Arc of light fixed by Adam Block's EZ Selective Rejection - But why the arc? :-) PixInsight Addicts · Paul H · ... · 46 · 607 · 10

elmirage001 0.90
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We conducted the extended baffle tube experiment last night with interesting results. Black paper strips 2.5 inches in width were made and for the experiment simply secured around the baffle with a rubber band. The paper extended out very close to one inch beyond the end of the baffle tube. Ten one-minute exposures were taken in Sequence Generator Pro just as in a regular imaging session. Auto focusing and plate solving were the same as for the actual session Paul used for M13. The Lum filter was used for the experiment. Paul's scope was used but as noted before Jim experiences nearly identical arcs of light on his subs of M13.

pauls_scope_test.jpg

To our delight the first sub upon downloading showed no arc next to M13. None of the ten test subs showed arcs even using the extreme STF stretch in PixInsight. We then went out to the observatory and removed the paper extension from the baffle tube and shot another 10 subs without the repair. On the very first sub the arc was back. It was important to us that there was no diminishing of the light from the primary mirror or vignette creation by doing the baffle extension. What we discovered doing side by side examination of the subs with was that Paul had been getting subs all along with a lighter ring around the edges and a darker central area. It was faint and had been perfectly corrected by his flats, but was quite noticeable when blinked with subs using the black paper band. The frames with the band were very noticeably smoother and flatter, but not darkened or vignetted at all. The subs with the band were much improved beyond just repairing the phantom arc problem. In his attached photos of the master created from the stack of 10 subs it looks as if there are darkened, perhaps vignetted corners but this is because he has no correct flats to go with the paper band repaired subs. The darkened corners indicate just how much lightness was being corrected before and how much over correction those flats now cause. New flats will be taken of course.

results.jpg

Today more permanent and durable baffle extensions will be made. We were both worried about putting a plastic 3D printed baffle on which might fall down and damage our primary mirror surface. So better black paper baffle extensions will be made for both of our scopes. A double thickness of paper will be used around the outer edge to provide a depth stop when pushed onto the scope's baffle tube and a layer of adhesive-backed velvet around the other edge to provide some holding force to keep the baffle extension in place. We are hopeful the extensions will solve the issue for both scopes though Jim who has actually got two phantom arcs has yet to test it on his identical 10 RC Truss.

baffle_extension.jpg

We'll be testing the new setup on both scopes tonight and we'll update with our results.

We wish to thank all of you who shared input to Paul's thread. And a special shout out of thanks to Rich for sharing his baffle extension solution.

Jim and Paul
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elmirage001 0.90
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Now for the PixInsight side of this post...

This image had the problem arc showing in the L, R, G, and B subs. I was able to remove the arc in the Lum subs using Adam Block's Selective Rejection method as some of the L subs did not have the arc. All of the R, G, and B subs contained the arc so I removed it using the LinearStarNet script and the clone tool.

M13_v1.jpg
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Alan_Brunelle
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Well done!  Being that I wish to own one of these in the near future, thank you for your service!
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elmirage001 0.90
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Alan Brunelle:
Well done!  Being that I wish to own one of these in the near future, thank you for your service!

Hi Alan,  this was definitely an Astrobin community win :-)
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plane 3.10
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I like your black paper extension Paul.  I think I will copy yours for sure.  MUCH better looking.  BTW, I too had multiple arcs in some of my subs so Jim will be pleased that they will be gone too.

Rich
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pfile 1.81
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well that is fantastic. i had plans to extend the primary baffle when i put the RC10 back into service but i should try this first. my flats look exactly like the one shown with the dark spot. do you mind posting a similarly stretched flat with the extension in place once you have them?
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plane 3.10
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·  1 like
I sure will.
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elmirage001 0.90
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Rich Sornborger:
I like your black paper extension Paul.  I think I will copy yours for sure.  MUCH better looking.  BTW, I too had multiple arcs in some of my subs so Jim will be pleased that they will be gone too.

Rich

Hi Rich, that's all Jim's doing on the extension.  He's a mechanical wizard and has made many amazing astro tools and fixes over the years :-)
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elmirage001 0.90
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well that is fantastic. i had plans to extend the primary baffle when i put the RC10 back into service but i should try this first. my flats look exactly like the one shown with the dark spot. do you mind posting a similarly stretched flat with the extension in place once you have them?

I'll be doing new flats soon and I'll post the lum side by side comparison here.  Thanks for the suggestion!
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talbotj 2.41
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Hi Paul,
Great detective work.  I was going to reply with my solution to a similar problem but then read the entire thread and see you found the issue or at least how to get rid of it.  

 Its very similar to a problem I had with my refractor and my guess is it probably has to do with the anodized retaining ring holding your secondary mirror.   I can see the brighter reflection of the retaining ring in your image of the Secondary.  The issue I had was with my reducer which I was using at the time.   The small spacers which hold the reducer lenses separated within the reducer housing were black anodized.  Black anodizing looks very nice but is also very reflective depending upon the incidence of the light hitting it.  My arcs were much more curved than yours and thicker and only happened when I was "somewhat near" a brighter star.  The fix was to take the lenses out of the reducer and carefully put self stick felt on the spacer sides facing the lenses and put it all back together.  The curved rings where totally gone!  And, as a bonus the background of the image was also much darker.   My hunch is if you were able to put some felt on the retaining ring of your secondary mirror it would probably go away.  

