Dimmer/Smaller stars are round, Brighter/Larger stars are elongated

Discussion in 'Off Topic' started by JoshuaHufford, Nov 7, 2021.

  1. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Not sure what section to put this is so I just put it in off topic, please move if needed.

    Please take a look at my images, it seems to me that the brighter stars have an elongation at a diagonal from top left to bottom right, and the dimmer stars look round. This seems to happen with all of my filters or even without a filter (blank spot in my filter wheel, although obviously bloated stars).

    My guiding was excellent during all of these images, and the length of each image seems to have no real impact on the star shapes.

    Collimation of the telescope was just checked and is good.

    Field flattener was removed for testing and while the stars in each corner were elongated out in each direction pointing towards the corners as would be expected, the brighter stars in the center still showed the diagonal elongation and the dimmer stars looked good.

    The image train consists of this equipment,

    Stellarvue 130mm F7 Triplet with a Stellarvue field flattener
    OAG-8300 with STi guide camera
    FW8-8300 with Astrodon LRGB filters
    ST-8300M

    Also of note there is something going on in the bottom left corner of the images, I'm not sure if this is possibly a shutter error, or possibly frost? My desiccant has not been replaced in several years.

    Any idea what could be causing this?

    https://www.dropbox.com/s/c7o6dfwulnyq627/600 S L.fit?dl=0
    https://www.dropbox.com/s/dc89437rwb235vc/600 S R.fit?dl=0
    https://www.dropbox.com/s/dc89437rwb235vc/600 S R.fit?dl=0
    https://www.dropbox.com/s/3l4jy6b0kaafmxy/150 S R.fit?dl=0
    https://www.dropbox.com/s/3l4jy6b0kaafmxy/150 S R.fit?dl=0
     
  2. Colin Haig

    Colin Haig Staff Member

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    There's only 3 images - you've posted 2 of them twice.

    Hey @Doug what do you think?

    Desiccant:
    My rule is: If you think it's been too long, then it's been too long.
    Usually you will get tendrils, halos, or bigger areas affected the longer the camera runs.
    There's a hint of halo around several of the stars.
    What was the humidity and wind like? Any chance it was near the dew point?

    Diagonal:
    The faint stars don't show much elongation becuase there aren't enough photons to measure it.
    It's sub-pixel, and the star flatness is pretty good using the information window.
    Xi Cyg really dominates the frame.

    Bottom left corner:

    It's hard to say what is going on - possibly an issue with the shutter - it's almost a perfectly straight line.
    Simplest check would be to do a sky flat or t-shirt flat and see if it shows up there.
    Frost usually shows up all over.
     
  3. Doug

    Doug Staff Member

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    In these situations typically ALL of your stars are actually elongated, but you just can't see it in the tight cores of the fainter stars. In fact the only stars that look elongated are the ones that are saturated, so you're looking deep into the tails of the star's energy distribution.

    Sometimes occasional guider excursions can cause the stars to be slightly elongated in the tails, but this does not line up with your telescope axes. PA is 177 degrees according to PinPoint - pretty closely aligned E-W. I even tried rotating your frame 3 degrees but the elongation was nowhere near horizontal, so that's not it.

    I would have to go with an optical effect. I don't think your collimation is off because I don't see much difference between the corners, and it is unusual for a good refractor to be badly uncollimated.

    Perhaps your issue is a slight tilt to the focal plane.

    Try rotating the camera 90 degrees and see if the effect changes with the camera rotation.
     
  4. Colin Haig

    Colin Haig Staff Member

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    I checked the images with CCDInspector, and collimation seems to be really good.
     
  5. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Thank you. This evening we are going to try rotating the camera and a different ST-8300 as well.
     
  6. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Tried another ST-8300 camera and a ZWO camera as well with the same result. If the camera is rotated 90 degrees the elongation rotates 90 degrees as well so your thought of the focal plane being tilted seems accurate.

    The focuser is a 3" Feathertouch and all of the connections between the focuser, field flattener and camera are threaded, no set screws or nosepieces. The camera rotation was done via the built in rotator on the focuser.

    Any thoughts on how to figure out if the optics are causing the tilt or if a mechanical problem is causing it?
     
  7. Colin Haig

    Colin Haig Staff Member

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    If the rotator on the focuser is part of the problem, this isn't the best test. Rotate the camera without the use of the rotator.
    At this point, I think you may want to work with an artificial star and eliminate mount motion from the situation.
     
  8. Colin Haig

    Colin Haig Staff Member

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    BTW obviously if the camera is rotated and the elongation rotated with it, then it's not optical from the scope. It's mechanical.
     
  9. Colin Haig

    Colin Haig Staff Member

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    One other thought - do you have a clear (empty) slot in the filter wheel? That would take out one more optical surface.
     
  10. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Yes I do have one blank spot in the filter wheel, we did the test in that location and the stars were larger as expected but showed the same elongation.

    Thanks for all of the help, were going to inspect all of the mechanics of the scope when we have time.
     
  11. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    My observatory partner and myself tested the scope with an eyepiece last night, for whatever reason there seems to be astigmatism, an out of focus star is obviously not round, if you go to the other side of focus the out of round shape changes 90 degrees. The scope has an adjustable lens cell for collimation and was bought used so who knows what might of happened to it, we are hoping it is possibly just a pinched optic. Were going to call Stellarvue and see what they suggest.
     

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