Wavelength dependant reflection ghosts

Discussion in 'STF Series CCD Cameras' started by Christopher Duffey, Dec 30, 2016.

  1. Christopher Duffey

    Christopher Duffey Standard User

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    All,

    I'm not sure exactly where to post this thread but since it concerns an STF-8300M with a FW8 and an OAG but this seems as good a place as any. I purchased the package as a bundle with a set of Baader 36 mm LRGB and Ha, OIII, and SII filters. I have been happily using this wonderful package with an 8" Ritchey-Chretien equipped with a Teleskop Services Flattener with no complaints. On to the issue, I recently decided to image the Flame Nebula which of course includes the very bright star Alnitak. During processing I noticed a series of regularly spaced ghosts which look like out of focus stars - the brightness of the ghost images is dependent on their closeness to where the bright star is located in the FOV - the ghosts overlap or not depending on what filter I am using, it very much looks like an interference pattern. The effect can only be reproduced on very bright stars like Vega, Capella, etc... I do NOT see this on normal images I shoot. I have relentlessly searched for reflective surfaces in the scope and the rest of the imaging chain and have painted a few of the surfaces which seemed more reflective with flat black chalkboard paint. I have verified and re-collimated the scope and recently took some images with the scope and flattener attached to my Canon T5i which did NOT produce the artifact under the same exposure settings. I have talked to the smart guys at OPT and posted on Cloudy Nights and this seems to be unique except for the case of a completely different camera/scope combination which was traced to dew formation on the camera - I have checked this as well and have reproduced the effect on a very dry nights (southern California) with the cooler set to far above the dew point. I have also, based on Baader info on the Internet, flipped the filters around in the wheel - this had a minor effect - maybe.... but did not even come close to eliminating it. I have also tried removing the flattener and still see the effect. So at this point since the only way to eliminate the issue is to change to a different camera (although it does not have filters or an OAG) I am assuming it is related to some internal camera thing. I am wondering if anyone else has seen anything like this or has a similar configuration and would repeat the test on Capella. I have attached one image that shows the issue (Blue Filter) the least effected filter seems to be the Luminance filter but that could just be the background is not as dark. I am stumped.....
     

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  2. Doug

    Doug Staff Member

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    Wow, that's spectacular. I've never seen a repeating pattern like that before. Looks like light is bouncing back-and-forth in the system somewhere.

    The issue is obviously optical in nature. You have an out-of-focus image of the telescope aperture, so that suggests a pupil ghost. That would readily explain the brightest ghost superimposed on the star itself. Usually that is caused by a curved optical surface near the focal plane. It picks up a reflection off the CCD (or window or filter) and almost-focuses it back on the sensor.

    I understand you removed the flattener to no effect. Do you have a focal reducer in the system? If so it would be a prime candidate. Does the RC have any refractive optics near the focal plane, other than that flattener?

    I realize this might be rather hard to do, but is there a way you could change the distance between the camera and filter wheel?

    Does anything like this happen if you use a blank filter slot, i.e. no filter at all?
     
  3. Christopher Duffey

    Christopher Duffey Standard User

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    There are no other optics (focal reducer) of any type in the path - the setup is the standard SBIG configuration with the FW8 plate attached directly to the camera - I can do the blank position test but conditions here are bad for the near term - it does also happen on at least the Ha filter. I suspect it won't happen with no filter. I'm bummed you have not seen this before, I was hoping...

    I am going to attempt an "artificial star" test and see if I can reproduce in my house
     
    Last edited: Dec 30, 2016
  4. Colin Haig

    Colin Haig Staff Member

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    Christopher - sorry so say it - but that is REALLY COOL. You should send it in to Sky & Telescope or APOD. It's truly bizzarre. Are the Baader's flippable? e.g. you've confirmed shiny side facing OTA?
     
  5. Christopher Duffey

    Christopher Duffey Standard User

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    LOL, Yeah - it would be cool if I had not been troubleshooting this for several weeks - I have flipped the filters so that the shiny side points out and in - the Baader website says that in MOST cases the shiny side out is correct but in some cases shiny side in is appropriate, especially if there is a flattener or a reducer between the filter and the telescope aperture - weird
     
  6. Christopher Duffey

    Christopher Duffey Standard User

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    I was able to put some extensions and get my telescope to focus down a long hallway and into my garage - the reflections appear to go away when switch to the open slot, but I am going to wait until my garage is dark before calling it for sure, the good thing is I have a nice repeatable test setup I can use to troubleshoot without dragging everything outside, I'm actually shocked at how well this works, I always laughed at those artificial star flashlights. This is just an LED flashlight with an eyepiece cap in front of it
     

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  7. Christopher Duffey

    Christopher Duffey Standard User

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    OK definitely goes away with an open filter slot here are Blue and None, so I took the camera apart and verified the filter wheel screws where not very tight on the off chance the filters were "stressed" - all good. After looking at the blue image it looks like two mirrors facing each other with infinite ghost getting dimmer - this MUST be reflections between the CCD and the Filter face - but why doesn't EVERY setup like this do this ???
     

