Dreadful flat field image quality ST-8XME camera - any suggestions?

Discussion in 'Legacy Models - Community Support' started by Dave Young, Apr 28, 2015.

  1. Dave Young

    Dave Young Standard User

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    I am having great trouble obtaining decent quality images and I was hoping somebody may be able to suggest corrective action. I am a little out of my depth.

    I am operating a SBIG ST-8XME camera which I want to use for photometry. My sixty second exposures of the star fields around my targets are pretty awful, with a lack of contrast, poor FWHM. The camera was attached to a 5" refractor for this flat field. I have taken flat fields with and without filters, the attached fits file and png exported (after applying screen stretch in maxim dl) are from using no filter, but the effects are similar.

    I have been taking repeated flat field exposures during twilight (which passes quite quickly here at 32S) of between five and sixty seconds to try to obtain between 30% and 70% saturation levels of the camera.

    When I apply flat field corrections to my images, they degrade rather than improve. The flat field also appears to contain bizaare artifacts which persist between exposures. I have replaced the dessicant recently and the camera is kept house in an air tight pelican case when not in use.

    If anybody has any suggestions to improve the quality of the flat fields (and hence the subsequent images) they would be warmly received.
     

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  2. Tim Puckett

    Tim Puckett Guest

    Dave,
    at first glance you have moisture in your CCD chamber. You need to bake the desiccant plug and get the moisture out of the camera.
    Check the o-ring seal on the desiccant plug. I think it's damaged.
     
  3. Joseph Zeglinski

    Joseph Zeglinski Standard User

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    Dave,

    About the leaky or flattened O-Ring.

    When I baked the desiccant plugs on my STL and RGH cameras, I decided to not only replace a very old, somewhat stiff and flattened ring, I also bought some MOEN faucet "silicone grease", in the local Home Depot hardware plumbing supplies, and coated the rubber O-ring. It seemed a bit pricey for a pillbox amount, but it provided the simplest solution to possible leaks, as the camera metal changes shape with rapid & severe temperature cycles.

    I think that "just a tiny seed-sized dob" of the silicone grease not only rejuvenates older O-Rings, but keeps the rubber pliable, preventing premature stiffening or flattening, it also gives the plug a "tighter seal" against the CCD chamber wall. This may even lengthen the time periods between re-baking the desiccant, owing to the tighter seal. Likewise, screwing in the hot & dry plug, against a dry chamber wall, might also slightly "warp" the O-Ring circular shape, into an oval or worse - an old ring might even contort itself, outside the plug's underside. Meanwhile, the grease allows the ring to adjust & slide enough to keep its perfect shape, as the plug is tightened down.

    Finally, I also (very lightly) lubricate the plug threads with the same silicone grease, to make the plug screw in more easily. If the "silicone grease" is intended to make a faucet's O-Rings swing more smoothly and help prevent O-Ring warp and high pressure water leaks, it should do the same for the desiccant plug's O-Ring chamber seal.

    Of course, you must always remove the ring before baking the desiccant, and now ALSO give the threads a wipe - since you don't want to ... "carbonize" any remaining grease on the plug threads in the 375 deg-C oven.

    Joe Z.
     
    Last edited: Apr 28, 2015
  4. Dave Young

    Dave Young Standard User

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    Dear Tim,

    Evidence backs you up Tim!! I've taken a couple of flat fields indoors, just placing a couple of sheets of white card over the camera aperture. Cooling the camera to 5C is just fine, but cooling it much past 0C causes the weird image artifacts. The brown o-ring in the desiccant plug cap looks ok, if it was damaged, would that damage be visually obvious? I'll refresh the dessicant. Appreciate your reply.
     
  5. Tim Puckett

    Tim Puckett Guest

    Dave,

    I suggest picking up some new o-rings. Joseph has some good suggestions as well. .
     
  6. Dave Young

    Dave Young Standard User

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    Dear Tim, Joe,

    Thank you both for you input. I'll definitely take up Joe's suggestion of the silicon grease. Although I've never baked the O-Ring, there does appear to be a dark spot on it, as per the slightly out-of-focus attached photo of the desiccant plug head. Tim, are you able to point me in the right direction to source replacement 0-rings please? I've had a look on the sbig accessories site, but have not had any luck. They do sell an entire replacement desiccant plug, which I am prepared to buy if it is what is necessary to get my camera running properly again.
     

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  7. Joseph Zeglinski

    Joseph Zeglinski Standard User

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    Dave,

    That looks like a chip in the O-Ring, though it could be along term compression dent, but certainly not a burn. Maybe it was a defectively molded ring to begin with. I can't imagine how it could have been "compressed" that way from the bottom of the cap upwards. Perhaps you, or a former owner used a screw driver tip to pry it out of its groove, before baking and later inverted it when placing back in. But then again, if so, the rubber would have immediately sprung back into perfect shape.

    Anyway, when I found that my (used) RGH plug was missing its O-ring, I went to Home Depot plumbing section, and bought a small packaged "repair" kit of faucet replacement O-rings, and one of the five or six sizes inside, was perfect for the desiccant plug. That's when I also found the O-Ring silicone grease and perhaps improved the potential leakage problem. So, I used this O-ring rather than losing imaging nights, waiting for an SBIG replacement. I have heard SBIG has been quite good about taping a couple of O-rings to a return letter, free of charge. So typical of this really professional company. Contact them.

    Joe
     

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