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10" iDK Star Flaring Issue

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#1 rockstarbill

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Posted 08 April 2021 - 12:32 AM

Hey folks,

Thought I would come get some more opinions of what I am seeing in my data. For some background, I am working on getting my 10" AGO iDK collimated and ready to do some Galaxy Season imaging, starting this next New Moon cycle. The link below is to a stack of 59 x 120 second Lum frames, taken with an Astrodon L filter and a FLI ML16200 camera on the 10" iDK:

https://1drv.ms/u/s!...3KRWgA?e=eAo02O

The strange behavior I am seeing is that there is a flare on the brighter stars that is opposite of their position against the center axis of the frame. If anyone has experienced this or may know what I can look at on the scope to eliminate this, I would appreciate knowing. Lighter stretches reveal the stars are in excellent shape throughout the frame, but this flare is very troublesome and too visible to just leave be.

Edited by rockstarbill, 08 April 2021 - 11:59 AM.


#2 Tapio

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Posted 08 April 2021 - 04:05 AM

Hm, don't see the problem.



#3 aaube

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Posted 08 April 2021 - 05:04 AM

Tangential astigmatism ?  It looks a bit like it...

https://www.handprin...tml#astigmatism

 

Alain

 


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#4 lucam

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Posted 08 April 2021 - 07:29 AM

Bill,

 

Have you imaged with this scope before? Is the flare new or was it already there? I see what you mean in the bright stars. Just looking at the frame it appears there may be two effects compounding. The stars at the edge of the field of view appear to be just a hint more elongated than in the center, perhaps field curvature that can be further tuned with spacing of the corrector or secondary mirror. I think that may be the reason why see an asymmetric flare. If you look at the bright star just below M101, the flare looks almost perfectly symmetrical.

 

As for the flare itself. I was asking if you had noticed it before because it could be caused by atmospheric conditions (if it's new) or the mirror (if it was there before). You can try masking the edge of the mirror and see if it makes a difference. How is the mirror supported? By the center only or does it have clips?

 

Regardless, the data looks excellent!

 

Luca


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#5 rockstarbill

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Posted 08 April 2021 - 07:43 AM

This isn't the first time I've used it but older data shows the same behavior. The spacing from the chip to the corrector isn't likely the problem as the corrector is fixed in place and the chip moves independently as the focuser tube moves.

I don't believe there are any clips involved in the cell design. I've contacted AGO to get their thoughts. The data was quite good, if I can get rid of the flare this is ready for prime time.

#6 rockstarbill

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Posted 08 April 2021 - 10:05 AM

Hm, don't see the problem.


If you look at the bright off axis stars you'll see a flaring effect pointing away from the center of the frame. You'll also see some minor star elongation in the corners. My current suspect is mirror spacing error, but the manufacturer knows this better than I, so we'll see what Dave thinks. Last hurdle before this puppy is ready for some galaxy work.

#7 buckeyestargazer

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Posted 08 April 2021 - 10:07 AM

That is a strange one for this scope.  If it were a refractor I would say the backfocus between the corrector and image plane was off, but that obviously isn't the case here.  So that makes me wonder if the distance between the secondary mirror and corrector needs to be slightly increased or decreased.  IDK (I don't know LOL).  I'll be interested to hear what AGO says.  I don't think I see this with my 12.5" but I'm going to be on the lookout now.  


Edited by buckeyestargazer, 08 April 2021 - 10:08 AM.

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#8 rockstarbill

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Posted 08 April 2021 - 10:16 AM

That is a strange one for this scope. If it were a refractor I would say the backfocus between the corrector and image plane was off, but that obviously isn't the case here. So that makes me wonder if the distance between the secondary mirror and corrector needs to be slightly increased or decreased. IDK (I don't know LOL). I'll be interested to hear what AGO says. I don't think I see this with my 12.5" but I'm going to be on the lookout now.


The spacing is my suspicion. The scope plate solves at 1720mm, spec is 1674mm, and other users I've talked to in the past say 1702mm.

My 12.5" iDK is next up for collimation work and preparation for duty. I can say I've never seen other iDKs displaying this issue so you're likely fine.

