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TS GPU Ghosts/Reflections? TS-Optics TSGPU · John Walsh · ... · 9 · 238 · 2

Cassiopeia1 0.00
John Walsh
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Any fellow Newtonian users here..
Has anyone ever had to deal with "ghosting" or ghost reflections from their coma corrector and its interaction with the camera sensor?
I had a lot of blue/red blobs in my M45 image and while testing the other night on IC63, with Navi in the image I had another one of the same nature.
After a bit of head scratching and some helpful insight from an astrophotographer, online, I drew a line between Navi and the blob, and the line passes virtually through the centre of the image(sensor).
I repeated this idea on my M45 unprocessed stack and each blob is a symmetrical transform of a bright star through the centre of the image. Remarkably symmetrical in fact...
So it seems that the bright stars are reflecting from my camera sensor back to the CC and then back again to the sensor.
The coma corrector is a TS optics GPU.
I never had any of these types of reflection prior to using a CC.
Do I just live with this or is it possibly that the DSLR sensor glass sucks?
I can always go back to corrector-less imaging anyway.
The only other thing I can think of is to dither aggressively between every sub and hope that the blobs get rejected.

Equipment:
Skywatcher 150PDS, flocked with primary mirror mask and CNC spider.
Canon 700D modified.

Many thanks in advance.
JohnIC63 Blobs1.jpgM45 Blobs1.jpg
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andreatax 9.89
andrea tasselli
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The DSLR glass sucks. Never had with either of my CCs, one of which is a copy of the TS one.
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Cassiopeia1 0.00
John Walsh
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andrea tasselli:
The DSLR glass sucks. Never had with either of my CCs, one of which is a copy of the TS one.

Well that’s actually good to know Andrea, thank you.  
If I can afford an AstroCam in future I suppose the AR coating would be superior
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MikeHuerto 2.39
Mike Dobres
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I have the same TS-GPU coma corrector.  I have used it on a 14 inch Dobson and a 130 PDS with an astrocam (ZWO-ASI 294MCpro) and have not seen such reflections.  So perhaps its your DSLR.
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Cassiopeia1 0.00
John Walsh
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Mike Dobres:
I have the same TS-GPU coma corrector.  I have used it on a 14 inch Dobson and a 130 PDS with an astrocam (ZWO-ASI 294MCpro) and have not seen such reflections.  So perhaps its your DSLR.

Thanks Mike. Hopefully it is the DSLR so. Another thing to add to the list of reasons why I will eventually need a Astro Camera
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messierman3000 7.22
Oscar
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I haven't seen such issues with my GPU; I always knew my GPU as the best CC in terms of having no reflection issues whatsoever
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Cassiopeia1 0.00
John Walsh
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Oscar:
I haven't seen such issues with my GPU; I always knew my GPU as the best CC in terms of having no reflection issues whatsoever

Thanks Oscar. So either it is my DSLR as others have suggested or a defective unit. Hopefully it’s the camera but I have no way to determine this
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andreatax 9.89
andrea tasselli
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I think the halo surrounding Gamma Cassiopeiae is pretty telling in where the fault is. Sure, not a 100% guarantee, but all the astro-modded DSLRs I have/had some issues like that which disappeared once an astro-cam was used instead.
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Mau_Bard 4.06
Mau_Bard
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It is always difficult to diagnose where these subtle optical reflections originate, and that requires a lot of patience and attempts.
I did rarely experience reflection issues, only with narrowband filters and only in presence of extremely strong stars inside or very close to the framed field, but my reflections had a completely different appearance than yours.

I would dare suggest checking/trying/reasoning around the following ideas:

- is the coma corrector protruding, even just a little, in the main scope tube when in focus?
- is there any not-blackened part in the tube or in the optical train in general?
- is the main mirror ring a precisely metal machined and darkened one or a 3d printed one? You might try and remove it, as last resort, if you do not find any other cause.
- has your camera been modded by swapping the IR filter with an AR glass, or just simply removing the IR filter, and leaving the sensor "naked"? I think I am in the second instance, and never experienced issues with DSLR
- can you try insert a filter between the coma corrector and the camera to see if the situation is changing? By the way, are you already having a filter there? If yes try and remove it.
- do you have a different DSLR camera at hand, even not modded, maybe from a friend, to make a check with a different configuration?

These are just a few ideas. Hope this helps.
Ciao, Mau
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Cassiopeia1 0.00
John Walsh
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It is always difficult to diagnose where these subtle optical reflections originate, and that requires a lot of patience and attempts.
I did rarely experience reflection issues, only with narrowband filters and only in presence of extremely strong stars inside or very close to the framed field, but my reflections had a completely different appearance than yours.

I would dare suggest checking/trying/reasoning around the following ideas:

- is the coma corrector protruding, even just a little, in the main scope tube when in focus?
- is there any not-blackened part in the tube or in the optical train in general?
- is the main mirror ring a precisely metal machined and darkened one or a 3d printed one? You might try and remove it, as last resort, if you do not find any other cause.
- has your camera been modded by swapping the IR filter with an AR glass, or just simply removing the IR filter, and leaving the sensor "naked"? I think I am in the second instance, and never experienced issues with DSLR
- can you try insert a filter between the coma corrector and the camera to see if the situation is changing? By the way, are you already having a filter there? If yes try and remove it.
- do you have a different DSLR camera at hand, even not modded, maybe from a friend, to make a check with a different configuration?

These are just a few ideas. Hope this helps.
Ciao, Mau

Thanks very much Mau!
At the moment I don’t have access to another camera to test but will ask around and hopefully can source one for a short time.
The primary mask is a CNC machined one by backyard universe.
The draw tube protrudes about 5mm or so into the draw tube and the corrector is recessed another few mm from the end of the draw tube.
The secondary mirror sides have not been blackened and on or 2 black screws have not been covered in flocking but everything else is black.
To my eyes it looks like the reflection is coming off the sensor glass but no harm to check those other things.
When the camera was modded, the rear filter was removed while the front filter was left in place. I think there is also a glass protective cover over the sensor itself.
is it the case that the AR coating in Astro and is so much superior that theses issues don’t arise?
many thanks again
John
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