In any case,  you have a wonderful solution that doesn't seem to affect anything else in the light cone.
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elmirage001 0.90
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Jon Talbot:
Hi Paul,
Great detective work.  I was going to reply with my solution to a similar problem but then read the entire thread and see you found the issue or at least how to get rid of it.  

 Its very similar to a problem I had with my refractor and my guess is it probably has to do with the anodized retaining ring holding your secondary mirror.   I can see the brighter reflection of the retaining ring in your image of the Secondary.  The issue I had was with my reducer which I was using at the time.   The small spacers which hold the reducer lenses separated within the reducer housing were black anodized.  Black anodizing looks very nice but is also very reflective depending upon the incidence of the light hitting it.  My arcs were much more curved than yours and thicker and only happened when I was "somewhat near" a brighter star.  The fix was to take the lenses out of the reducer and carefully put self stick felt on the spacer sides facing the lenses and put it all back together.  The curved rings where totally gone!  And, as a bonus the background of the image was also much darker.   My hunch is if you were able to put some felt on the retaining ring of your secondary mirror it would probably go away.  

In any case,  you have a wonderful solution that doesn't seem to affect anything else in the light cone.

Hi Jon,

Thank you very much for your addition!  Jim and I did some more testing last night and we'll have an update most likely tomorrow. 

Thanks again! 

Paul
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elmirage001 0.90
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Jon Talbot:
Hi Paul,
Great detective work.  I was going to reply with my solution to a similar problem but then read the entire thread and see you found the issue or at least how to get rid of it.  

 Its very similar to a problem I had with my refractor and my guess is it probably has to do with the anodized retaining ring holding your secondary mirror.   I can see the brighter reflection of the retaining ring in your image of the Secondary.  The issue I had was with my reducer which I was using at the time.   The small spacers which hold the reducer lenses separated within the reducer housing were black anodized.  Black anodizing looks very nice but is also very reflective depending upon the incidence of the light hitting it.  My arcs were much more curved than yours and thicker and only happened when I was "somewhat near" a brighter star.  The fix was to take the lenses out of the reducer and carefully put self stick felt on the spacer sides facing the lenses and put it all back together.  The curved rings where totally gone!  And, as a bonus the background of the image was also much darker.   My hunch is if you were able to put some felt on the retaining ring of your secondary mirror it would probably go away.  

In any case,  you have a wonderful solution that doesn't seem to affect anything else in the light cone.

Hi Jon,

Thank you so much for your information!  Jim and I did some more testing last night and we'll post an update by tomorrow.

Thanks again!

Paul
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elmirage001 0.90
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Hi Jon,

Thank you so much for your input!  Jim and I did more testing last night and we'll post an update by tomorrow.

Thanks again!

Paul
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elmirage001 0.90
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We did the experimenting with the secondary baffle extensions. Paul took three stacks of ten exposures each. We waited until late when the sky was really well darkened. The first stack was with the stock baffle and no extension. The second stack was made using the black paper device at 25 mm of  additional extension. The last stack of ten exposures was made the the paper baffle out only 12.5 mm.

The arc of phantom reflected light was seen on the stock baffle subs and we noted almost immediately that though the subs taken at 25 mm of extension were free of the band of light they were also flatter and smoother, yet darker and M13 was clearly dimmer. We finished the gathering of the data and then examined stacks made in PixInsight using WBPP which confirmed visually that the 25 mm of extension does clip off some light. We could see on the stacked master that the brightest was less than the stock baffle stack. The darkest was the 25mm extension length and the middle 12.5 millimeters of extension was nearly as saturated as the stock baffled stack.

Our theories and conclusions are these. Some of the brightness in the stock mask images may actually be scattered light created by the ground glass rim at the edge of the primary mirror. It has not been mentioned by anyone that they have had such phantom light bands when the scope was used at 2000mm of focal length. But is has been mentioned that they appeared after telecompressing the scope. It maybe that they also occurs at 2000mm. Has anyone seen these bands and false lights in 10 inch RC truss scopes when used without a telecompressor at 2000 mm focal length? I have no experience except using the telecompressor.  We think this ground edge on the primary mirror could be the source of the scattered light which forms the bright donut on the stock baffle images.

