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Issues with a 6" ritchey-chretien: pinching?

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#1 RJF-Astro

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Posted 13 April 2021 - 05:43 AM

Hello all,

 

I would like to get input on an recurring issue I have with my 6" ritchey-chretien. In my case it looks like an issue with triangular stars. This points at pinched optics, but I welcome other suggestions. Oh and I would only like responses from those who have actual experience with this scope or its bigger brothers. These seem to have specific mechanical issues, so experience with others types of scopes might not apply.

 

Here is what I know:

  • The triangle stars are not present in every session. Even if I have not touched collimation or anything else.
  • The issue does not seem related to this particalar scope. I had a similar 6RC last year but I sold it. Then I missed it and bought a new one this year. Both have had this issue with triangular stars.
  • There seems to be no relation to a particular orientation. I have had it when the scope was pointing at different directions.
  • Sub length does not matter. It is present at 4 second subs and long exposures.
  • Filter type does not matter. It is visible on LRGB and narrowband.

Here is what I think:

  • This is not related to collimation. It shows up and dissapears without changing collimation.
  • Temperature can be an issue. In the summer I had no issues. I have seen it mostly in spring and with cold weather, which might give a steep temperature drop in the evening.
  • Weight on the focuser can be an issue. I have a reducer, OAG, filterwheel and ASI-camera in the image train. Currently I use a heavier R&P-focuser and Celestron OAG. But with my previous RC I used a lighter ZWO OAG and standard crayford.
  • I have read about secondary pinching. But I did loosen that slightly with my previous RC and did not notice any difference.

I will continue testing, and waiting for different conditions like less temperature change. But any input is welcome! Could it be that the primary is too tight in the retainer for example? Does weight play a role, as the focuser is directly attached to the mirror cell and baffle? And can tight primary screws cause pinching, or is this not possible because they only affect the mirror cell?

 

And, ofcourse, some images:

Link to a complete 1 hour session (60x60s lum)

 

Link to some random other subs

 

Examples

 

pinched RC.jpg

 

20210413 M63 lum center crop.jpg

 

old rc pinch.jpg

 

Thanks for looking!



#2 WadeH237

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Posted 13 April 2021 - 12:09 PM

The trefoil shape of the stars is pretty indicative of pinching, as is the fact that it's temperature dependent.

 

To find the cause, you will need to look at all of the optical elements and identify any contact points that occur at 120 degree positions around the center of the element.  It could be that the collimation screws are too tight, although they would probably have to be gorilla tight to warp the plate to which the secondary mirror is attached.  Because of that, I would tend to discount this.  I would also rule out the collimation screws for the primary, since they are connected to the carrier and not the mirror itself.

 

That leaves whatever mechanism attaches the primary mirror to the mirror cell.  I've not looked at my RC6 in a while, so I don't remember if it uses mirror clips or not.  I suspect that it does, and I would think that these are the most likely candidates.  If they are too tight, you could get the symptom that you describe.  They need to be just tight enough to prevent the mirror from moving - but no tighter.

 

If you are using a reducer or flattener, that could also be a candidate.


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#3 RJF-Astro

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Posted 13 April 2021 - 12:56 PM

Thanks Wade! I dont think the 6RC uses clips, only a mounting ring. Here is a website where someone disassembles it. So the main culprit would be the mounting ring then.

 

I do use a reducer 0.67x, good idea to rule that out. The focuser has a compression ring, so that should distribute the tension. But there is room to slide it in further, so the ring clamps on a spacer. I will do that.



#4 RJF-Astro

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Posted 13 April 2021 - 04:02 PM

So I had a clear hour tonight and did some more testing. The reducer does not seem to be the issue. There was more than enough time for cooldown. Pointing straight up was no different than at 45 degrees.

 

However, when I defocused, the stars turned round and not triangle. I think with a pinched mirror the unsharp stars should be somewhat triangle as well? Instead, I saw what looked like on-axis coma. And that points at a collimation error of the primary.

 

So maybe what I am seeing is on-axis coma, turning stars into triangles. If I serach for pictures of coma, I see similar shapes, especially when combined with some mild astigmatism.

 

Unfortunately clouds rolled in so I could not work on collimation. But that will be my next to-do. Looking back at previous images I made with the new scope that I thought looked good, still showed mild triangular stars too. It does not explain the issue with my previous RC yet though, and I did not do any test with defocused stars at that point.

 

Single__Blue_2021-04-13_22-15-47_Bin1x1_20s__2C.jpg

 

Single__Blue_2021-04-13_22-17-53_Bin1x1_8s__2C.jpg

 

coma.jpg

 

These are WA lenses, but still...



#5 avarakin

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Posted 13 April 2021 - 05:51 PM

I am dealing with exactly the same issue with my 8" RC. 

It seems to be correlating with cold weather. 

I think it is caused by the retaining ring of the primary mirror being too tight. 

Here is my theory of what is happening:

Considering that thermal expansion of AL is more than glass, the AL parts of the scope shrink more than the mirror with colder temperature and thus the retaining ring starts pressing on the mirror and causing the distortion.

Here is what I did fix this problem: I tightened up the ring and then un-tightened it by half a turn.

So far I did not see triangular stars, but problem is that I did not have cold weather, so I guess I will have to wait till next winter to confirm that my fix is working.

In any case, I don't think there is any good reason for the retaining ring to be tightly screwed, I think that having a gap of 1/10" should not cause any trouble.


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#6 Dan Crowson

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Posted 13 April 2021 - 09:23 PM

This seems to be a common problem with these scopes. I see it as well with my AT12RCT until the mirror gets cool. In NM, that tends have me pointed up for 10-15 minutes before I start imaging.
 
