FW8G - Positioning issue

Discussion in 'Filter Wheels' started by Martin Pugh, Aug 11, 2019.

  1. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    Doug
    I am having some sort of problem with the FW8G whereby I am taking flats at the PA of the camera, and on the corresponding side of the mount where the lights were taken and I have remeasured the linearity of the STXL11002 to see if that had changed somehow. However, all of the light frames, regardless of filter are showing the edge of the filter holder in the top left of the image with red being the worst. But the flats are not correcting this. I have not seen this problem before.
    I can confirm that the scope is not obstructed when taking the lights, so its not that.

    Any suggestions as to how I can resolve this?
    thanks
    martin
     
  2. Colin Haig

    Colin Haig Staff Member

    Joined:
    Oct 27, 2014
    Messages:
    2,197
    Hi Martin,
    Can you confirm I'm understanding this properly?
    a) A light frame shot through a filter such as Red is showing the edge of the filter holder.
    b) Are the corresponding flats (eg Red) showing the edge of the filter holder?
    Am wondering if the home position sensor is slightly offset from the normal position (loose screw on the optical sensor).
    Could you post a sample FITS of the red filter flat?
    Are you doing sky flats (scope pointing up) or panel flats (scope horizontal)?
     
  3. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    Hi Colin
    a) yes, I believe it is the edge of the filter holder, strong in the top left, but weak in the lower right.
    b. Annoyingly, no. If it did I think I would be able to flatfield it out, but the flats do not show it and therefore do not remove it.

    Yes, doing sky flats using CCDAutopilot which calculates the null point automatically.
    I took fresh flats last night having remeasured the linearity of the camera, and took it at the PA of the target, still the same.

    I have your email address I think so I will send the FITS via wetransfer. too large to post here.
    cheers
    Martin
     
  4. Colin Haig

    Colin Haig Staff Member

    Joined:
    Oct 27, 2014
    Messages:
    2,197
    Hi Martin - got the images.
    Using Analyze... Pixel Math... I divided the one by the other, and got this:
    mpdonutshift.png
    If you look at the bottom of the image, there is a fairly big donut that shifts. By the size of the donut, am thinking it is on a filter. The smaller donuts don't shift, and those are likely on the front of the chamber window.
    If you blink between the two flats, you can see a shift going on in the position of the image circle on the chip.

    Am going to think about this a bit. Until I get my morning coffee, am thinking possibilities might include (feel free to rule out):
    - some kind of mechanical flexing going on with the way the camera+FW is attached to the scope.
    -- The null point is going to move a bit, so wondering if its camera orientation (camera vs gravity tugging down)
    - I don't think its the filter carousel flopping around - that's pretty unlikely
    - the bearing/spring positioner that puts the carousel in position should be doing its job or it would be obvious

    Am thinking about how to troubleshoot.
    Is this one down in Australia?
     
  5. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    Hi Colin - not following. I think you need to see a light frame to see the issue I am having. I will send that and when you get it take a look top left and lower right.
    FWIW - The configuration I have on this scope has been sound for a very good while now, and I have not seen this issue before i.e Flat fields not matching the optical noise in the image.

    this is the scope in Chile.
     
  6. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    Colin - just sent you two red frames, one raw, the other calibrated. Here is the other puzzling thing, while the shadowing is present in LRB images, its almost non-existent in the green. I will send you one.
     
  7. Colin Haig

    Colin Haig Staff Member

    Joined:
    Oct 27, 2014
    Messages:
    2,197
    Thanks Martin - I got the files.
    Agree, the green looks near-perfect.
    I'm puzzled. It's not obvious (to me) why this is happening.
    Let me ponder this some more.
     
  8. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    5,841
    Martin,

    That's really unusual. Can you pop the lid off the filter wheel and run it through some different slots? Maybe it's binding somewhere and you can see it happen - check for obstructions etc. There is a mechanical indexer so it will be very obvious if it doesn't land in the right place.

    Doug
     
  9. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    I think we have resolved this but I am waiting to get some new flats and light frames on the same target to confirm.
    If this proves to be the case, this is something I have flagged to you before. To recap, the washers you provide are too small to securely reach the edge of a 50mm filter. On top of this, the FW chain catches the top of the screws that secure the shoulder piece/washer in place. So having removed the FW cover we found that over time, the chain has caused some movement of the shoulder/washer to the point that nearly all of the filters are no longer secured in place by a washer. So what I have been seeing is not the edge of the filter holder, but the edge of the filter itself and this clearly explains why I cannot duplicate it in flat, because the filters are moving around.

    Martin
     
  10. Martin Pugh

    Martin Pugh Cyanogen Customer

    Joined:
    Oct 27, 2014
    Messages:
    157
    Furthermore, the support guys in Chile have confirmed that other STXL cameras in the same observatory have the same issue of the chain catching the top of the screws.
     
  11. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    5,841
    Martin,

    Thank you for the feedback. We'll review the filter wheel design and see what we can do to mitigate this.

    Doug
     

Share This Page