The stack using the full 25 mm of extra baffle shows M13 to be dimmer. The core of the globular was noticeably less exposed. The stack of subs taken with the paper baffle extender out just 12.5 millimeters showed an M13 that was nearly identical to the brightness and spread of recorded stars as the stock baffle stack but with no arc of light. We are not sure at what point the light cone will begin to be clipped using the extension. But it seems clear from our work and our set up of the telecompressor that it is being clipped at an extension of 25mm. At an extension of 12.5 mm the amount of light reaching the sensor is essentially the same as with the stock baffle but without the arc of light and with improved overall flatness. I need to say at this point that all of these conclusions are based on our careful examination of image stacks made with the greatest care. We did our best to capture and integrate them identically. Ten subs seemed a reasonable amount to mitigate any issues of seeing in individual subs.

no-25-12-5-extension-test-2.jpg

We would be delighted if others would collect similar experimental data and relate it to us through this thread. There remains a bit of mystery about the origin of the false light arcs. Neither Paul or I are optical designers or persons who understand much about ray tracing in mirror systems employing hyperbolic surfaces. But we can see differences in stack of subs.

Thanks again for all of the contributions that have been made to this thread.

Jim and Paul
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pfile 1.81
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just to clarify - jon mentions above that his problem was the black anodization, which implies that you would need to flock the inside of the baffle, or make the baffle extension start inside the baffle at the secondary and stick out either 12.5mm or 25.mm from the original end of the baffle.

from your photos it looks like the extension is wrapped around the outside of the baffle - is that correct?

rob
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plane 3.10
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Thank you Jim and Paul for your attention to analyzing the data so thoroughly.  I'll be scooting my extension inward 1/2" so as not to clip any more signal than necessary.   Great work guys.
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meteoritehunterjim 0.00
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That is correct Rob the thing that we made is on the outside of the secondary baffle.  It may indeed be as Jon said that one of the sources or even the main source of the reflection is the polished anodized retaining ring holding the secondary mirror. It should be a flat non-reflective black but is not. To repair it would be best accomplished by doing something like Jon suggests. But I am not convinced that I can do it without mucking up the secondary. The ground edge of the primary mirror should also not show glaringly as it does. It is there to prevent chips but should have been blackened with paint. I do not know right now if the edges of the glass element in the CCD67T telecompressor are blackened or not. It is a quality telecompressor that is doing great things for us and many others but right now seems to perhaps bring this problem of light bands to our images with the .75X spacing we are using. I hope that others will pick up this problem and make a final determination as to the source of the light causing the arcs in our system. The baffle extension seems to be fixing it for Paul. I have not really had a chance to fairly test it on my scope. Thanks.
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jerryyyyy 9.03
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Don't recall what this looks like, but on my old Tak everything that might be shiny was painted non-reflective black... can you carefully paint that offending ring?  Of maybe just put some electrical black tape over it to see if that is the problem and it goes away with the tape.
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elmirage001 0.90
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Hello Everyone!  One more test...

It dawned on me that just using a 60 second exposure on M13 may not go deep enough so the image below compares 1 minute, 5 minute and 10 minute subs.  I see no trace of the arc using the 12.5mm extension in any of the subs.  Paul

2-5-extension-1-5-10-minutes.jpg
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crfrancis 0.90
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The ground edge of the primary mirror should also not show glaringly as it does. It is there to prevent chips but should have been blackened with paint.

I'm not familiar with these scopes, but do you have sufficient access to temporarily put an annulus of black paper to hide the edge? It would have to be supported on something above the mirror itself (not in contact with it). If that is possible it could be tested without your baffle extension to see if your idea is right.

cheers,
Richard
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meteoritehunterjim 0.00
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Hi Richard, We looked at that briefly but don't want to touch the primary in any way. It is held in place by substantial clamps and any paper ring  would be well above the mirror. At this point I think we may have solved the false light bands and reduced the scattered light. Hopefully this will make image processing much easier. If we are ever forced to pull the the primary mirror for some reason we would probably carefully blacken out the ground edge. But it is going to have to be a pretty serious reason to mess with the primary.
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talbotj 2.41
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Hi Paul,

Sounds like your 12.mm extension may be the ticket, at least in this field of view.  It may be wise to point near a target that is also near a bright star.  Keep the bright star just out out of the field of view but close and take some 3-5min exposures.   If your arc is not there then you're probably good.  If it is,  it would suggest that the extension may need to be lengthened.  Then I suppose you have a choice of clipping some of the light from the primary or living with it.

The reason I even mentioned the black anodized retaining ring was my experience seeing these types of arcs on my refractors  caused by anodizing  parts and a great presentation at NEAF many years ago by Michael Barber of  SBIG where they highlighted this exact problem.  At the time they were able to show how reflective black anodizing is by showing some great examples of images taken looking through the optics and how to see it in your flats, just like you showed.

  Just look through your focuser with your scope pointing at the bright sky.  You can really tell where parts/pieces of your optics train are really bright and areas very dark.  You should be able to see the brighter edge of your mirror if its causing a problem or the thin ring of the retaining ring if its the cause.  Either way its very enlightening to see how bright some things are.   SBIG's  possible solution was to paint the offending parts/ areas  either ultra flat black or black using a high heat resistant paint (like for grills)  which had non reflective properties in the bandwidths that our sensors are typically sensitive to.   I have tried the Krylon Ultra flat black and it works well but the very thin self stick felt is even better if you can find a way to apply it.  

While I haven't used a RC type reflector, I would think the same issues could be fixed using the same techniques.

Best of luck as you nail down the offender...

Jon
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