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Field rotation issue

Pawel Kolano
edited March 8 in PixInsight
Hi,

I've asked on other forums but I can't find a solution, here is a weightImage of 10hr of M81, done with 2600MC Pro, Asiair Plus, ZWO 120mm guide scope on AM5 mount. Drizzled 2x.

I seem to have a lot of field rotation, and in my case there isn't much on the left side and basically none at the bottom.
What could be the cause?

My PA was quite good, my guiding was around 0.90" to 1.40".
I've tried to dither every 1 and 3 frames, dither distance set to 5.

Everything in my imaging train seems tight, I'm really lost what could be the issue?

Thanks

Comments

  • BlockHeadBlockHead
    How can you tell you have rotation? I do not see that... instead just dithering. 
    Did you dither your images?

    Did you blink your calibrated images to see if there was actually rotation?

    Looking at rejection to understand this might be confusing.

    -the Blockhead
  • Pawel Kolano
    Well when looking at the weightImage I can see lot's of what I was told is a field rotation. 

    Here is rejection_low screenshot, I took the master stack that isn't drizzled, at least I think so since WBPP outputs 2 images, one drizzled and the smaller one I guess isn't?


    I've tried blinking calibrated frames but it's really hard to tell.

    In this case I've dithered every 3 frames.

    Both output images require quite a bit of crop, when I saw less of these white tilted spaces in previous stacks I had to crop less, because the sides didn't look as bad.

    I'm lost, what else I can try to do?
  • Pawel Kolano
    I can upload some of the frames if you would be kind enough to look at them, I've tried asking on other forums but didn't receive much help. :(
  • BlockHeadBlockHead
    Yes! This is correct!!
    The rejection map should show that if your frames are rotated- it will reject the areas where there is NO overlap. This means you will get a stacked image *that does not show* the rotation. This is true with proper local normalization. 

    But Pawel, if you are observing image rotation this has nothing to do with processing. Your data IS rotated because of how you acquired the data. This means your polar alignment is not perfect or your camera is moving... or something like equipment stuff. I asked you earlier if you blinked your images... you would see the rotation (in addition to your dithering). 

    The cause is bad polar alignment if I had to guess. There are a handful of other reasons.
    I do not typically handle acquisition issues...too many different ways and all I can do is guess.

    -the Blockhead
  • Pawel Kolano
    Thanks for all your help.

    I'm lost because lately Asiair was showing my PA as quite good, but I don't really trust it anymore.
    I've a new Redcat 61 (previously RC51), everything is very tight.

    My guiding scope is ZWO 120mm mini and main camera 2600MC Pro, my guiding is usually around 1.0" to 1.5", could this be the issue?

    Also I don't have my guiding scope focused 100%, but the focus is quite good, do you think it makes sense to get a perfect focus with the guide scope? I've heard it doesn't matter much since it's analysing star centers anyway. I'm not sure if it would improve the "drifting" anyway, I guess not.
  • BlockHeadBlockHead
    If the guide optics and main optics are not aligned... you will get field rotation.
    It isn't a focus issue.
    -the Blockhead
  • Pawel Kolano
    Sorry for a silly question but what do you mean exactly?
    I have my guide scope attached to the bracket on top of my scope, guide scope for my eye seems aligned with the RC61, can it be that the guide camera is rotated a bit? 

    Or do you mean something else?

    Thanks!

  • BlockHeadBlockHead
    No I mean something else... the guide scope and main scope need to be co-linear precisely. There also cannot be any flexure between them. You need to align both on the same star. But as I say...this will not always be enough. Flexure will be an issue. This is a well-known down side to having a separate guide scope.

    -the Blockhead
  • Pawel Kolano
    Hmm I'm not sure what I could do to fix that, the guide scope seems to sit nicely, but who knows, maybe there is temp. change or something.

    Do you think it would make sense to get a bigger guide scope for my Redcat 61?

    Thanks
  • BlockHeadBlockHead
    Larger would make it harder...not easier I think.
    -the Blockhead
  • Pawel Kolano
    I was wondering about something like that:


    With current setup I can only move the guide scope left/right since there is no rings.

    But if you say bigger will be worse, then I guess I will stick to what I have.

    And do you think there is any benefit from achieving perfect focus with bahtinov mask? Or none at all?

    Thanks Adam
  • BlockHeadBlockHead
    An ADM adapter provides up-down and left-right. It is what I installed on my telescope.
    In term of field  rotation due to guiding... focus has nothing to do with it,
    -the Blockhead
  • Pawel Kolano
    Out of curiosity why do you think 50mm guide scope would make it possibly worse?
  • BlockHeadBlockHead
    Bigger probably means a longer focal length.
    That might make it easier to see the issue with co-alignment- but perhaps harder to keep aligned.
    But the issue is keeping the two telescopes aligned. 

    I am not an expert on acquisition nuances (actually..I am...but specific to the equipment I have used). 

    There are hundreds..if not thousands, of threads on forums like cloudy nights that discuss OAG and guide scope pros and cons.  

    Here is ONE example:

    I reccomend reading these forums and the discussions.

    -the Blockhead
  • Pawel Kolano
    Yeah I was looking into OAG while you were posting this, I might try to go this way instead.

    Thanks :)
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