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Combining calibrated images

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  • Kevin Dixon
    Happy New Year folks! After calibrating the M42 images I acquired a few nights ago, I am ready to combine the images in Maxim DL. Is there a preferred option
    Message 1 of 2 , Jan 1, 2001
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      Happy New Year folks!

      After calibrating the M42 images I acquired a few nights ago, I am
      ready to combine the images in Maxim DL. Is there a preferred option
      for aligning the indivivual images prior to median combining them?
      The available options for alignment are: none; auto-correlation; auto-
      star matching; manual 1 star shift only; manual 2 stars; and, overlay.

      I have the same question for RGB color combining as well.

      Clear skies,
      Kevin Dixon
      ksbtk@...
    • Ron Wodaski
      Different methods seem to work better on different images. I like to use the Process | Align menu item to do my aligning separately from color combining,
      Message 2 of 2 , Jan 1, 2001
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        Different methods seem to work better on different images. I like to use the
        Process | Align menu item to do my aligning separately from color combining,
        because I can check the result. Here's how I do it:

        * Use Process/Align with your preferred (or random) alignment method.
        * Do another Process/Align, but choose overlay. Now use the Next button to
        step through the aligned images. If they are well-aligned, you are done. If
        there are problems, the method you chose won't work well for these images.
        Close everything without saving, and try a different method.

        NOTE: you can also use the "Overlay all images" button to view all of the
        images simultaneously for a quick check.

        If nothing else will work, careful use of the Overlay method, with manual
        shifts as small as 0.1, and rotational shifts as small as 0.01, will get you
        perfect alignment.

        NOTE 2: I usually use Pixel Math to match the average background level of
        the images before I align them. Otherwise, if there are major differences in
        background level, the background of one image might obscure the stars of
        another image. I usually make the average background level about 50 units
        before aligning and/or combining. I do this manually, one image at a time,
        and since there is usually a gradient of some kind in most images taken at
        my location, I carefully look for the background that is the darkest, and
        the subtraction I do with Pixel Math is an amount that will take that
        darkest area to ~50 units.

        As to the meaning and utility of the available options, here are my comments
        on each:

        Auto - correlation
        ---------------------------------------
        Uses "fancy math" to match images. If stars are present, they are used to
        refine the accuracy of the correlation. No rotation; vertical and horizontal
        shifts only. Only works with images that are already fairly close in
        alignment. Usually the only auto option that will handle planetary images;
        you may need to use "Overlay" to get the planetary images close enough for
        auto-correlation to work.

        Auto - star matching
        ---------------------------------------
        Maxim looks for patterns in the stars, and uses them to match images.
        Applies both rotation and horizontal/vertical shift as needed.

        Manual 1 star - shift only
        ---------------------------------------
        If there are no rotational problems between images, this is the fastest
        method to get an alignment. You click on one star per image, and Maxim
        aligns the images by shifting vertically/horizontally. Use Auto Next for
        fast operation.

        Manual 2 stars
        ---------------------------------------
        This method is like the previous one, but adds a second star to support
        rotational corrections when needed. To use, turn on Auto Next. Click on the
        same star in all images the first time through, then click on a second star
        the second time through each image. For good results, pick stars that are
        away from the center and on opposite sides. Never use a saturated or bloomed
        star!

        Overlay
        ---------------------------------------
        If none of the other methods give satisfactory results, you can use
        "Overlay" to do a fully manual registration. You can make horizontal
        movements as small as 0.1 pixel, and rotational adjustments as small as 0.01
        pixel -- just type the number in manually instead of using the scroll
        arrows. (You could probably use even small numbers for each of these, but
        the change is so small it won't improve anything).

        First you set a reference image; then you view each image in turn and adjust
        it to match the reference. Move among images with the Next and Previous
        buttons.

        I usually choose a star near the center of the image as the center of
        rotation. Use the shift buttons to get a perfect alignment on this star,
        then click on it to mark it as the center of rotation. Most of the time, you
        will use small numbers (like 0.05 or less) to make rotational adjustments.
        If you took images with wildly different rotations, of course, you can use
        much larger numbers.

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

        -----Original Message-----
        From: Kevin Dixon [mailto:ksbtk@...]
        Sent: Monday, January 01, 2001 11:23 AM
        To: ccd-newastro@egroups.com
        Subject: [ccd-newastro] Combining calibrated images


        Happy New Year folks!

        After calibrating the M42 images I acquired a few nights ago, I am
        ready to combine the images in Maxim DL. Is there a preferred option
        for aligning the indivivual images prior to median combining them?
        The available options for alignment are: none; auto-correlation; auto-
        star matching; manual 1 star shift only; manual 2 stars; and, overlay.

        I have the same question for RGB color combining as well.

        Clear skies,
        Kevin Dixon
        ksbtk@...



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