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  1. #1
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    Default Movement during imaging session?



    I noticed this morning when looking at my images from last night that during the entire imaging session the stars had shifted a little. I've never noticed this until now and have taken several sessions with successful images. I'm done anywhere from 60 second subs to 360 second ones with no star trailing in the single subs. But when I stack the whole set of subs there is a dark border around the image, especially around the 360 ones I did. I went back and looked at the first and last image of all my recent sessions and over an hour or two the stars shifted a little??? Is this normal with guiding sessions? My guiding has always been great with PHD. Maybe I'm over reacting but just wanna make sure its normal or not. It's funny it doesn't seem to affect my single subs, but I guess if I took longer ones, like 10 plus minutes it might.

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    Default Re: Movement during imaging session?

    What are you using to stack them?

    It is 100% normal and OK for the stars to shift A LITTLE from one sub to the next - the stacking software will "register" (align) them regardless. In fact this is a VERY GOOD THING - it is called "dither" and will ensure that when the subs are stacked the stacking software can correctly identify hot/cold pixels because they won't move from one sub to the next, whereas the stars will. This improves the noise reduction processing.

    If the stars move a lot over the course a long set of subs you can end up in a situation whereby when the images are stacked you lose a lot of image because only the central area will stack properly. However, as I said a movement of a few pixels between subs is fine and good.

    Having said all of that, I suspect you should look at your PHD guiding setup because unless you are intentionally dithering between subs - either by manually slewing the scope a little or using imaging software such as BYE which can send dither signals to PHD - really the guiding SHOULD be holding the image steady!





    What you don't want of course is for stars to move (streak) within a single sub.

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    Default

    Yeah its enough to move the stars a few pixels over say 20-30 minutes. I have no trailing in my six minute subs though. My PhD setup is reporting perfect numbers as well. I am not dithering so that's not the shift. What else could be shifting it? My Dec has always been out of balance a little so maybe that's it? But if I'm getting away with six minute subs then its not a huge deal. Just wanted to know if its normal and if not where to look for issues. Thanks.

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    Default Re: Movement during imaging session?

    Quote Originally Posted by whitenerj View Post
    Yeah its enough to move the stars a few pixels over say 20-30 minutes. I have no trailing in my six minute subs though. My PhD setup is reporting perfect numbers as well. I am not dithering so that's not the shift. What else could be shifting it? My Dec has always been out of balance a little so maybe that's it? But if I'm getting away with six minute subs then its not a huge deal. Just wanted to know if its normal and if not where to look for issues. Thanks.
    If you are not intentionally dithering, the DEC drift is most likely caused by very small polar alignment errors. What kind of gap/pause do you have between subs?

    If there is no visible streaking in the subs I'd tend to ignore it provided your stacks look OK. As I said, dithering is a good thing, streaking is bad.
    Personally I'd like to know why this is happening because I'd rather be in control of the dithering than leave it to (happy) chance - but that's just me :-)

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    Default

    I have no pause in the subs. Never have needed it. The final stacked image always looks great. I'll try and and look at the Dec balance more and adjust it. Every since I got my camera cooler it has weighted down the one side of my scope.

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    Default Re: Movement during imaging session?

    I agree you don't need a pause between subs - but if you had a longish one, that would explain your results.

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    Default Re: Movement during imaging session?

    It sounds pretty normal for long subs. I get a black fringe on the edges when stacking 'longish' subs, I just do a corner to corner crop when processing to eliminate the little tail on the histogram to the left in the levels histogram. Over a period of 6 minutes, there is substantial movement/rotation of the imaged stars. 15 arc-seconds a minute, if I remember correctly? So that equates to a total movement of 1.5 degrees over 6 minutes. Sounds normal?

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    Default Re: Movement during imaging session?

    Ok, here is an example of what is going on. These are 6 minute subs of M101 that are 2 hours apart during the whole imaging session. Notice the drift in the subs in 2 hours.

    LIGHT_360s_800iso_+24c_536stdev_20120810-00h23m05s045ms.jpg

    LIGHT_360s_800iso_+27c_577stdev_20120809-22h20m14s274ms.jpg

 

 

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