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I've seen an interesting issue with my first few captures; the dark frames and light frames I took with the camera mounted on the scope featured this odd glow towards the middle/left of the frame. Conversely if I take a dark frame with a cap after the EFW with the camera on my desk, I get a perfect dark frame with the ASI183's usual amp glow. Is this likely to be a light leak, or something else altogether?

Skywatcher 200PDS, ASI183MM-PRO, Mini ZWO EFW, all clamped up in the standard Skywatcher focuser.

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M33_120sec_1x1_Lum_frame8_c_r.jpg

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Edited by discardedastro

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It looks like an internal reflection coming from the bright star that's on the very right-hand edge of the image.

Tracking down where the light is reflecting from could be tricky....it could be the inside of the OTA, the focuser draw-tube or any extension tubes. It could even be reflections between the camera sensor, cover glass or filters.

 

images where you have a bright star in the frame can be tricky.

 

Edited by Zakalwe

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I concur. Filters edges are also a consideration - have been instances of unmounted filters creating reflections if the edges aren't darkened, not sure if it is possible for mounted filters, suppose it depends on the mounting material.

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I should clarify though that the dark frame (second image in the post) was taken _with the telescope cap on_ - the really bright star on the right, do you mean in the last frame? If so, that's a dark frame taken with the lens cap on the EFW, rather than the telescope cap - I repeated dark frame capture with the camera cooled on the bench after I saw the first dark frames and the lighting issue, and the bright star in that frame is just the amp glow the ASI183 tends to suffer from. So I don't think it's a bright star causing issues - my thinking has mostly been local pollution, lights from near the scope etc.

The Baader filters have a black mounting disc and I can't immediately see anyone reporting similar issues with them. I've got some flocking material I haven't yet fitted to the tube, so I might give that a shot - slightly terrified of removing the primary mirror cell to get at it all but hopefully it'll all go back together without too much issue! My suspicion is mostly that there might be light leaking in around the focuser's drawtube (on the outside) that's then reflecting off the secondary.

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The second image is a dark frame with the lens cap on the scope, so it can’t be anything to do with what is being imaged in the light frame eg bright star at the edge of the FOV.

I’ll start the betting and say it’s light coming in from behind the primary, maybe your computer screen or a light. 

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You have the usual ASI183 "starburst" amp-glow on the right hand side.  This will calibrate out.  The big oval shaped bright patch in the upper centre is probably some kind of light leak.  Does it happen if you take darks with the lens cap on in a totally darkened room?  If not then it's definitely a light leak. You will just have to experiment putting a black cloth over possible entry points until you narrow it down.

Mark

 

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Can't really add much to what has been said, except you say you did the dark away from the scope with a cap on the EFW, so it's not light leak from the secondary.

I must admit I do tend to do my darks when it is dark to avoid any chance of light leak.   I used to get light leak with a manual filterwheel, but yours is EFW so should be enclosed and leak proof.   But my primary leaks light so I do them in the dark. 

Carole 

Edited by carastro

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Right - thanks for all the tips. The scope is set up in my garden but still close enough to the house there are some potential sources of light.

I've got the scope off the mount while I fit a focusing motor to the focuser (which has turned out to be Exciting, the Skywatcher focus motor mount does fit, but blocks up the parallelising adjustment screws, so had to expand some holes in that plate with some files and a drill), so I'll go around with a torch on the bench and see where there's potential for leakage.

I'm guessing something as simple as a blackout cloth over the back of the primary would be enough to stop any leakage there, though I might be brave enough to pop the mirror cell off and flock around the interior near the primary.

The focuser's the other potential entry point - I've now applied flocking around that area to try and minimise the impact of any leakage, but might also paint the (non-contact) external surfaces of the drawtube with some matte black enamel I've got lying around to mitigate bounced light there. Everything upstream of the focuser is sealed and I can take perfect dark frames with the lens cap on the EFW in a brightly lit room, so that lot's OK!

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I've flocked the inside of my tube now - took out the primary and very carefully cleaned it while I was at it. Once I had it all back together on the bench I assembled the camera and took some 60 second and 5 minute exposures.

With the scope in a bright room there were similar-looking halos in the frames. Covering the focuser made little difference, but covering the rear of the scope by the primary with a dark coat resulted in perfect frames showing very no sign of any light. So it looks to me like I need to find a way to properly cover the rear of the tube to minimise the risk of light entering the tube and causing these problems again - either I can find an appropriate plate pre-made or get a friend to CNC some sheet aluminium. Thanks all for the help!

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I had the same problem with my 150pds and like you traced it to light leak around the primary.  My solution was to use a bungy loop to hold a piece of black polythene over the back of the scope, this can easily be taken on/off for collimation and star tests etc.

Martin

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I find I can't do darks on the scope at all and that's that. I have to do them with the camera out of the system and the metal screw-on chip cover fitted. And this is on refractors with fully sealed electric filterwheels. How does the light get in? Not a clue - but it does. I compared 30 minute darks done on the scope and off. They were certainly very different from each other.

There do seem to be a lot of threads discussing reflections in CMOS cameras.

Olly

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