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Re: RGB and flats

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  • John Smith
    This discussion has indeed been interesting and raised a question in my mind. I have been routinely taking flats in each color (LRGB) and decided to analyze a
    Message 1 of 14 , Oct 1, 2001
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      This discussion has indeed been interesting and raised a question in
      my mind. I have been routinely taking flats in each color (LRGB) and
      decided to analyze a recent set. The flats were taken with a light
      box that I had built. All pixel math was done in Maxim. I first
      normalized each flat to an average of 15,000 ADU. I decided to use L
      as the reference and computed L-c+2200 where c= R,G,B. The offset of
      2200 was an attempt to keep the difference positive. If flats were
      truly consistent over color, then the result of the subtraction would
      be an average value of 2200 with minimum and maximum very close to
      2200. Here are the statistics as reported by Maxim:

      Color Min Max. Averag SD
      L-R+2200 733 2598 2199 154
      L-G+2200 76 3016 2199 410
      L-B+2200 683 2590 2199 108

      Based on this data taken with my f/5 scope, I don't believe I can get
      away with a single set of L flats for all colors. It would be
      interesting to look at similar data for different scopes and optical
      trains. I also need to take either some twilight flats or overhead
      flats to certify my light box radial uniformity but I don't think
      this changes my conclusion.

      Feel free to tell me what I'm missing in this analysis. Thanks.

      John

      --- In ccd-newastro@y..., "Kevin Dixon" <ksbtk@h...> wrote:
      > Good evening folks:
      >
      > This has been a most interesting thread. I see that several people
      have
      > stated that flats for each color channel are not necessary.
      >
      > When I examine the dark-subtracted flats that I produce with my
      light box,
      > they each have a substantially different appearance from one
      another. Based
      > on this, I would conclude that indeed, flats should be created for
      each
      > channel. However, that is based strictly on the appearance of the
      flat.
      >
      > Does the physics behind flat fielding render the different
      appearance of
      > each channel's flat, a mute point? I look forward to the opinions
      of others
      > in the group.
      >
      > Clear skies,
      > Kevin Dixon
      > ksbtk@h...
    • Ron Wodaski
      It would be useful to compare the subtracted frames visually as well. Ron Wodaski The New CCD Astronomy http://www.newastro.com ... From: John Smith
      Message 2 of 14 , Oct 1, 2001
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        It would be useful to compare the subtracted frames visually as well.

        Ron Wodaski
        The New CCD Astronomy
        http://www.newastro.com

        -----Original Message-----
        From: John Smith [mailto:johnzonie@...]
        Sent: Monday, October 01, 2001 7:31 AM
        To: ccd-newastro@yahoogroups.com
        Subject: [ccd-newastro] Re: RGB and flats


        This discussion has indeed been interesting and raised a question in
        my mind. I have been routinely taking flats in each color (LRGB) and
        decided to analyze a recent set. The flats were taken with a light
        box that I had built. All pixel math was done in Maxim. I first
        normalized each flat to an average of 15,000 ADU. I decided to use L
        as the reference and computed L-c+2200 where c= R,G,B. The offset of
        2200 was an attempt to keep the difference positive. If flats were
        truly consistent over color, then the result of the subtraction would
        be an average value of 2200 with minimum and maximum very close to
        2200. Here are the statistics as reported by Maxim:

        Color Min Max. Averag SD
        L-R+2200 733 2598 2199 154
        L-G+2200 76 3016 2199 410
        L-B+2200 683 2590 2199 108

        Based on this data taken with my f/5 scope, I don't believe I can get
        away with a single set of L flats for all colors. It would be
        interesting to look at similar data for different scopes and optical
        trains. I also need to take either some twilight flats or overhead
        flats to certify my light box radial uniformity but I don't think
        this changes my conclusion.

        Feel free to tell me what I'm missing in this analysis. Thanks.

        John

        --- In ccd-newastro@y..., "Kevin Dixon" <ksbtk@h...> wrote:
        > Good evening folks:
        >
        > This has been a most interesting thread. I see that several people
        have
        > stated that flats for each color channel are not necessary.
        >
        > When I examine the dark-subtracted flats that I produce with my
        light box,
        > they each have a substantially different appearance from one
        another. Based
        > on this, I would conclude that indeed, flats should be created for
        each
        > channel. However, that is based strictly on the appearance of the
        flat.
        >
        > Does the physics behind flat fielding render the different
        appearance of
        > each channel's flat, a mute point? I look forward to the opinions
        of others
        > in the group.
        >
        > Clear skies,
        > Kevin Dixon
        > ksbtk@h...



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