#9 Peter in Reno

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Posted 08 April 2021 - 10:21 AM

I'll be very happy to accept as it is. I am not surprised the plate solve resulted a little different than the spec probably because it's really difficult to precisely grind huge mirrors and have 100% success rate for all mirrors to result exactly the same.

 

What I'm seeing is the upper half appears to be darker than lower half as if it was gradients at bottom half unless the bottom half may be IFN.

 

Peter


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#10 rockstarbill

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Posted 08 April 2021 - 10:25 AM

No flats applied to this as it was just a set of testing frames. The data looks good and if I can get the flare to go away even better. As is, I'll be collecting data with it at new moon.

#11 Peter in Reno

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Posted 08 April 2021 - 10:55 AM

Good point about flats. I supposed flats should be able to correct optical imperfections including your image.

 

Peter



#12 rockstarbill

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Posted 08 April 2021 - 11:02 AM

Yes, flats will not correct the flare though. I have older data that is flat corrected and shows the same issue.

#13 Peter in Reno

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Posted 08 April 2021 - 11:08 AM

Yes, I realized that.

 

Are the flares more pronounced with stacked image than single RAW sub?

 

Peter



#14 rockstarbill

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Posted 08 April 2021 - 11:12 AM

Yes it's much easier to see in a stack. If I put a bigger sensor on the scope I suspect I'd see it progressively get worse the further off axis the bright stars get. That's speculation though, I have not actually tested that yet.

#15 Eric Benson

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Posted 09 April 2021 - 07:29 AM

Hi Bill,

 

The flares around bright stars are common for CATs due to the tight baffling required for the two mirror system. I think the uneven flares come from non-concentric baffling - i.e. misaligned optical path wrt to the mechanical baffles. Note if baffles were 'perfect' you would not see any flares, the diffracted light would be spread evenly throughout the field. But unavoidable micro-ripples in the metal baffle edges are like hundreds of really short spider vanes at all angles, and we get flares.

 

Since you have some strange star shapes in the periphery I suspect collimation and/or primary-secondary spacing is off too, maybe the two things are related.

 

What does an out of focus star look like, is it cutoff near one side?

 

HTH,

EB


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#16 rockstarbill

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Posted 09 April 2021 - 08:14 AM

Out of focus stars look nice round and concentric. The mirror spacing is my suspect. The to specifications focal length of the telescope is 1674mm and my scope plate solves to 1720mm which is a substantial difference. I've reached out to Dave from AG Optical to get his thoughts.

#17 sarmen2

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Posted 09 April 2021 - 10:14 AM

Hello Bill,

 

Thanks for posting this. I also image with an idk10 and have not noticed flaring as in the example. I will look closer at some combined and single frames of the past two years. The idea about tangential astigmatism is interesting. If present it may be detectable. Maybe next time out, test with a wide star field that nearly fills the frame and dial in fine focus manually on frame center and corners and see if there is any detectable difference in focus point. Maybe with guiding and 30 sec, or 60 sec exposure that produce the intensity peaks similar to your combined image with the same filter. Have to say, overall the star shapes are wonderfully round though in that combined frame.


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#18 rockstarbill

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Posted 09 April 2021 - 10:18 AM

Hello Bill,

Thanks for posting this. I also image with an idk10 and have not noticed flaring as in the example. I will look closer at some combined and single frames of the past two years. The idea about tangential astigmatism is interesting. If present it may be detectable. Maybe next time out, test with a wide star field that nearly fills the frame and dial in fine focus manually on frame center and corners and see if there is any detectable difference in focus point. Maybe with guiding and 30 sec, or 60 sec exposure that produce the intensity peaks similar to your combined image with the same filter. Have to say, overall the star shapes are wonderfully round though in that combined frame.


Yeah good rich star fields are not very common this time of year.

If you can, plate solve an image of yours and let me know what the focal length comes out at. I'd be curious to compare that, especially if you are not seeing any bright star flaring.

Seeing was very good on the night I took the test frames and finished up collimation work.

