FW8G position accuracy

Discussion in 'Aluma Series' started by Michael Brown, May 2, 2019.

  1. Michael Brown

    Michael Brown Standard User

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    I'm seeing some dust doughnut patterns in flat-field images moving when comparing flat-field images taken on different nights. What repeat positional accuracy is expected with the FW8G? Is there an estimate available in mm or micron?

    I'm going to look into other potential causes of this issue, including extreme focus positions and cross checking that the filter is absolutely secure in its housing, but I want to check what accuracy is expected out of the filter wheel regardless.
     
  2. Doug

    Doug Staff Member

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    The wheels are supposed to be repeatable within 5 microns.
     
  3. Michael Brown

    Michael Brown Standard User

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    I'm seem to be seeing bigger shifts than that, which I haven't seen previously when using the older FW8G-STT. I am using the inserts and from memory they did not feel as snug as they did for the FW8G-STT.

    I will do some tests, including extreme focus values soon. Given there's been some updates on the software/firmware side of things in recently, do they have any impact on filter wheel positioning/settling?
     
  4. Doug

    Doug Staff Member

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    None whatsoever. The detent mechanism is entirely mechanical. If you hear a "click" every time it changes filter position then it's working.

    Perhaps the filters are simply loose in their cells. Give the wheel a light shake - do they rattle around a little? If so then a little shimming may be required. Is this an Aluma, STT, or STXL wheel?
     
  5. Michael Brown

    Michael Brown Standard User

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    Aluma, using the inserts and o-rings provided.

    As I noted before, from memory the fit didn't seem as snug as with the STT equivalent filter wheel (that I have on another telescope), but memory is a poor substitute for the tests I need to do.
     
  6. Doug

    Doug Staff Member

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    The Aluma filter wheel's carousel is 100% identical to the STT carousel, so they really should behave exactly the same.
     

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