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Re: [ccd-newastro] Total exposure time question: (was Re: ngc7331 7 hrs Lum)

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  • Bert Katzung
    Hi Steve: You re on the right track with your interpretation and it probably would be better if people specified the particular method of combining that they
    Message 1 of 4 , Oct 1, 2004
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      Hi Steve:
      You're on the right track with your interpretation and it probably would be
      better if people specified the particular method of combining that they use.
      I'm certainly one of the sinners in that regard, although I do usually
      specify the number of frames and individual exposure time for each rather
      than just the total time.
      I don't think anyone does individual frames longer than 30 minutes and most
      use 5 to 15 minutes. The method of combination gets into theory and practice
      that I'm not yet familiar with other than the obvious advantage of
      eliminating cosmic hits and airplanes when anything like median or the more
      complex procedures are used. I usually use sigma clip or SD mask (tools that
      are new to MaxIm 4 but have been available from Ray Graylak, Russ Croman,
      and others for some time). If I have only a few very thin exposures I will
      sometimes sum, but that's only for exploratory stuff. If the results look
      promising I'll go back and get more frames and sigma clip or SD mask.
      Hope this helps....
      Bert

      Bert Katzung
      katzung1@...
      http://www.astronomy-images.com
      http://www.visionlightgallery.com/katzung/

      ----- Original Message -----
      From: <steve.holak@...>
      To: <ccd-newastro@yahoogroups.com>
      Sent: Friday, October 01, 2004 12:32 PM
      Subject: [ccd-newastro] Total exposure time question: (was Re: ngc7331 7 hrs
      Lum)


      > I've only been CCD imaging a month, with an ST-2000XM, and I'm getting
      > progressively better with tuning my system and mount (and processing), so
      > I'm not too far away from mustering up my first post. I've read Ron's
      book
      > and lurked here for about a year (and I've done film AP for several.) I'm
      > getting a handle on CCDSoft (don't have Maxim yet), and have good chops
      > with Photoshop from film experience.
      >
      > I have a question though, on the meaning of the total luminescence time
      > that imagers post. I'm aware of the different methods of image
      > combination, and their reasons (averaging reduces noise, median and sigma
      > rejects the odd pixels, etc.), but when someone speaks, such as this
      > thread, of "7 hours lum" or "180 minutes total." If the combine method
      > isn't explicitly stated, and someone posts "10 x 6 mins for 1 hour total
      > lum", does that imply the images are averaged or summed?
      >
      > For example, I shot the supernova in NGC6946 last night at 12 X 5 min
      > (until the *** moon came up), but haven't processed yet. If I average the
      > 12 frames, do I post my lum time as 60 mins, or 12 x 5 averaged? If I
      > median combine, what would be the protocol for the total exposure time I
      > post?
      >
      > --thanks
      >
      >
      > Steve Holak
      > Senior Software Architect
      >
      > Brokerage Concepts IS Dept.
      > 610-491-4879
      >
      > email: steve.holak@...
      >
      >
      >
      >
      >
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
    • Wodaski Yahoo
      In nearly all cases, the exposure time stated is the total exposure time. The combine method will affect the S/N of the final image. A mean will have the
      Message 2 of 4 , Oct 1, 2004
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        In nearly all cases, the exposure time stated is the total exposure time.
        The combine method will affect the S/N of the final image. A mean will have
        the lowest noise, and best S/N, but may leave cosmic ray and other artifacts
        (e.g., satellite tracks). Sigma reject, min/max clip, and other more
        sophisticated algorithms remove outliers and then average (typically).

        So it is total exposure time that is commonly reports, and this has some
        usefulness and of course it's inexact without information about the combine
        method. But it's still a useful number in and of itself.


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

        -----Original Message-----
        From: steve.holak@... [mailto:steve.holak@...]
        Sent: Friday, October 01, 2004 1:33 PM
        To: ccd-newastro@yahoogroups.com
        Subject: [ccd-newastro] Total exposure time question: (was Re: ngc7331 7 hrs
        Lum)

        I've only been CCD imaging a month, with an ST-2000XM, and I'm getting
        progressively better with tuning my system and mount (and processing), so
        I'm not too far away from mustering up my first post. I've read Ron's book
        and lurked here for about a year (and I've done film AP for several.) I'm
        getting a handle on CCDSoft (don't have Maxim yet), and have good chops
        with Photoshop from film experience.

        I have a question though, on the meaning of the total luminescence time
        that imagers post. I'm aware of the different methods of image
        combination, and their reasons (averaging reduces noise, median and sigma
        rejects the odd pixels, etc.), but when someone speaks, such as this
        thread, of "7 hours lum" or "180 minutes total." If the combine method
        isn't explicitly stated, and someone posts "10 x 6 mins for 1 hour total
        lum", does that imply the images are averaged or summed?

        For example, I shot the supernova in NGC6946 last night at 12 X 5 min
        (until the *** moon came up), but haven't processed yet. If I average the
        12 frames, do I post my lum time as 60 mins, or 12 x 5 averaged? If I
        median combine, what would be the protocol for the total exposure time I
        post?

        --thanks


        Steve Holak
        Senior Software Architect

        Brokerage Concepts IS Dept.
        610-491-4879

        email: steve.holak@...






        Yahoo! Groups Links







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