Cleaning Filters

Discussion in 'General' started by Charles Cynamon, Jun 9, 2019.

  1. Charles Cynamon

    Charles Cynamon Cyanogen Customer

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    Jun 3, 2017
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    Location:
    Mount Airy, MD
    I have a set of UBVRI 50mm round filters purchased from you. While taking new flat images, I noticed a very unusual image for my V filter. I removed my camera to inspect the filter and saw that it was fogged over. I tried cleaning the filter with 91% alcohol but the fog persists. Can you recommend the best way to clean my filter?

    My observatory is in Maryland and I have an automated system,which I leave in the ready state (without the OTA cover on). I have the camera (STXL-6303e) and filter wheel (STXL-FW8G) connected to a PlaneWave CDK 300. The most used filter for me is the V and the system is open but parked in a configuration to prevent dust collection on the mirrors. Could this issue have likely resulted from humidity? I constructed the observatory in Oct 2018 and this is the first summer. I'm trying to figure out how best to avoid this happening in the future.
     
  2. Doug

    Doug Staff Member

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    Filters can degrade over time especially if exposed to humid conditions. It may need to be replaced. I would inspect the other filters as well.

    If that is the only filter that is damaged, then there is an obvious solution to reduce exposure - rotate to a clear filter when parking the instrument.

    Another thought is to put a small heater in the wheel. Don't thermally connect it to the shell. The goal isn't to warm up the wheel, it's to make a pocket of warm air inside the wheel so you can't get condensation on the filters.
     
  3. Charles Cynamon

    Charles Cynamon Cyanogen Customer

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    Location:
    Mount Airy, MD
    Thanks for your rapid response Doug. I have inspected the other filters and there appears to be no damage. Their respective flat images were as expected too. Your response did not recommend a cleaning method. Should I regard that as you do not believe the V filter is salvageable?
     
  4. Doug

    Doug Staff Member

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    If 91% isopropyl alcohol didn't work, then I'm afraid it's not dirty. It's etched.

    I'd take it out of the wheel. Wash it in dish soap and water, and try alcohol again. But I don't think it will work.
     
  5. Charles Cynamon

    Charles Cynamon Cyanogen Customer

    Joined:
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    Location:
    Mount Airy, MD

    From a cost benefit perspective, is it worth re-polishing?

    Also, if I replace the filter, do some brands/coatings do better in my climate than others?
     
  6. Doug

    Doug Staff Member

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    The coatings are what do the filtering.

    I don't believe you'll find anything better. The coatings are tough but don't survive long-term "acid dew" exposure.

    A small (couple of watts) heater warming the air inside the wheel should prevent a recurrence. All you need is a couple of degrees. And as I said, rotate the wheel to a cheap-and-tough clear filter when not using it.
     
  7. Doug

    Doug Staff Member

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    For replacement filters, I'd recommend Custom Scientific:

    https://customscientific.com/astronomy.html

    (Note - they've raised their prices but apparently not updated their web site yet. The price is $360 per filter.)
     

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