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Thread: Unguided imaging on 2000mm focal length sct

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    Default Unguided imaging on 2000mm focal length sct



    Hi,

    I’ve been trying to take 30 second subs of Andromeda on an 8” sct with a 2m focal length without guiding. The images look OK; not great. The ones I do not discard have somewhat elongated stars. They are probably 50% longer along one axis than the star’s diameter. I drift aligned very carefully, but I cannot eliminate an observable periodic error with amplitude of something like 2-3 arcmin and period of around 10min. I focus as best as possible with a bhatinov mask through the preview screen of my dslr.

    I cannot get deep sky stacker to register these images. I have tried on four evenings with the same results. My speculation is that the lack of autoguider together with the long focal length makes 30 second exposures impossible. Does this sound like a reasonable hypothesis to others?

    Here is one of the images:
    https://www.dropbox.com/s/jmgdeugib3...%20PM.jpg?dl=0
    Last edited by j.gardavsky; 01-04-2019 at 06:15 PM.
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    Default Re: Unguided imaging on 2000m focal length sct

    Do you Barlow your reticle EP when doing the drift alignment?
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    Default Re: Unguided imaging on 2000m focal length sct

    What are you using for a mount?
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    Default Re: Unguided imaging on 2000m focal length sct

    If you use the Celestron of the Meade 8" SCT either one will benefit from the .63 Focal Reducer. It takes your 2000mm FL and reduces it to 1260mm.

    Cheers,
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    Default Re: Unguided imaging on 2000m focal length sct

    Hi Ozman, I do not use a barlow for the drift align. However, I do think the polar alignment is quite good. I see clear periodic motion actross 40 images.

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    Default Re: Unguided imaging on 2000m focal length sct

    The mount is a celestron avx

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    Default Re: Unguided imaging on 2000m focal length sct

    The failure to stack in DSS will be due to the elongated stars. It needs round stars in order to stack.

    Trying to go unquided at 2000mm is definitely a challenge. Given your observed periodic error, you need either guiding or PEC. If your mount doesn't do PEC, then you need a guider.

    Any time you have elongated stars, it is good to know what direction the movement was in. If the movement is in RA, then the problem is periodic error or some other tracking anomaly. If the movement is in dec, then the problem is polar alignment.

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    Default Re: Unguided imaging on 2000m focal length sct

    Imaging at 2000mm with a SCT is pretty hard to start with. The AVX mount isn't all that good of a GEM mount. There are things you can do to try and make the mount better like Hypertune/Supertune and better polar alignment. But personally I wouldn't put a lot of hope into these. You are probably going to have to add on guiding using an OAG (Off Axis Guider) since it's an SCT or go with shorter exposures.
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    Default Re: Unguided imaging on 2000m focal length sct

    I wonder if there is a problem with your power supply to the mount ? Such trailing occurs if there is not enough power (low voltage); have you checked this aspect??
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    Default Re: Unguided imaging on 2000m focal length sct

    Thanks KathyNS, Noting the direction of trailing will be useful for confirming the problem. Great suggestion!

 

 
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