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Thread: Stacking images from alt-azimuth mount (image shift)

  1. #21
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    Default Re: Stacking images from alt-azimuth mount (image shift)



    i think the other thing you can do for field rotation is try to process out some the trails before stacking, PS can do this to a degree, im not sure what other programs can- youll probably lose some dimmer stars in the process
    if at 1st you dont succeed, screw it
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    A motorized (tracking) dob will NOT eliminate or reduce field rotation. A dob is basically an AZ mount and all AZ mounts suffer from field rotation. An equitorial mount is the only practical solution to getting longer exposures.

    What Jennifer and Sxinias are saying is that you must limit your AZ tracking exposures to short times (Jennifer has good results with 30 second exposures) and then rotate each image before stacking them. Sxinias says that DSS does a good job of de-rotating and then stacking the images for him. Not sure how Jennifer aligns/stacks her images.

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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Sxinias,

    You mention that GIMP can be used to stack increments (are increments the individual pictures?).
    1) How do you translate and rotate the individual images?
    2) Once each image is translated and rotated to match some "master" image, how are they then stacked together?

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    Default Re: Stacking images from alt-azimuth mount (image shift)

    @Neal_mlc,

    AFAIK, to eliminate field rotations, you'll either have to use short samples (actually just minimizes it, depending on how short you go) like most alt-az mount users do, use your dobsonian on a wedge (effectively making it an equatorial mount), or use a field rotator to compensate.

  5. #25
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Quote Originally Posted by STEVE333 View Post
    Sxinias,

    You mention that GIMP can be used to stack increments (are increments the individual pictures?).
    1) How do you translate and rotate the individual images?
    2) Once each image is translated and rotated to match some "master" image, how are they then stacked together?

    Steve
    If you just take a series of shortish exposures (such that each exposure does not show any trailing) then DSS will de-rotate and stack them for you just fine.
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    I just stumbled on this thread. I find that Nebulosity is another program that is very easy to use for stacking (and capturing) even with Alt Az images. It is not free but not expensive either. I'm surprized it has not been mentioned yet.

    All my images to date have been captured with an Alt Az and OSC CCD. When my target is something bright like a cluster, I typically shoot 40 to 60 exposures of 25 seconds. I use higher number of frames when shooting near zenith because I know I will discard about 1/3. For galaxies or faint nebulas, I usually shoot 150 frames of 25 seconds.

    Alt Az does work! I have several examples on my Smugmug AP page (Astrophotography - Zortax).

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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Stacking with a program like GIMP is tedious and really not feasible if you have more than a few images to stack. The process varies slightly dependent upon what program you are using but basically you load two images, copy one then paste it on top of the other. You will have to manually rotate one image so the two match (this can take some time). The Difference (subtract) command will let you know when you have the two aligned. Use either dodge or normal to combine the images at 50% transparency then flatten the two layers.

    Load another image, copy it, and past it on the combined image from the first two images. Rotate until aligned then combine using 33% transparency, repeat again but at 25% transparency for the 4th image, 20% for the 5th, etc.
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Paul,

    Thanks for mentioning that all your astrophotography photos were taken with an Alt Az mount. Since that is all I have at this time that is very encouraging. Unfortunatly I only have an F/13 4" aperture at the moment. However, Sxinias has convinced me that getting an F/5 scope is really a good idea for all this DSO astrophotography and I think I am going in that direction.

    I have created a little program that creates a graph of Max Exposure Time vs times before or after transit for any object and any observer location. If you are interested I would be glad to share it with you. It consists of two files (one .exe file and one .ini file). There are pictures in my album (Steve's Pics) that show the input screen and then the output screen. The program may show when you could take longer exposures if you wanted to to reduce the number of images.

    Clear skies,
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Quote Originally Posted by STEVE333 View Post
    Paul,

    Thanks for mentioning that all your astrophotography photos were taken with an Alt Az mount. Since that is all I have at this time that is very encouraging. Unfortunatly I only have an F/13 4" aperture at the moment. However, Sxinias has convinced me that getting an F/5 scope is really a good idea for all this DSO astrophotography and I think I am going in that direction.

    I have created a little program that creates a graph of Max Exposure Time vs times before or after transit for any object and any observer location. If you are interested I would be glad to share it with you. It consists of two files (one .exe file and one .ini file). There are pictures in my album (Steve's Pics) that show the input screen and then the output screen. The program may show when you could take longer exposures if you wanted to to reduce the number of images.

    Clear skies,
    Steve
    Steve, you're on the right track if you get an f/5... It will make a huge difference. I currently own both a 10" f/10 and a 4" f/5. The camera also makes a big difference. I'm a luck owner of a good CCD camera.

    Your program sounds interesting. What do you mean by "time before or after transit"?

    Thanks,
    Paul
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Paul,

    Consider an example where you wanted to view M33. The declination for M33 is about 30.7 deg. Putting that value into my program along my latitude of 37 deg will produce a graph (on my laptop) showing the Max Exposure vs. Time Before and Time After transit. At my location (Santa Cruz, CA which is at longitude 121 W) M33 will transit (be at it's highest elevation in the sky) at 39 minutes after midnight. Three hours before transit would be 9:39 pm, two hours before transit would be 10:39 pm and so on. The graph shows the Max Exposure for all times when the object is above the horizon. See an example of this in the picture below



    I hope you can see the picture as this is my first attempt to include a picture.

    Hopefully this makes sense.

    Steve
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