#19 sarmen2

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Posted 09 April 2021 - 12:32 PM

You can use the same m101 field with same exposure times 120s and see if the flaring is detectable in a single frame. There are a few open clusters in Auriga (fairly well placed now) that might work. Then try the fine focus test.

 

After plate solving an idk10 image and using reported image scale, the focal length comes out to 1672mm.

 

That's computed the focal length given plate solved image scale and known pixel size.

 

Which plate solver did you use that reported focal length? I'd like to give that a try. 

 

I don't know if using a Ronchi eyepiece can be a reliable method to tune the primary to secondary distance. We know AG optical instructs to only adjust 2 of the 3 collimation screws on the primary in order to not change the P to S distance tuned at the factory. Maybe this distance is very slightly off.



#20 rockstarbill

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Posted 09 April 2021 - 12:40 PM

You can use the same m101 field with same exposure times 120s and see if the flaring is detectable in a single frame. There are a few open clusters in Auriga (fairly well placed now) that might work. Then try the fine focus test.

 

After plate solving an idk10 image and using reported image scale, the focal length comes out to 1672mm.

 

That's computed the focal length given plate solved image scale and known pixel size.

 

Which plate solver did you use that reported focal length? I'd like to give that a try. 

 

I don't know if using a Ronchi eyepiece can be a reliable method to tune the primary to secondary distance. We know AG optical instructs to only adjust 2 of the 3 collimation screws on the primary in order to not change the P to S distance tuned at the factory. Maybe this distance is very slightly off.

The flares are detectable in a single frame. I have 59 of them I can upload if anyone is interested in looking at them. Auriga is in a tree for me, so unless its in the north, high east, or extremely high in the sky toward zenith, it will be off-limits to me. 

 

All of the folks I have asked about plate solved FL report very close to the spec for their scope. The spec is 1674, so you are well within that and you are not seeing the flaring problem I am. I used PixInsight's ImageSolver script to solve the image.

 

Let me ask you another question, since you own a 10" can you see the center white ring and spider set in the below image? The very inner one, not the larger two. Even with the secondary baffle removed on my scope, I cannot see that.

 

iDK_125_Rings.JPG

 

Lastly, my version of the iDK does not have a black screw on either mirror, and the version of the manual I have does not talk about leaving screws alone. The older ones do, but my version does not.



#21 sarmen2

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Posted 09 April 2021 - 02:17 PM

I'll take a look, have to get the idk10 out. I'll also try PI plate solve to see what it comes up with.



#22 rockstarbill

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Posted 09 April 2021 - 02:27 PM

Dave got back to me. He's going to model that 3% variance detected in plate solves on Monday to see if it would cause optical performance issues. His past modeling he could recall wasn't with the 10" specifically.

A few other good points. The inner ring isn't visible on the 10" at all. The black screw is no longer put on the scopes as it proved to be a non issue.

Mirror spacing error according to Dave would show in corner stars being out of round. This is only a 35mm diagonal chip, so I suspect going further out would show a pattern of worsening performance.

I do have a 16803 I can put on the scope and that has a big 52mm chip. While the specifications on the scope would dictate some performance issues (52mm chip on 50mm corrected field) it may be useful to show the field quality overall. I'm mulling this over vs just completing my m101 project as is and worrying about further testing later.

#23 sarmen2

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Posted 09 April 2021 - 03:49 PM

Looking at this relative to sensor size seems relevant. My claim about not experiencing this is with micro 4/3 sensor, not the larger aps-h of the 16200, or even larger 16803. Good idea!



#24 rockstarbill

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Posted 09 April 2021 - 04:15 PM

Looking at this relative to sensor size seems relevant. My claim about not experiencing this is with micro 4/3 sensor, not the larger aps-h of the 16200, or even larger 16803. Good idea!

This would not be visible on 4/3 and would barely be visible on APS-C. 



#25 rockstarbill

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Posted 12 April 2021 - 08:03 PM

Dave got back to me and I am going to adjust the mirror spacing a minor amount (while marking the existing position) by moving the secondary mirror outward a very tiny amount to see if that resolves the issue.

I am mid project on m101 right now so this tweak will have to wait until this has completed. I'll let folks know how it goes once I get to it.


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