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Celestron OAG Curved artifact

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#1 Linwood

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Posted 24 February 2021 - 09:39 PM

I have a celestron OAG.  I love how large it is, lots of stars usually to pick from.

 

I have a problem I cannot seem to solve.  When there is a bright star on the main sensor, I get a weird circular artifact on the OAG frame.  PHD2 star selection identifies parts of it, sometimes, as a star.  if it gets elected as the guide star, either my guiding becomes perfect or more frequently it decides it needs to move a tiny bit - - and a bit more -- and a bit more... never actually seeming to move and it guides way off the target.

 

I can tell you what it is not -- it is not something on the prism, or the sensor of the guide camera. I have taken it apart many times and found nothing.

 

Someone else who had this said moving the stalk further out (away from the frame) solved it -- I tried that today and it actually got worse.  I moved it far inside, just barely outside the full frame sensor.  A bit better but it is what you see below.

 

On many star fields there is no issue.  The artifact does move, depending on where on the make frame the brightest star is.

 

The prism is rectangular, but the stalk looks round.  I suspect the stalk is involved in this, perhaps the stalk/prism interface.  But it looks clean and properly attached.  I cannot figure out what to do to the OAG.

 

I've tried PHD2.  I was lightly chastised in that group for not setting the Minimum HFD, but the HFD of the star-like pieces of the artifact compete with stars.  I cannot find a value that excludes it.

 

I tried the ROI parameter (hidden) in the start-guide API from NINA.  I think it is ignored for multistar though not sure.  But it definitely does not work, it still searches the whole frame.  The regular subframe option is not available with multistar. 

 

I'm about ready to punt and shift to another OAG, though I hate giving up all that real estate (plus I have special precise part plumbing for two OTA's, which I hate to throw away as they are near perfect spacing). 

 

Does anyone have any ideas? 

 

oag.jpg

 

This is what I get centered on the horsehead near rotation zero.  Unfortunately there is often a bright star in many of the more interesting objects.

 

PS. If this belongs in SCT or somewhere else, moderators, I would appreciate it being relocated.


Edited by Linwood, 24 February 2021 - 09:39 PM.


#2 andysea

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Posted 24 February 2021 - 10:15 PM

I see that sometimes  with my Celestron OAG that I only use with my RC scope. It has never been an issue before but I can see how it could be with multi star guiding, which I have not used yet with that OAG.

It's clearly a reflection. I'll study the OAG this evening and see if I can come up with any constructive suggestions. 

 

 

Added question:  what guide camera?


Edited by andysea, 24 February 2021 - 10:15 PM.


#3 Linwood

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Posted 24 February 2021 - 10:20 PM

Guide camera: ASI174MM Mini

 

Multistar is great, it definitely improves my guiding considerably.  But it will jump around (even aside from the fact that I stop and start guiding during autofocus every filter change). 

 

Here's what happens when PHD2 locks onto it: 

 

BadGuide.jpg

 

 



#4 rgsalinger

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Posted 24 February 2021 - 11:31 PM

It may improve your guiding numbers but I'd be curious if it improved your actual images in some measurable (or simply obvious manner).

 

Given that the measurement of RMS error has to be different from that of a single star isn't it likely that it's just another way of expressing the same amount of error? 

 

MaximDL has had this for some years now and I've used it and found that while the numbers got better the images didn't. YMMV, of course.

 

Rgrds-Ross


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#5 Dan Crowson

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Posted 24 February 2021 - 11:41 PM

I get those same curved reflections using my RC with a SBIG bolt-on OAG and ST-i. I can't say Maxim has ever used one for a star but it could be my settings (which might be options in Phd as well). Since the programs usually use the centoid, since it appears to be moving around, I'm surprised Phd would use that 'big' of a star.
 
Dan



#6 Linwood

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Posted 24 February 2021 - 11:42 PM

It may improve your guiding numbers but I'd be curious if it improved your actual images in some measurable (or simply obvious manner).

 

Given that the measurement of RMS error has to be different from that of a single star isn't it likely that it's just another way of expressing the same amount of error? 

I think that's an excellent question and I honestly don't know.  But are you sure that the latter is true? 

 

I'm not exactly sure how multi-star is working, since there is still a primary star.  It COULD still measure the error against that star, and be doing the guiding differently.  It could be measure the error differently.  Not sure.  Did you check?  Ask over in their enclave? 

 

But the total RMS error is definitely better, which has a definite placebo effect.  smile.gif

 

It's similar to changing the scale from 2" to 4", the 4" looks much more soothing. waytogo.gif

 

Horsehead is too low, I switched to M3 hoping the moon is far enough away, and no curved artifact.  Maybe I can go to sleep now.


Edited by Linwood, 24 February 2021 - 11:49 PM.


