Resolved Flex in imaging train

Discussion in 'STX and STXL' started by Russell Croman, Jun 8, 2020.

  1. Russell Croman

    Russell Croman Cyanogen Customer

    Joined:
    May 8, 2020
    Messages:
    37
    Hi,

    I'm seeing quite a bit of flex somewhere in my imaging train. Here's the full setup:
    • FSQ-106
    • Flange
    • Spacer
    • WR35
    • 1/4” spacer
    • 1/2” spacer
    • STX adapter
    • AO-X
    • SC-3
    • FW7-STX
    • STX-16803
    At first there was quite a bit of focal plane tilt that I was able to correct by putting shims under the camera adapter ring, although it took a surprising 200µm of shimming to get it flat. Performance at the zenith is as follows:

    zenith.jpg

    But when I go to 30° altitude, I see quite a bit of change in the focal plane as I rotate the camera. Here are CCDI reports from 0, 90, 180, and 270°:

    0.jpg 90.jpg 180.jpg 270.jpg

    Something is flexing, and not perfectly symmetrically… the focal plane tilt isn’t simply a function of the rotator angle, but it does move with it. Something is more flexible in certain directions. Since a significant change happens at 0° rotation after slewing from the zenith to 30° altitude, I'm pretty sure the flex is behind the rotator (although it's possible that the rotator draw tube itself could be shifting).

    Any ideas?

    Thanks,

    Russ
     

    Attached Files:

  2. Colin Haig

    Colin Haig Staff Member

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    Messages:
    3,604
    Is the WR35 a Moonlight focuser/rotator?
    Can you post a photo of your setup?
    That's a pretty thick shim. I think you're going to have to contact the focuser manufacturer for ideas too.
     
  3. Russell Croman

    Russell Croman Cyanogen Customer

    Joined:
    May 8, 2020
    Messages:
    37
    Yep... moonlite nitecrawler focuser/rotator.

    The flex could be happening anywhere from the rotator draw tube back. I'm working with moonlite on the draw tube possibility, but I wanted to see if anyone has experienced flex in the imaging train. This is a heavy setup with a lot of bending moment on the cases of the AO, filter wheel, and camera, and on a fast optical system. It won't take much flex to cause an issue.

    Again, the issue isn't so much the static focal plane tilt... I have that dialed out temporarily with the shims. I'll be replacing those with a moonlite sidewinder for a bit more precision. The bigger issue is with the remaining flexure in the system that causes that tilt to change as the scope slews around and the camera rotates.

    Thinking through this further, I think the flex has to be behind the draw tube... the bending moment on it is not changing as the camera rotates with the scope pointed at one spot as in the CCDI reports I posted. The roller bearings it rides on are staying in the same place, etc. One possibility is that the draw tube is not collimated, but if that were true, I would expect the focal plane tilt to rotate strictly with the camera, so the CCDI reports would show no change with rotation, i.e., it would be a static tilt taken care of by the shims.

    At least I think I'm thinking about that correctly. Let me know if I have some wires crossed in my calculus... it's easy to think with the wrong reference frame when analyzing this stuff.

    BTW, I checked the scope collimation vs. pointing altitude... it seems solid.

    Here's a couple of pictures of the setup... you can see my temporary aluminum-foil shims in the close-up.

    IMG_7885.jpeg IMG_7888.jpeg

    -Russ
     
  4. Doug

    Doug Staff Member

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    By thought would be to get a dial indicator attached to the tube assembly, and use it to probe at different points in the assembly.
     
  5. Colin Haig

    Colin Haig Staff Member

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    Was the AO energized when doing these tests?
     
  6. Russell Croman

    Russell Croman Cyanogen Customer

    Joined:
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    It was. Interesting thought... but any tilt in the AO window would just cause an image shift, right? Not a focal plane tilt.

    -Russ
     
  7. Russell Croman

    Russell Croman Cyanogen Customer

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    Good suggestion. Amazon truck with dial indicator showing up tomorrow. :)

    -Russ
     
  8. Russell Croman

    Russell Croman Cyanogen Customer

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    Messages:
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    I think I found the issue...

    I noticed that focus was shifting by ~650 counts from low to high pointing altitudes. At 0.27µ/count, that's almost 0.2mm... something had to be loose. Sure enough, if I push on the focuser drawtube with my thumbs, it shifts visibly. Getting that addressed now. :)

    -Russ
     
  9. Doug

    Doug Staff Member

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    That will do it! Will mark as resolved.
     

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