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66162Re: [ccd-newastro] Manually reduction paradox

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  • Wodaski - Yahoo
    Nov 3, 2009
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      Well, let's consider the obvious first. Are you using automatic
      contrast adjustment? If you don't, then applying a dark will radically
      alter the appearance of the image. Depending on the new values, yes,
      you could see all or most of the data 'disappear'. It's still there;
      you just need to adjust contrast/blackpoint/whatever your software
      calls it. <g>

      Ron W

      On Nov 3, 2009, at 1:00 PM, lbarneo2003 wrote:

      > Ron,
      > Recently I bought a SBIG-402 ME ccd camera, and it woks very well. I
      > known that for scientific purpose the best option for image
      > reduction it is those that you explain in your book (section 2:
      > using dark frames): "…to take one or two dark frames before your
      > light exposure, then another one or two after, and average the dark
      > frames…" And this is handy when your exposure time is less than 3
      > min (usually in photometry/,astrometry). But if you want to take a
      > "postal" image that requires a lot of exposure time, this procedure
      > is not very practice, and one solution is to use dark frames that
      > previously have been taken (same temperature, exposition time). And
      > this is my problem: I have a paradox when I perform a manual
      > combine. When I apply it in drak substraction (CCDOPS), the dark
      > frame seems to"eat" the image frame. I have some test in a dark
      > room. When I apply the automatic reduction, all works very well.
      > When I take a dark frame, and perform the dark substraction, the
      > image disappear or almost: all is black. This is very significant
      > using short exposures (less than 120 seconds). Could you explain me
      > this difference? With CCDSoft this effect is similar. You can see
      > the images in album photo: Testdarkroom. Thanks
      > Luis
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