Light leak in FW8S-STXL filter wheel

Discussion in 'Filter Wheels' started by Tom Bash, Oct 16, 2020 at 7:38 PM.

  1. Tom Bash

    Tom Bash Standard User

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    Hello,
    I purchased an STXL-6303 and FW8S-STXL filter wheel at the last AIC. I've used it a few times and noticed there was always stray light in the images, which would vary from target to target but almost always would leave a pattern in the lower left corner area of the image. And the pattern would be different for each target. I initially thought it was an internal reflection from a bright star near the field, but targets with this issue are not near a bright star. And my old STL never had this problem using the same OTA (Celestron HD 11). This week I decided to dive into this to see what is happening. As part of my testing I shot a simple 1 frame through each filter LRGB of M33, and the pattern was seen in all filters at the same location on the chip:
    http://www.sbastro.net/Astroimages/M33-LRGB.jpg
    I took the camera assembly off the scope and sealed the entry port of the filter wheel, and took some 5 minute images all binned 2x2 to try and see where the light is getting in. The 5 minute dark frame looked normal, but does appear slightly brighter on the left side:
    http://www.sbastro.net/Astroimages/5_minute_Dark.jpg
    Next I shot a 5 minute light, using Lum filter, with the sealed camera, and got this:
    http://www.sbastro.net/Astroimages/5_minute_light_Lum_filter.jpg
    I took another 5 minute shot through an empty filter slot, and the pattern was much stronger:
    http://www.sbastro.net/Astroimages/5_minute_light_empty_filter.jpg
    Next I remove the filter wheel from the camera and put the red camera top back on, and sealed the entry port, and all the lights I took looked just like darks, there was no evidence of any light leak at all. And I did some test images using just the camera on the scope last night, and none show this issue, they are all clean. So the issue is with the filter wheel. I have checked all the seems and joints and there are no gaps. Any idea what is causing this?
    If you want to see the corresponding FITs files, just edit the links above and change the file type to .fit.
     
  2. Colin Haig

    Colin Haig Staff Member

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    Hi Tom,
    Although we're not back to work till monday, I thought I'd comment.
    Definitely odd, and looks like stray light is getting to the sensor.
    I'm confused why it would be happening with the M33, since I assume it is dark around the camera - eg no observatory lights are on.

    Could you provide:
    Photos of your setp - eg FW mounted to camera
    fw+camera and nosepiece and however it mounts to the scope
    photo of your setup?

    Serial number of camera and filter wheel.
    What nosepiece is in use - can you cap the side of the FW that normally attaches to the scope.

    Can you check whether all screws are in place around the mounting point between the FW and focuser?

    Other things to try:
    Put a black garbage bag around the FW+camera part. leave the nose piece side out of the bag.
    Black electrical tab around the seam between the upper and lower half of the FW.
     
  3. Tom Bash

    Tom Bash Standard User

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    Hi Colin,

    I will try to answer all of your questions. The camera serial number is X18070015, and the filter wheel serial number is FXG191202.
    I use the SBIG STXL 2" nose piece to attach the camera to my focuser, here is a picture of the setup which has the light leak:
    http://www.sbastro.net/Astroimages/With-filterwheel.jpg
    The telescope is not in an observatory but it is in a dark area where the nearest porch light is about 200 feet away and down hill. The yard illuminated by the light is visible from the telescope location, so there is a source of light present while imaging at night.
    When I removed the filter wheel and attached just the camera, this was the setup - which had no light leak:
    http://www.sbastro.net/Astroimages/Without-filterwheel.jpg
    I have used that focuser with my old STL camera on other telescopes with no light leak issues. All the fasteners holding the filter wheel to the camera are there and snugged tight.

    This is how I bench tested for the light leak, I used the telescope side adapter that was supplied with the camera, and when used with just the camera there was no light leak:
    http://www.sbastro.net/Astroimages/Bench-testing.jpg

    I tried both your suggestions with the bag and taping the seam on the filter wheel, and the light leak is still there. With the bag the light was attenuated but still visible:
    http://www.sbastro.net/Astroimages/Bag-test.jpg
    Taping the seam on the filter wheel had no effect on the light leak.