#7 Linwood

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Posted 24 February 2021 - 11:43 PM

I get those same curved reflections using my RC with a SBIG bolt-on OAG and ST-i. I can't say Maxim has ever used one for a star but it could be my settings (which might be options in Phd as well). Since the programs usually use the centoid, since it appears to be moving around, I'm surprised Phd would use that 'big' of a star.
 
Dan

It doesn't really.  If you look really closely (and I didn't give a high res enough shot probably) it's all broken up, not continuous. Those broken pieces look enough like stars to PHD2 that I can't find a way to tune them out.



#8 andysea

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Posted 25 February 2021 - 01:39 AM

I took a closer look at the OAG. The inside wall of the stalk is pretty shiny and I wouldn't be surprised if that was the cause of the artifact. Tomorrow I am going to paint it flat black.

Why manufacturers don't black out anodized parts that are in the light path is beyond my comprehension. 



#9 Linwood

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Posted 25 February 2021 - 12:00 PM

The metal surrounding my stalk is black.  Could be flatter, but black.  Square blocky thing that slides up and down.  The inside of the stalk -- I can't see how to get to it.

 

Whatever this is has to cause a round-ish result, the only thing I see like that is the glass stalk that attaches to the prism.  I just don't see how to do much with that.



#10 petercoxphoto

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Posted 25 February 2021 - 12:49 PM

I had that issue with my OAG. It was a custom-made adapter ring between my reducer and the OAG that was brass and shiny.

I painted it black and no problem since. I'd say it's your shiny stalk alright.

Cheers,
Peter

#11 Linwood

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Posted 25 February 2021 - 07:39 PM

I need to tear mine apart again.  I stared into it (from the star side) and didn't see anything obvious but there was something that looked bright down inside, probably the guide camera itself.  I didn't want to screw up my focus right then so left it, but I'm wondering if there's something shiny inside the guide camera's nosepiece.  That's for tomorrow.  

 

I would sure hate to give up that great big prism, I have yet to find a spot where I could not get a guide star, never needed to move anything around, etc.



#12 Linwood

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Posted 02 March 2021 - 10:47 PM

So I've torn mine down three separate times trying to find a remedy without any luck.

 

I've moved it further out of frame, further in frame; further in seems less bright but not absent in any motion.  Further in actually is better in terms of PHD2 picking one less often, but worse in that I get two now.

 

I've shifted the camera 90 degrees, the object moves on my screen (i.e. it's not in the camera or anything that turns with the camera). 

 

I just don't see anything inside that is shiny, other than the prism itself.  I think it's the edges of the prism either generally (though why curved) or where the prism is bonded to the glass (?) stalk.  The latter is my guess, and I have no idea what to do about it.

 

Since others have it also getting a replacement (even if Celestron would oblige) is not likely to help.

 

Choosing targets without a bright star just off frame is only a partial solution; there are a lot of bright stars.

 

Staying up all night and fixing PHD2 when it grabs the wrong thing is not high on my list of fun tasks.

 

The PHD2 group had no useful ideas beyond "find where it comes from". 

 

My best idea was using the ROI API but that just doesn't seem to work.

 

I think I need another OAG.  bawling.gif

 

Unless anyone has a better idea.

 

Or an idea for a good large prism OAG. 

 

I may take one more run and literally paint the edges of the prism black.  That's the only thing I can think still to try, and I really hold no hope that will work.



#13 Linwood

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Posted 02 March 2021 - 11:05 PM

PS. I did just open a ticket with Celestron.  I have no experience with them and their response, but hopefully they have some suggestion. 



#14 petercoxphoto

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Posted 03 March 2021 - 07:51 AM

Are you sure there's nothing elsewhere in your image train that's shiny? Something on inside of an extension ring, for example? A brass screwhead?



#15 Linwood

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Posted 03 March 2021 - 09:44 AM

Are you sure there's nothing elsewhere in your image train that's shiny? Something on inside of an extension ring, for example? A brass screwhead?

Nothing in front of the OAG, no.  I have a 3.28" to OAG adapter from Precise Parts.  It's 3.28 inches inside and nice and black inside, but more importantly 3.28" - light basically doesn't touch it.  That goes straight into the OAG. 

 

Now if it's coming from the back side... there's a filter wheel and filters there, but I see it with all the filters the same, and the NB filters are MUCH more reflective than (say) luminance.  I'll give it a try next time out switching back and forth just to see, in fact I can force it to loop fast while switching filters and see if anything moves. I think it's much more likely something is reflecting back from the filter wheel than coming from the OTA side, but I don't see it then showing up on the sensor from the back side of the prism.  but... maybe.



#16 Linwood

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Posted 03 March 2021 - 12:27 PM

I've already had 3 responses from Celestron with questions, so kudos them for responding to tickets quickly and on point.



#17 Linwood

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Posted 11 March 2021 - 10:22 AM

I thought I would update this and also ask anyone who has the problem to open a ticket with Celestron also.  They seem interested, responsive and helpful but at the moment have no actual results.  Refer to ticket CSL-324-95391 as related. 

