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Internal Reflections?

Calibration Vignetting
Fast Newtonians •15 replies•201 views
Jack Groves avatar
Jack Groves avatar
Looking for some feedback on what could be the cause of issues seen in my calibrated master light frames.

Scope is a 800m f4.5 ONTC with a 2600mm and TSGPU coma corrector.

Flats are taken with a Primalucelab Giotto and N.I.N.A's Flat Wizard. I have taken flats of various exposure times, ADU values etc and still having the same results. Gain/Offset values are the same as the lights. Flats are calibrated with matching flat darks.

The flats seem to be overcorrecting the vignetting but are removing the dust. I have generally found lower ADUs to work better in the past, but even with an ADU as low as 600 they still seem to overcorrect.

My opinion is that the is issue is due to some internal reflections, however I am using a cover to block the rear of the tube, there is no direct light facing the tube, and the light frames were all taken during new moon. I can only think that the issue is to do with the coma corrector being slightly too close to the main tube itself.

I've attached a master light as well as master flat for reference.
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Tom Marsala avatar
Tom Marsala avatar
Hi Jack-
I'm no expert, and though I have never seen this in my Newtonian,   I have experienced similar problems with using a coma corrector in my C 9.25.  It does look like a ringing artifact from the corrector.  Very weird about the overcorrection in the vingnetting. It sounds like you have reworked many options for your flats so I would say that isn't the problem.  That is a beautiful sub, by the way! (sorry-not much help!)
Tom
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Dale Penkala avatar
Dale Penkala avatar
Hello Jack,

Can I ask what software are you using to process your data? I haven’t experienced this in any of my Newtonian’s in the past with APP but have seen something like or similar to this in PI’s WBPP. If memory serves me correctly there are some different algorithms in WBPP that can help with this situation. Someone that is more verse in WBPP might be able to chime in more with this for you that is if you’re using PI’s WBPP.
Jack Groves avatar
Jack Groves avatar
Dale Penkala:
Hello Jack,

Can I ask what software are you using to process your data? I haven’t experienced this in any of my Newtonian’s in the past with APP but have seen something like or similar to this in PI’s WBPP. If memory serves me correctly there are some different algorithms in WBPP that can help with this situation. Someone that is more verse in WBPP might be able to chime in more with this for you that is if you’re using PI’s WBPP.

Hi Dale, I used WBPP. I did run a quick stack through APP and it was slightly better, but definitely still visible.
Dale Penkala avatar
Dale Penkala avatar
Jack Groves:
Dale Penkala:
Hello Jack,

Can I ask what software are you using to process your data? I haven’t experienced this in any of my Newtonian’s in the past with APP but have seen something like or similar to this in PI’s WBPP. If memory serves me correctly there are some different algorithms in WBPP that can help with this situation. Someone that is more verse in WBPP might be able to chime in more with this for you that is if you’re using PI’s WBPP.

Hi Dale, I used WBPP. I did run a quick stack through APP and it was slightly better, but definitely still visible.

Hello Jack,
Have you tried rerunning WBPP with some of the other algorithms? Boy I can’t remember what thread I read this on but (I think it was here on AB) I’m pretty sure it was in the processing algorithm that was changed. You might want to try rerunning it with different settings and see if it will change the result. 

I’m sure someone that is more familiar with WBPP than I am would be able to add more insight then I am.

Dale
Wes Smith avatar
Wes Smith avatar
A friend was taking the Leo trip last weekend with a 120mm skywatcher refractor with 0.7 reducer flattener and had the identical centred round  artifact. We have no explanation so i would say it is not a “ dob prob” . …
Jacob Heppell avatar
Jacob Heppell avatar
I find if I calibrate with only flats and no dark/bias, then I see overcorrecting.
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Jeff Coldrey avatar
Jeff Coldrey avatar
Hi Jack. My initial thought is that it would be good to test without the corrector, to rule that out of the diagnosis. I would also try calibration without the use of BIAS frames in the calibration (of master-flats, or lights), assuming that your dark durations already match your light durations and do not require "scaling". Hope that helps, and good luck!
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Jack Groves avatar
Jack Groves avatar
Jeff Coldrey:
Hi Jack. My initial thought is that it would be good to test with the corrector, to rule that out of the diagnosis. I would also try calibration with the use of BIAS frames the calibration (of master-flats, or lights), assuming that your dark durations already match your light durations and do not require "scaling". Hope that helps, and good luck!

