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Re: [ccd-newastro] Re: Darks, Flats, Camera rotation, and binning 2x2

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  • Yahoo - Wodaski
    I forgot that there are some setups where the filter wheel, for example, wouldn t get rotated with the camera. Even so, there are still situations involving
    Message 1 of 1 , Feb 27, 2009
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      I forgot that there are some setups where the filter wheel, for example,
      wouldn't get rotated with the camera.

      Even so, there are still situations involving dust and non-rotating,
      dust-collecting surfaces where rotation won't matter:

      * The signal to noise ratio of the image where the dust donuts are is
      low enough to hide the dust donuts in the noise;

      * The distance from the chip to the location of the dust is too large to
      allow the dust to cast a discernible shadow

      or any combination of the two.

      The bottom line still remains: try it and see.

      Ron Wodaski



      Lenny Shaffer wrote:
      > At 02:25 PM 2/27/2009, you wrote:
      >
      >> <SNIP>
      >>
      >> The concept of 'imaging with the CCD vertical' doesn't really exist. The
      >> chip is read out exactly the same way every time. The rotation of the
      >> image has no effect whatsoever on how the chip is read.
      >>
      >> Since the chip is read the same way every time, there is nothing for you
      >> to worry about with respect to this aspect.
      >>
      >> If you do rotate the camera, however, sometimes vignetting may change.
      >> Some telescopes will give you very similar flats no matter how the
      >> camera is rotated; some will not. The only way to know is to take flats
      >> at different rotation angles and compare them (e.g., subtract one from
      >> the other and see what's left over).
      >>
      >> It's nice to prepare in advance, but there are many things you won't
      >> know until you try them both ways. <G>
      >>
      >> Ron Wodaski
      >> <SNIP>
      >>
      >
      > What you are saying doesn't relate to dust or dirt on the optical
      > components not inside the camera. You cannot remove the dust donuts
      > in one position while shooting the image then arbitrarily rotate the
      > camera and shoot flats in some alternative location. While part of
      > the rational in taking flats is to compensate for pixel to pixel
      > response to light and vignetting it's also to remove dust donuts. No
      > processing package I'm aware of attempts to align dust donuts during
      > the reduction phase so it's best to shoot flats in the same
      > orientation as the image frames.
      >
      >
      > Clear skies-
      > Lenny
      >
      > Your Murphy's Law of the Week:
      > "The less intelligent the idea, and the person stating it, the more
      > likely it will be funded"
      >
      >
      >
      >
      >
      > [Non-text portions of this message have been removed]
      >
      >
      >
      > ------------------------------------
      >
      >
      >
      >
      >


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