Debugging oval stars

Discussion in 'STX and STXL Series Cameras' started by EricC, Oct 29, 2017.

  1. EricC

    EricC Cyanogen Customer

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    I'm getting oval stars with my STXL-16200 and Optec Gemini rotator/focuser on a C11 scope, regardless of the exposure length. I'm pretty sure that rules out guiding or anything with the mount, so I assume it means something isn't on straight.
    As far as I can tell, the Gemini is on the scope flat, and the camera is on the Gemini flat. While the scope was pointing straight down, I loosened all the screws holding the Gemini and camera on to make sure things were seated correctly, then tightened the three screws part way, then all the way.

    What's the best way to troubleshoot this, and fix it?
    Is it possible to troubleshoot/fix during the day?

    Attached is a 30-second exposure. A 10 minute exposure has similar stars.
    In the picture, when I slew west the stars move to the left, and when I slew north the stars move up. A plate solve shows I'm at 0 degrees.
     

    Attached Files:

  2. Colin Haig

    Colin Haig Staff Member

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    I'm just another customer, and had a fast look at your image.
    It could easily be an optical problem.
    Start with improving the focus of your stars in the centre of the field.
    Do you have a focal reducer/field flatener ? Is this an old C11 or an EDGE HD version?
    Have you collimated the scope with the camera on it ?
    When you talked about "flat", keep in mind the optics aren't necessarily parallel to the mechanics. E.g. the main mirror will shift around a bit.
    You might want to try a utility called CCD Inspector from CCDWare, that can map the field and show you what is going on. I think there is a free trial version for 30 days or so.
     
  3. Doug

    Doug Staff Member

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    Have you checked your scope collimation?
     
  4. EricC

    EricC Cyanogen Customer

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    Doug,
    Are you asking about collimation because the shape of the stars on various parts of the picture indicate out of collimation? Or is that a "standard" question to ask when dealing with oval stars?
     
  5. EricC

    EricC Cyanogen Customer

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    I haven't yet checked collimation. Need to read up on how to do it with a C11 EDGE HD.

    I noticed that depending on where I'm pointing in the sky, the size of the ovals and the location of where they are round changes, which I assume implies something is sagging. I have the mirror locked down as tight as I can make it.

    Colin, by "flat" I meant the Gemini and the camera appear to be seated correctly and aren't tilted.
    If I recall correctly, there was a tiny washer on one of the four bolts holding the adapter to the camera, that came with the camera.
     
  6. Doug

    Doug Staff Member

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    It's a standard question when the stars are different shapes in different parts of the image. Which is the case with your instrument.

    If you have flexure then you'll need to sort that out first.
     
  7. EricC

    EricC Cyanogen Customer

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    I checked collimation using an out-of-focus star and it was very good, but I tweaked it a little. However, it didn't change the oval stars.
    What other things can I check?
    As far as I can tell, the Gemini rotator/focuser is screwed into the C11 nice and tight, and the 16200 is on the Gemini tight. Looking at them visually, they appear to be on flat, i.e., I can't see any tilt.
     
  8. Colin Haig

    Colin Haig Staff Member

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    Eric, if you've got a local astronomy club, you might want to find an expert to have a look at your setup.

    Mechanically rigid attachment has next to nothing to do with optical perfection - if the mirror, corrector, lenses, and focal plane of the chip are not perfectly collimated, your images will be poor.
    Even if the mirror is locked down, its still gonna move to some extent. You've indicated that the oval stars move around the field when the scope is pointing in different directions.
    You may be able to improve this with better collimation, however, you MAY have a lousy C11EdgeHD. You might want to contact your dealer for advice.

    Better collimation may help - collimating on a single star isn't gonna do it. You need to verify that the edges of the field are good too.

    Do you have the focal reducer?
    https://www.celestron.com/products/reducer-lens-7x-edgehd-1100

    I had a closer look at the JPG you provided above, and all of the stars are out of round - not even the ones in the centre of the field.
    Focus could be better as well. Are you using Focusmax or some other tool to automatically focus?

    From what I can tell, really, these issue are nothing to do with MaxIm or SBIG, and they are the sort of things we all have to deal with as we ramp up our astrophotography skills and equipment.

    What mount are you using, and how are you guiding?

    Again, I suggest you might want to check out CCD Inspector.
     
  9. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Have you tried any exposures unguided? It could be an problem with autoguiding. I would start taking exposures of a bright open cluster unguided, start at 1 second, 5 seconds, work your way up and compare the star images. If your stars are out of round even in a 1 second exposure then you probably have an optical problem. Start autoguiding when you can at least get 30 second unguided images with your setup.
     
  10. Doug

    Doug Staff Member

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    Autoguiding errors would result in blurred stars that are fairly consistent across the field. These aren't.
     
  11. Colin Haig

    Colin Haig Staff Member

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    Eric, one other thought for you. Any chance you've turned the main focus knob that moves the mirror with the mirror locks on? You might try pointing the OTA vertical so that the mirror points at the zenith, release the locks, and then tighten them again. Am wondering if the mirror is under a little bit of stress from the focus/lock mechanism.
     
  12. EricC

    EricC Cyanogen Customer

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    Colin, I can check that. I have a Gemini focuser but after using an eyepiece I need to first refocus with the main knob.
    Thanks for the suggestion.
    A couple months ago it worked fine and nothing changed that I know of.
    Eric
     

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