Hi Jeff - just to clarify, do you mean to test without the corrector?

I can certainly try calibrating with bias frames, but I have always been able to use flat darks with this setup and it has worked just fine.
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Capturing Ancient Photons avatar
Capturing Ancient Photons avatar
I flocked the upper part of my Skywatcher 10" Quattro 250P and that seemed to solve my internal reflection problem. On moonlit nights I add on a dew shield for additional protection.
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Rob Johnson avatar
Rob Johnson avatar
I had a lot of trouble with a very similar problem, I posted at the end of the thread what the solution was for me - a very slightly loose focuser tube.

https://www.astrobin.com/forum/c/astrophotography/equipment/achieving-precise-newtonian-collimation-for-flats/?page=3


Hope this helps, Rob
Jeff Coldrey avatar
Jeff Coldrey avatar
Jack Groves:
Jeff Coldrey:
Hi Jack. My initial thought is that it would be good to test with the corrector, to rule that out of the diagnosis. I would also try calibration with the use of BIAS frames the calibration (of master-flats, or lights), assuming that your dark durations already match your light durations and do not require "scaling". Hope that helps, and good luck!

Hi Jeff - just to clarify, do you mean to test without the corrector?

I can certainly try calibrating with bias frames, but I have always been able to use flat darks with this setup and it has worked just fine.

Hi Jack. Yes, apologies for my fat-fingers when messaging from my iPad.
Have corrected by post above.

Testing "without" a corrector may be worth a try (with new flats).
I doubt it's the cause, but would be good to rule it out.

I also now get better flat calibration "without" the use of BIAS frames, albeit with my Esprit 120.
But the overcorrection in my case was symmetrical, so unlikely to be your issue.

A final long shot is to ensure you are not getting "frosting" on part of your sensor window (even with the 2600mm anti-dew enabled).
This has impacted me sometimes in the past with some ZWO cameras. If the problem goes away at say 10C, then that's your problem.

I think you're on the right track with stray light reflections.
Good luck. Keen to hear how you go.
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Jack Groves avatar
Jack Groves avatar
Rob Johnson:
I had a lot of trouble with a very similar problem, I posted at the end of the thread what the solution was for me - a very slightly loose focuser tube.

https://www.astrobin.com/forum/c/astrophotography/equipment/achieving-precise-newtonian-collimation-for-flats/?page=3


Hope this helps, Rob

Hi Rob - thanks for sharing this thread and your solution! I will need to do some testing but the focuser could definitely be a cause of my issues too. The issue was fine a few months ago until I installed a new focus motor for my Feathertouch. The Feathertouch is solid but unfortunately uses a compression ring which I have always worried would cause these types of issues if not adjusted correctly. 

I saw in your thread many people saying they’ve had similar experiences with odd reflection when using the TSGPU too. It is a good corrector but I am debating trying the Paracorr or a TS Maxfield.

I’ll report back soon once I’ve had a look at my focuser!
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Jack Groves avatar
Jack Groves avatar
For anyone curious, here is an uncalibrated master light frame. It's heavily stretched to show the issue. I do see some reflections, and I assume the darker patch in the middle is the shadow from the secondary.

Jack Groves avatar
Jack Groves avatar


Took some time to inspect my equipment this evening and try to diagnose potential areas for a light leak or reflection.

I’d previously had issues with the TSGPU being too close to the main tube so I moved the primary further up the tube and was able to push the TSGPU further out. It is a good 1.5” out of the tube at focus now, it could still be a culprit given the very notorious reflections it is prone to. 

One thing I had never noticed was the awful machining for the base of the focuser to attach to the tube itself. I was checking to ensure that it was level and there was no gaps and in doing so was able to see the whole way through to the other side! I would never have thought of this but 100% it could be one of the causes. I’ve used some electrical tape tonight to seal the gaps and hope this will help.
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Frank Szabo avatar
Frank Szabo avatar
Well, from your first post, I couldn't help myself not to grab your .jpeg image and see if I could salvage it.  Here is how GraXpert fixed it. I also ran a mild denoise on it. 

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