FW8-8300 and dew on filters?

Discussion in 'Filter Wheels' started by Clayton Bownds, Jul 20, 2015.

  1. Clayton Bownds

    Clayton Bownds Standard User

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    I live and image in a very (read:extremely) humid part of the south. Texas this year has been inundated with rain and the relative humidity has consistently been above 90% the few times I've been able to image this year. I am relatively new to imaging with a cooled, monochromatic CCD, so often when things go sideways while imaging, I don't know or understand how to troubleshoot them... I'm getting better, but still a "newbie". The last several imaging sessions have been cut short do to some phenomenon where my images get fuzzy and out of focus similar to when there is dew on the OTA, but the dew heaters I was using were doing their job on the front elements, so I didn't know where it might be. I thought that maybe the cooling was an issue, so I lowered the setpoint from -10C to -5C and watched to see if that helped. It may have helped a little, but that could have just been my wishful thinking. Anyway, I ended up packing it in scratching my noggin. Last night, I think I had a eureka moment. I was setting up to image and took a set of Luminance subs with the blank spot on my wheel (no filter) and everything looked great... so far so good. I moved to the red filter for the first of my RGB channels and it was a blurry mess... green and blue were the same... just for fun, I tried the narrowband filters (these were all just short test subs at this point) and they were the same. I moved back to the blank spot and took a relatively crisp and clean looking image. Eureka! Something is going on with the filters. So my first and obvious question is: Is this normal for higher humidity areas? What is the preferred way to manage this? I borrowed a long, velcro dew heater strip from another nearby imager and wrapped it around the protruding portion of the FW8 rectangle (the part of the wheel extending beyond the camera body) and ran it on high the rest of the evening. It took some time... and McGyver would by proud... but it seemed to help.

    I don't think there is anything I can do to get away from the dew problem where I live, so I'm pleading for some wisdom here on how to improve my defenses. I'm ready for your tribal knowledge (please)! :)

    Thanks so much,
    Clayton
     
  2. Doug

    Doug Staff Member

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    It's unusual to have the filters fog up... but we have terrible humidity here also and I've had every optical surface imaginable fog up at one time or another. My main scope has heaters on the primary and secondary, and they're needed.

    What sort of telescope do you have? An open truss tube scope would expose the filters directly to humid outside air. If so possible corrections include putting a shroud over the instrument, or perhaps you might even have to wrap a heater around the filter wheel.

    If it's a closed tube - refractor or SCT, then there is excessive humidity inside the telescope. The simplest solution is to store your tube with some desiccant inside. I remember encountering a small company that made filtered desiccant plugs for storing your SCT; don't know if they're still around but it's something you could cobble together for yourself.
     
  3. Clayton Bownds

    Clayton Bownds Standard User

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    Thanks, Doug! I will look into it. In the mean time, I think strapping a dew heater on the FW8-8300 all night might help keep the filter chamber warm enough to serve as a work-around. Wish me luck!

    ps. Here is the result of last night's dew-interrupted imaging. The stars are fuzzy and the tracking isn't the best, but I'm learning to work through issues and getting slowly better over time.
    [​IMG]
     
  4. Doug

    Doug Staff Member

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    If that's one of your bad images... it looks like you have a lot of potential there!
     
  5. Gunny

    Gunny Cyanogen Customer

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    Guiding looks good, but hard to really tell without magnification . Are you perhaps referring to the nebulosity in the center of the image due to the gas surrounding the Iris ? When I've had fog on the main lens of my refractor, it appears more like bands of moisture.....I'm like you so far as a high humidity area. When the dessicant plug starts to fill with moisture, pictures don't look as good as this. Others who know a lot more than I may want to chime in, but I see more processing artifacts than a humidity problem. Regardless, it's still a good idea to incorporate strip heaters on a scope, especially an open truss reflector. They definitely require a shroud where I live.
    Not bad for a beginner at all..................Gunny
     
  6. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    I live in Missouri which can be very humid, I've imaged on nights where the humidity is 100% and water is dripping off of everything except for the camera housing (from the heat it generates) and what is near my dew heaters, but I've never had a filter dew up before. I'm using an ST- 8300 and FW8-8300. I don't think I saw an answer to Doug's question about your scope being open or closed tube. I image with a Mak-Newt, which is a closed tube, although it is vented, but that is the only thing I can see different from my setup to yours. I could see how an open tube scope could possibly allow a filter to dew up.
     
  7. Doug

    Doug Staff Member

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    Open tube would be worse... but I've had the primary mirror on my Mak-Newt fog over before in high humidity conditions. If humidity is in the tube, it can condense on optics if the ambient temperature drops enough.
     
  8. Clayton Bownds

    Clayton Bownds Standard User

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    I apologize for being out of pocket and not responding sooner. I had to spend some time in Toronto on business. Anyhoo, I have unfortunately deleted the worst of the subs that really show the contrast between before and after putting the dew strip on the FW8-8300, but here is an example of what I'm talking about (before on top and after on bottom, but not quite resolved yet): https://www.dropbox.com/s/uhqj4uyo679mkcj/dewfilterquestion.jpg?dl=0 When/if it happens again, I'll get more example photos. These are just cropped subs with nothing more than an autostretch and histogram transform done so I could save them as nonlinear. The before photos I had to show is really a lot better than the ones I deleted - which were really, really bad.

    As for the tube question - I normally shoot with an Orion EON 130mm APO triplet refractor, so closed. The night I shot the Iris Nebula, I swapped out the 130mm refractor for a camera lens because I wanted to try out the CLA-83-CN that I'd bought at the time I purchased the CCD and never got around to it because it requires some dainty unscrewing/screwing that I'd rather not do at night in the field. In that case, I was using a Canon EF 600mm F/4L IS telephoto lens... also closed.

    My normal dew heater set-up is one large strip on the front element dew shield just in front of the lens element plane and another in a similar position on my guide scope. I have never used one on the camera end of the scope because I never thought it necessary. I shot with a DSLR mounted via T-ring to prime focus adapter prior to buying the SBIG CCD and never experienced dew on that end of the scope... even on the most humid of nights. Based on this, I just guessed that it had something to do with the cooling on the camera now that I didn't have before... or that I was using a filter wheel now that I never used before. Since I didn't know what it could be, I figured I'd ask here... I'm sorry... I am really a newbie.

    Thanks!!
    /c
     
  9. Doug

    Doug Staff Member

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    The camera will not cool the filter wheel down. If anything it will warm it up (the body of the camera is warm when operating).

    I'd suggest you store your tube with some desiccant tucked inside the eyepiece cap, and see if that resolves the problem. (Just tape it in place or something to make sure it can't fall into the tube.)
     
  10. Clayton Bownds

    Clayton Bownds Standard User

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    I will give it a shot. Thanks!!

     

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