    Another experiment I tried was turning the camera upside down (no bag), and that did make a significant difference in the strength of the light leak. The next two images taken with the camera are scaled the same, they are 5 minute lights through an empty filter slot. This is the light leak with the filter wheel on top, as illustrated in the bench testing photo:
    http://www.sbastro.net/Astroimages/testing.wheelup.jpg
    The light leak became much stronger when I flip the camera upside down:
    http://www.sbastro.net/Astroimages/testing.wheeldown.jpg

    I think the source of the light leak is at a seam along the spacer that is used to attach the filter wheel to the camera. When I flipped the camera that spacer is exposed to more light, as it is in the shadow of the filter wheel when the filter wheel is on top. I did tape the seam between the spacer and the camera body and that didn't appear to improve the leak. I can't see any obvious gap on either the camera or filter wheel side, but it's the only thing that makes sense to me.
     
  4. Doug

    Doug Staff Member

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    How about the nosepiece adapter at the front of the wheel? Our adapters have a groove and O-ring on the back side. The original adapter plate that came with the camera should have come with an O-ring underneath it. Did you install that O-ring behind the 2" STXL adapter?
     
  5. Colin Haig

    Colin Haig Staff Member

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    With the bag test, did you put EVERYTHING in the bag? And is that bag actually black (no light gets through)? The pelican casewithout the foam might be a better "dark room".


    What's the "spacer" you are referring to? Maybe a photo.
    It seems like we still don't know where the light is getting in.
    Another test idea:
    run a flashlight around the outside, shoot short exposures continuously, go around it with the flashlight.

    If you power everything off, and disconnect the two cables between the filter wheel camera, do you get anything? Am justr trying to eliminate internal sources.
    I'll probably need my colleague @Doug to chime in.
     
  6. Tom Bash

    Tom Bash Standard User

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    Hi Doug,

    The O-ring is in the nose piece adapter. When I used the nose piece adapter with the O-ring on just the camera without the filter wheel there are no light leaks.

    The spacer is what I am calling the red rectangular adapter that connects the filter wheel to the camera body. It replaces the front red cover of the camera when installing the filter wheel.

    For the bag test I put the entire camera in the bag since I couldn't easily tape it around the nose piece adapter, and the bag is not perfectly opaque. So my results are not a surprise. I will do the test with the filter wheel attached and the two cords disconnected and let you know what happens.
     
  7. Doug

    Doug Staff Member

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    We've not had any issues with light leaks with the red "adapter". It nests inside the black pieces at top and bottom. From your picture they look to be installed correctly, but maybe have a closer look.
     
  8. Tom Bash

    Tom Bash Standard User

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    Hi Doug,

    The testing I did yesterday confirmed the light leak is not related to the red adapter spacer. I also powered off the camera and removed the two cables so that there was no power supplied to the filter wheel and guide camera, and the light was still there. Another test I did was put electrical tape over all the holes where there were screws on the filter wheel housing, and that had no effect on the light leak either. The final test I did was to try and tape around the various knobs and connection ports on the filter wheel housing:
    http://www.sbastro.net/Astroimages/covered.ports.jpg

    That made a big difference. The two images below were scaled the same in processing.

    The light leak with the electrical tape on the knobs on the filter wheel housing:
    http://www.sbastro.net/Astroimages/filter.knobs.taped.jpg
    Then after I removed the tape from the knobs:
    http://www.sbastro.net/Astroimages/filter.knobs.tape.removed.jpg

    Considering how difficult it was to ensure the electrical tape prevented any light from getting in, I think we have our answer.
     
  9. Doug

    Doug Staff Member

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    Interesting... no one has ever reported a problem with this before. You must have significantly bright stray light for it to work its way in like that. Do you have an outdoor security camera nearby? The ones with "night vision" have built-in IR illumination LEDs that spray out a ton of light that is invisible to the human eye. Also black anodizing isn't very effective at absorbing IR light, so it can bounce around inside the equipment more than visible light. (The innards of your camera, filter wheel, most telescope metal parts, and all your adapters will be anodized black.)

    It definitely will NOT be due to the lock knob on the side. If you pop the lid off and look at it you'll see why.

    As for the focus and mirror position knobs, it would be difficult for light to get in if they're installed properly. On the inside there is a washer and a lock nut, which should be snugged up to minimize backlash when the knob is turned. Basically we snug them up until it's hard to turn the knob, and then back them off slightly. I think it would be very hard for light to work its way through that.

    The filter wheel control cable should be pretty snug as that entry point acts as the cable strain relief; it should be hard for light to get in there.

