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

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    Question Stacking images from alt-azimuth mount (image shift)



    Hey everyone,

    I went out last night to do some astro pics and I kind of dedicated most of my exposures to M81 & M82, the problem I am having is that I have around 178 of 14 second exposures (41mins 30secs) and now I'm trying to process these with DSS and the end result is coming out almost twisted with and certainly not stacked as I thought, I know this is because I only have an alt-azimuth mount and an EQ mount would be better suited however are there any ways of getting rid of this effect via processing??


    Could I perhaps process a range of say 40 a time and then collaborate these after?

    Many thanks and kind regards
    Adrian
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    There are a few programs that excel at stacking many photos:

    - Registax (haven't really tried it for Deep sky objects)
    - PixInsight (has a 45 day trial - amazing stacking capabilities even from mosaics and differing views ie Alt-AZ possible drift)
    - Deep Sky Stacker (DSS which you mentioned - may need to carefully check settings)
    - ccdstack (has a trial)

    Some like DSS and PixInsight will remove the longer trailing exposures (or you can set it to leave these in) and keep only the best ones.
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    Thanks Gus!

    Much appreciated, I've requested a trial for PixInsight, just going through my settings in DSS.

    Much appreciated as always!
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    Default

    You can also round up oblong stars in PS.
    It can involve the use of layers and some find it a bit daunting, but it certainly can help the quality improve afterwards.
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    Adrian,

    I also use an AZ mount and stack 100+ exposures with DSS frequently. The image in each of your exposures is slightly rotated due to the rotation of the earth (field rotation). DSS selects certain stars and then rotates each image to align these stars. Looking at the final image you can see the outer edges and tell that the images were rotated; like a deck of cards that has been twisted. There is a central part of the image where all the photos you took overlap. Unless the object you are photographing was at the edge of your exposures and dropped off, it should within this area where all the images overlap. You simply crop the image and remove the ragged edges. This effect is more pronounced if your output mode is the Standard Mode. It's not near as noticeable if you select the Mosaic Mode for your output, however, the memory requirements for the Mosaic Mode are much more.

    Breaking the job up in increments and stacking the increments. I've tried that with out success so far. Theoretically possible but difficult to do. The problem is that DSS wants each photograph to be identical (width, height, resolution, etc.). Because of the rotating effect you mentioned, the image sizes of your increments will be different. I've had no luck trying to adjust the incremental images to a point where DSS liked them. There is another method however that does work .... use a image processing program like PhotoShop, Gimp, etc. and manually stack the increments. Since you will not have many increments, the job is not that difficult.
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    This pic was made up of 44 stacked 33 second images - it wasn't guided and half of them were from alt az and half from a wedge polar alignment.
    The stars at the edge are still round and yu can see where my camera alignment has left noise behind!
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    Hi Everyone!

    Here's a real good tutorial for DSS. It may help.

    http://www.stargazing.net/david/DSS/S3P2.7DSSV2HQs.pdf
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    So, it looks like that software can override the problems that come with an alt-az mount, dob mounts in particular. Is that a fair assumption? Is it safe to assume that longer, more than 5 minutes, exposures are now possible with dob style mounts?
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    no - field rotation will get you after a minute - my exposures were all about 30-40 seconds
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    Ok, That answers that question but as a follow on question, does a motorized (tracking) dob eliminate or reduce field rotation or is the only way to avoid it is to use a GEM?
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