An Alan Chen reached out to me five or so years back after seeing my wonky stars and gave me quite a few pictures of steps he took to fix the issue. With the 12" scopes, the issue appears to be the mirror only being supported by three cork pads other than the center retaining ring. His fix was to work on getting more support under the mirror. See his picture that shows the additional black supports.
 
http://www.crowson.c...mirror cell.jpg
 
I'm not much of a tinkerer and my scope has always been 1,300 miles away so I have not implemented any of his fixes.

Dan


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#7 avarakin

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Posted 13 April 2021 - 09:49 PM

6 and 8 have totally different design of mirror support. 

10 and 12 have some kind of a mirror cell, although it has only 3 points.

I modded my 10" using a different mod, where instead of pads, mirror is attached to mirror cell using velcro. The mod worked very well for my 10" and it is consistently giving some really good images.

6 and 8 do not really have a mirror cell, the mirror is supported by a ring. 



#8 RJF-Astro

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Posted 14 April 2021 - 01:40 AM

Thanks both. Thats what I thought too. First I will try to eliminate that on axis coma, then see if the issue is still there. Hopefully tonight, it is still cold here (<5 degrees c). Next step would be opening the scope and adjusting the ring.


Edited by RJF-Astro, 14 April 2021 - 01:40 AM.


#9 RJF-Astro

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Posted 14 April 2021 - 04:40 PM

Ok so it appears to have been collimation after all. From what I have read it is typical for an RC to show coma spread evenly across the field with a misaligned primary. This is different from a SCT or newt. And the coma looks a lot like triangle stars, which gives the idea of pinching.

 

Looking back I think I made an error in the first collimation session of my new RC. I defocused the stars by too much. I use the DSI-method. There is a point where the coma becomes clearly visible in the out of focus star, and then it dissapears again. This time I collimated on the sweet spot and that worked.

 

I am not completely statisfied yet, as my previous RC was collimated when the triangles appeared. But for now my issue looks like its solved...

 

Single__Lum_2021-04-14_22-27-48_Bin1x1_10s__4C.jpg



#10 RJF-Astro

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Posted 18 April 2021 - 03:17 AM

Heres an update, if anyone has the same issue it might be usefull. Last night I had an unexpected clear night so when I had everything set up the RC was still cooling down. I took some images while cooling down to keep an eye on star sizes. And there were the triangles again.

 

These changed shape as the scope cooled down, but I did not get a lot of clear sky time to let them turn to normal.

 

So I think this is what makes the RC complicated. Both collimation error of the primary and thermal issues look the roughly same. You need to let it cool down properly and if there are still triangles in-focus, you might also have to deal with collimation.

 

thermal.jpg



#11 WadeH237

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Posted 18 April 2021 - 09:05 AM

Honestly, I don't think that this is a collimation issue.

 

The aberration in your images does not look like coma to me.  The coma samples that you showed earlier in the thread are pretty extreme.  A small amount of coma looks like a crisp edge on one side of the star, with a soft edge on the other side.  As the coma increase, you get the characteristic comet shape.  And if you continue to increase the coma, then you get the boat anchor shapes from the samples.

 

The triangular spikes in your actual images look much more like diffraction effects, which is what you get from pinched optics.  The fact that they are temperature sensitive further underscores this.  That is exactly what you would expect from pinched optics.


Edited by WadeH237, 18 April 2021 - 09:06 AM.

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#12 RJF-Astro

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Posted 18 April 2021 - 03:37 PM

Could be. It is difficult to really separate those issues. In the images in post #4 the scope was outside for 3-4 hours. It should be at equilibrium.

 

But tonight I had the chance to do another quick test. This time with a cooldown of 2 hours. Stars looked good enough, no triangles. Let's hope it stays this way and if not, I'll report back.

 

 



#13 avarakin

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Posted 18 April 2021 - 04:15 PM

Did you try un-tightening the mirror holder ring? 

So far it is helping my 8" RC.

 

Alex



#14 RJF-Astro

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Posted 19 April 2021 - 01:37 AM

No I haven't yet. I am a bit hesistant to open it up. For now I will see how this works out, but if it continues to be an issue I will.



#15 Mert

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Posted 19 April 2021 - 05:48 AM

On my 8" I can't see this behaviour. I check colimation 1 or 2 times a year

#16 AhBok

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Posted 19 April 2021 - 06:28 PM

My RC6 does not have this issue at all. If it were me, I would loosen the primary retaining ring about a quarter turn. A better idea as mentioned previously is to slightly tighten the ring if it will snug up under light pressure and then back off a half turn. Everything seems to point pinched optics as the main issue.

#17 avarakin

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Posted 19 April 2021 - 10:32 PM

No I haven't yet. I am a bit hesistant to open it up. For now I will see how this works out, but if it continues to be an issue I will.

Opening RC is super easy - remove all screws from the back of the scope and then pull the back from rest of the tube. As a bonus, clean the mirror as part of the process. 



#18 PaulE54

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Posted 20 April 2021 - 01:42 AM

I had the same problem as you on my RC6, and as Avarkin says, loosening the primary mirror lock ring fixed it. It does not need much - 10 degrees at most. You need to remove the primary cell, remove the primary baffle tube, loosen the hex head lock screws on the primary mirror lock ring, then very slightly back off the lock ring - too much and you will allow the mirror to "flop". Then put everything back together. You won't disturb collimation by much. 

 

Clear skies.

 

Paul


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#19 RJF-Astro

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Posted 20 April 2021 - 01:44 AM

Alright, it looks doable. The mirror is still clean though, as my current RC is only a few weeks old. But it is a good way to get to know your scope right, which I think is always important.


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