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    Last edited: Dec 30, 2016
  8. Colin Haig

    Colin Haig Staff Member

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    Is the scope f8 or faster ? Is it well collimated? Do you have a nose-piece filter, ahead of the filter wheel? (e.g. some people - usually planetary imagers - put an IR-block filter in there, and that is sometimes part of the problem). Can you adjust the spacing/tilt of the camera to OTA ?
    One thing I'm wondering - what happens if your light source is in the exact optical centre, do you get the circle pattern symmetrically around the point source? In your images, it seems that the star is not reflected in the exact centre of the shadow of the secondary, and am wondering if that has something to do with what is going on.
     
  9. Colin Haig

    Colin Haig Staff Member

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    Also, have you tried an aperture mask (block off part of the input to the OTA) and see if it gives you a hint as to what and how many reflections?
     
  10. Christopher Duffey

    Christopher Duffey Standard User

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    The scope is an f/8 scope - I believe it to be well collimated, stars are round, checked with a Cheshire and a Howie Glatter laser collimator - the point source is never in the center - the relationship of the ghost images to the point source is constant - that is if the point source moves the pattern moves with it - there are no other filters or optics other than the flattener - when removed the weird pattern is still there. I do have a tip-tilt, I can adjust the camera tilt with respect to the OTA.
     
  11. Christopher Duffey

    Christopher Duffey Standard User

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    So two more data points for what it is worth, I can bring the ghost images into focus so I measured the distance that the focuser needed to move from being focused on the star and focused on the ghosts - it is 39 mm. Also verified problem still exists with the SBIG OAG assembly removed
     
    Last edited: Dec 31, 2016
  12. Christopher Duffey

    Christopher Duffey Standard User

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    Have painted a few internal surfaces flat black and was putting the whole thing back together and noticed "infinite" internal reflections in the Baader O-III filter that came with my camera/filter wheel/OAG combo that I was easily able to capture with my iPhone. Does this look normal ? Can anyone take a similar picture and verify that another filter does this or does not ? Now I am wondering if the filters have defective AR coatings, seems unlikely that 7 filters could all be bad but who knows ?
     

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  13. Doug

    Doug Staff Member

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    Hard to say; I've not seen that before but I haven't looked for it. Interference filters don't work properly at a large angle like that.
     
  14. Christopher Duffey

    Christopher Duffey Standard User

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    it looks like that at any angle, if I shine a flashlight right into it, I get the infinite mirror thing going on
     
  15. Doug

    Doug Staff Member

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    Do they actually work as narrowband filters?
     
  16. Christopher Duffey

    Christopher Duffey Standard User

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    yes - the filters all work - I re-collimated my telescope last weekend and ran some test shots - the camera and filters take beautiful images as long as there are no very bright stars in the FOV, attached is an image of the Running Man Nebula I shot 30 minutes RGB (2 minute subs) and 1.5 hours of Luminance (5 minute subs) - also included is a blue filter shot of Capella, a single 2 minute sub - it has been stretched in PixInsight to show the ghosts. I have reached out to OPT to see if they have a different filter I can try to eliminate or finally confirm the issue is the filters themselves - they are Baader 36 mm filters

    Thanks
     

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  17. Doug

    Doug Staff Member

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    Trying a different filter sounds like a good plan. Let us know what you discover.
     
  18. Christopher Duffey

    Christopher Duffey Standard User

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    Update - so Doug, I tried a different filter as well as putting a separate flat filter (a very high quality 2" Astronomik UHC filter) - in the imaging chain and switching the SBIG FW to a no filter position - the effect still happened in all cases, as you would expect an external filter changes the size of the aperture ghosts. At this point I have to ask if it is possible there is something wrong with the glass that covers the CCD. Is it possible that the glass is missing the AR coating ? At the advice of a friend with a PhD in electro-optics as well as experience with multi-spectral camera systems I tilted one of the filters in the FW assembly so that it was not parallel to the imaging chip - this did not fix the problem either. Is there any physical inspection I can do to give you any clues ? Attached is picture taken with an external filter - interestingly if you also switch in one of the filters in the FW with the external filter, the resulting image has a superposition of both reflection sources
     

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  19. Doug

    Doug Staff Member

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    This is baffling. Even if the CCD window wasn't coated it would not lead to this sort of artifact. It's far too close to the image plane to result in a huge reflection like that.

    Are you certain that the flattener isn't involved? It seems like the most likely culprit. I know you said you tried it without, but please try again and make sure it's really doing the same thing.

    Is there any other kind of refractive element inside the telescope? I would expect an RC to only have two mirrors...

    At this point I'm also going to suggest you try the camera on another telescope (borrow a friend's?).
     
  20. Christopher Duffey

    Christopher Duffey Standard User

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    I can try removing the flattener again, and yes there are only two mirrors in my RC :) I can get an adapter and put the camera on my 8" Celestron SC - I did do the opposite and put my Canon on the RC which worked normally. I also checked for frosting on the inside of the CCD window and observed something that could be evidence of the issue - but since my sample size is 1 I cannot say for sure - this is the reflection of the single white LED in my phone - this is exactly the pattern I am seeing, it could be it reflects back at the surface of the filter and is imaged. My Canon T5i has similar reflections but much less amplitude - same camera took these shots at same distance - STF-8300 has brighter returns off its cover glass
     

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    Last edited: Feb 5, 2017

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