 

We have together eliminated some things -- it's not anything on the filter wheel or behind.  I did some testing with PHD2 running and moving the filter wheel during exposures - no change.

 

I took some photographs with the OAG partially disassembled, and did find a couple possibilities.  The first is possible but doubtful.  This is looking down from the guide camera view into the stalk, with a bright light in front of the OAG where the OTA would be.  The surprise is the open channels around the stalk.  They are curved, at the sides and in view of my sensor, however they are also looking down at the OAG barrel (black) and seem unlikely to be illuminated enough in use with stars to have an impact.  But I've stuffed them with black foam and will see what happens next time out.

 

OAG Artifacts - A9B00724_111468.jpg

 

The other is looking into the prism from the front with focus on the stalk/prism interface.  You can see first that the coating on the overlap from the prism is flaking (this is only a few months old!) but at least in theory that isn't visible through the round stalk.  But there is a clear bright line on the stalk's edge lit by the flash I was using, which was more or less aligned with the camera.  Can that be somehow seen from above?  Not sure.

 

And there's the hybrid possibility that the flaking black on the square prism is allowing light to escape going upward into the guide camera direction, and going up those channels.  Though that would seem to be a pretty lucky alignment if so.  But possible I guess, after all the prism is designed to send light exactly there.

 

If I can get a clear night I can eliminate the channels from being an issue as they are completely opaque stuffed with black foam.

 

OAG Artifacts - A9B00719_111463.jpg



#18 petercoxphoto

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Posted 11 March 2021 - 11:04 AM

Thanks for posting this followup. As it happens, I saw this again with my OAG a few nights ago, so it wasn't the shiny adapter causing it after all. I'll get onto Celestron and let them know.

 

Cheers,
Peter


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#19 Linwood

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Posted 11 March 2021 - 11:18 PM

I didn't expect to get out tonight but about 10pm the wind died down and I got about 30 minutes to try the target that always shows this (Horsehead @ 2800mm with camera at 0 degrees (or 180 after meridian). 

 

I didn't see the artifact. 

 

I need to do it a couple more times, earlier in the evening when it is higher.  But I'm encouraged, I didn't see any sign of the curved artifact now that I stuffed the channels with black foam.



#20 petercoxphoto

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Posted 12 March 2021 - 05:50 AM

Thanks for the update. Can you share a photograph showing how you stuffed the channels?

 

Cheers,
Peter



#21 Linwood

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Posted 12 March 2021 - 06:35 AM

Thanks for the update. Can you share a photograph showing how you stuffed the channels?

Not effectively.  The shot above shows the holes.  Once stuffed all you can see is a bit of black texture where the wholes were (and right now it's assembled on the mount and I hope to image the next two days without tearing down). 

 

What I did was tear off about a quarter inch cube of pluck foam (it doesn't tear evenly so calling it a cube is a bit of an exaggeration).  I took tweezers and got it positioned over the whole and them with a very small allen wrench I had I just stuffed it in.  (Allen wrench as it was long enough and small enough with a flat head).  I kept stuffing until it was down inside the hole.  The hardest one was the 5th half-hole on the right above as it is a lot smaller.

 

I think you could do it from either end but I used the guide camera side because it was possible to get straight on with it, on the other side the OAG's case is opposite the hole.

 

I suspect it may be possible to remove the stalk and place something inside, then insert the stalk; I didn't try that.

 

I hope to get an earlier start tonight when the Horsehead is high. That's one I have been trying to build time with lately and it has universally given the artifact just past meridian, so if I do not see it tonight I think I'm good.

 

Once I tear down from these nights I'll take another photo but I really do not think it will help, the foam is inside the holes.


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#22 petercoxphoto

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Posted 12 March 2021 - 06:59 AM

Thanks Linwood, that description is very helpful. I'll go ahead and do that with my own and see if it resolves the problem.

 

Cheers,
Peter



#23 petercoxphoto

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Posted 12 March 2021 - 04:54 PM

So I stuffed foam into those semi-circular holes, I'm not sure what purpose they serve. They don't seem necessary for the mechanics of the system to work, and it certainly looks like an invitation to unwanted reflections. 

 

For the record, it was definitely easier to approach them from the prism side of the OAG rather than the guidescope side. And that smaller one was a real pain!

 

It'll be a while before we get a clear night here, so it'll be a while before I can test it. Looking forward to hearing your test results, Linwood.

 

Cheers,
Peter



#24 Linwood

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Posted 12 March 2021 - 04:57 PM

Ventilation?  For dew mitigation?  It was the only thing I could think of.



#25 Linwood

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Posted 12 March 2021 - 08:07 PM

So I'm on a target sequence that has had the artifact everytime before, and nothing.  No sign of it.

 

I'm thinking this was it.

 

Hopefully you get a clear night soon, Peter, to give it a try.


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