of the stars there have known magnitudes, I think that it would be not so difficult to characterice the film's HD curve (we are not making science... we can allow us some "large" errors), apply a transformation to make it "linear" in the whole range, then make the same with the flat, divide, and finally undo the first transformation. What do you think?

  I was thinking about lineariztion for a while then decided not to do so.
  It's just too much work.  I'm satisfied by the current results.
 
  Wei-Hao


> Ok. Thanks.

> I'll give it a try. I'll make them using a large white wall instead of the sky, and I'll try to shot it to be in the VI zone...

 

> Regards,

> Carlos Milovic F.
-------------------------
Astro & Photo - CMF
http://www.astrophoto.vze.com
-------------------------
Visita "AstroFoto", el foro de astrofotografía en español
http://espanol.groups.yahoo.com/group/astrofoto


		________________________________



-- 
________________________________________________________________
Wei-Hao Wang  :)

Institute for Astronomy at University of Hawaii

Address:                       
2680 Woodlawn Drive         Personal Website:
Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
________________________________________________________________
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo