Filter Reflection?

Discussion in 'Image Capture' started by Billrh, Jul 13, 2015.

  1. Billrh

    Billrh Standard User

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    I have been using my new SBIG STF-8300M camera and filter wheel for the last 6 months and I've been having an issue with some sort of reflection that seems to be originating from my filter set. I'm curious if what I'm seeing is normal. The issue is so strong that it makes it difficult to post process the image. I originally thought that I was getting IR reflection within the scope but I've painted part of the interior with flat black paint and covered the rest with black felt with no real difference in the images. The issue is seen in the R, G, B, Ha, and OIII filters. Less so in the CLS, Luminance and empty filter slot. I also shot an image though my telescope with the Orion StarShoot camera and didn't see any issue at all. Nor did I have this issue when using my modified Canon 40D using a CLS filter.

    I'm using a William Optics 90mm telescope with a 0.80x reducer, SBIG STF-8300m with the FW8-8300 filter wheel. The R, G, B, CLS, Lum filters are from Astronomik and the Ha and OIII filters are from Baader.

    When processing the images within Pixinsight it is difficult to remove the reflection. Either the inside circle (reflection?) is still in the image or too much of it is removed (or not enough of the outside area) as shown on the last two images below.

    Needless to say I'm currently not having much luck producing good images yet. Any advice would be greatly appreciated.

    Here are a few images of my issue:

    OIII flat
    OIII_Flat.jpg

    no_filter flat
    No_Filter.jpg

    Raw image of NGC6888 with Blue filter
    NGC6888_unprocessed.jpg

    Same image with Blue_flat subtracted using Pixinsight
    NGC6888_flat_subtracted.jpg
     
  2. Doug

    Doug Staff Member

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    Could be a pupil ghost perhaps - that sort of thing can depend on the exact optical configuration. What telescope are you using?
     
  3. Billrh

    Billrh Standard User

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    It's a William Optics 90mm Megrez doublet and I'm using a 0.80x field reducer.
     
  4. Doug

    Doug Staff Member

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    I was fully expecting you to indicate some kind of refractor, because the image is a big spot rather than a donut. It does sound like a pupil ghost. That's a reflection from a window or even the CCD sensor itself, which is reflected back into the camera by a concave surface not far from the focal plane. It ends up looking like an image of the telescope aperture.

    Try taking out the 0.8x reducer, and see if the problem goes away.
     
  5. Billrh

    Billrh Standard User

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    Bingo. That was it. Here is an OIII Flat image I just took with the 0.8x reducer removed. That's more like it.
    Thank you for helping determine the cause. Hopefully I can figure out a way to eliminate this effect so I can still use the reducer.

    Thanks again.

    OIII_no_reducer_HT.jpg
     
  6. Doug

    Doug Staff Member

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    These can be difficult to resolve without "redesigning" the optics. The reducer probably requires a certain spacing to the camera, which means you can't move it to another position where the unfortunate geometry does not occur. So if the reflection is from the CCD sensor itself you may not be able to fix the problem; the only thing you could do is try a different design of reducer.

    Might be worth a check to see if the filters are involved. Do you see the ghost at all when you use the empty filter slot?

    For what it's worth, you are not alone with this problem. Some major professional observatories have run into this.
     
  7. Billrh

    Billrh Standard User

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    The problem is greatly reduced when no filter is present. But I think the real problem was that I wasn't paying attention to the back-focus spec, which is 56mm +- 4mm for my TeleVue 0.8x reducer/flattener. With my DSLR setup it was at 55mm but with the new SBIG/Filter wheel it's at 40mm. This morning I put on an ad hoc 15mm spacer and shot another flat and the pupil ghost is gone. So I think I can get this working again once I get a permanent 15mm spacer installed. Thanks again for all your help.
     
  8. Doug

    Doug Staff Member

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    Ah, yes, the whole idea is to keep the pupil ghost so out-of-focus that it has negligible impact on the image. Most likely the focal reducer was designed with that in mind. Having the reducer at the wrong focal plane distance is likely the root cause.
     
  9. Jeff Kaufman

    Jeff Kaufman Standard User

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    Hello. I just came across this post.

    I also have a WO Megrez, but the 80mm FD. Also with an 8300M and the filter wheel. I am using a TeleVue 0.8x reducer (TRF-2008).

    I the past, I used to use this setup with a Canon DSLR and my field was rather flat (at least no significant star distortions on the image corners.)
    However, now that I have switched to the SBIG 8300M, the field does not look so flat (or perhaps something else is going on). The stars seem highly elongated at the corners. The CMOS sensor is about the same size between both systems so I don't think that is the root cause.

    The only difference between my old imaging system and the new system is i) using an 8300m vs. a Canon XTI and ii) I now have a Moonlight Focus system.

    I am attaching a picture hoping that someone has some idea what may be causing this issue.

    Note, my spacing from the TeleVue to the sensor is around 55mm, as TeleVue recommends. I also had to place a little spacer between the TeleVue 0.8x and the SBIG filter wheel to accomplish the 55mm spacing.

    Thank you!

    -Jeff
     

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