    The last thing is the opening for the HDMI cable. I'm willing to wager this is the source of your light leak. There is necessarily a small gap around the periphery of the connector. Although there's no direct path from there to the CCD sensor, it's certainly a place where light can get into the wheel. I'd concentrate on sealing that up. The most effective thing would be some black foam around the connector on the circuit board, top and bottom.

    And if you have a night vision security camera in the area... cover it or turn it off while observing!
     
  10. Tom Bash

    Tom Bash Standard User

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    Hi Doug,

    I do not have a security light at my observing site. There is a low level of ambient light from a security light lit up yard about 200 feet away, other than that the only lights are the various power supplies and PC illumination by the telescope. It's actually pretty dark at the telescope, which is why I was surprised to see these light artifacts in my images, and initially suspected internal reflections of starlight.

    I will take the filter wheel cover off and see what I can see, since if I recall the focus and mirror lock knobs are pretty loose on my unit. And I will really tape up the HDMI port to see if that will reduce the light leak further. In the test I did yesterday there was only 1 piece of tape over the HDMI port.
     
  11. Doug

    Doug Staff Member

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    It's weird because when we test these filter wheels in the shop we do it with the overhead lights on. You'd link we would see similar problems, but we don't.

    If the knobs are loose it's still a low risk, I think, as the light has to make two 90 degree turns to work it's way through. I'd still recommend tightening up the locknut, if nothing else than to minimize backlash. As I said it should be snugged up until the knob get a little hard to turn, and then backed away just a hair so it turns freely.
     
  12. Tom Bash

    Tom Bash Standard User

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    Hi Doug,

    The knobs were loose and I snugged them up as you instructed. And sure enough there is a pretty substantial light leak around the HDMI port, it looks to me like the cut out for the HDMI port is bigger than it needs to be:
    http://www.sbastro.net/Astroimages/HDMI.light.leak.jpg
    Even with the HDMI cable attached there is light leaking along the side:
    http://www.sbastro.net/Astroimages/HDMI.cable.light.leak.jpg

    The next test I did was to tape up the outside of the HDMI port, and I used a lot of tape to make sure this light leak was stopped, then take an image:
    http://www.sbastro.net/Astroimages/Taped.HDMI.jpg
    Unfortunately the light leak pattern is still there:
    http://www.sbastro.net/Astroimages/taped.hdmi.image.jpg
    As a reality check I next ran electrical tape along the joint between the two haves of the filter wheel, and the light leak is the same (this was scaled the same as the previous image):
    http://www.sbastro.net/Astroimages/taped.hdmi.and.seam.jpg

    I'm running out of electrical tape with these tests, and I'm not sure what to try next.
     
  13. Colin Haig

    Colin Haig Staff Member

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  14. Tom Bash

    Tom Bash Standard User

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    I'm thinking this might be better:
    https://www.autozone.com/sealants-g...Vkxx9Ch3Oug4bEAQYAyABEgIVKPD_BwE&gclsrc=aw.ds

    My question now is do all of these filter wheels have this kind of light leak, or is there something unique about mine? As it is, it is not really useful for long exposure deep sky imaging. From the testing I have done sealing around the HDMI port will reduce the intensity of the light leak, but there is still some light hitting the sensor from somewhere.
     
  15. Doug

    Doug Staff Member

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    That is not normal! We've sold a lot of these systems and no one else is reporting this problem.

    The intensity of the "leak" suggests it's something really very bright, which you are unlikely to encounter in typical observing conditions. It's supposed to be dark outside. It doesn't make sense.

    FYI if you are testing for light leaks in full daylight conditions, that is not going to be representative of what happens at the telescope. The camera is incredibly sensitive.

    Taping the seam will not do anything. Light would have to be able to navigate two 90 degree bends, with the two halves in close contact. I find it hard to believe light would be leaking around the knobs. You've covered the HDMI port. There really isn't anything else we can do to the wheel. And did I mention that this light spray is really bright?

    I'm wondering if we're barking up the wrong tree here.

    Let's try something very methodical:
    • Tape up the HDMI port, and whatever else you think might be needed
    • Put an opaque cap on the front of the filter wheel.
    • Take it out to your observing location and hold the camera in a typical observing position but not attached to the telescope
    • Take a typical light frame exposure
    • Do you see the light leak?
      • If not, then we're going to have to suspect something is somehow going on with the telescope or focuser
      • If you still see it, put the camera in a cardboard box and close the lid. Do you still see the light leak?
        • If so, it's not a light leak - it's something else
        • If not, we're really missing something obvious.
     

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