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Did I kill my RC6? Please help!

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#1 maxsid

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Posted 24 June 2022 - 01:12 AM

Hi All,

 

By accident, I handled my RC6 pretty roughly while mounting it on my newish CEM40 (I hate its saddle).

Now the scope is completely out of collimation.

I tried to collimate the secondary mirror, the stars would look OK in the middle but at the edges they are a mess.

I think I need to adjust the primary mirror.

This scares me. I watched a bunch of videos. They are not that helpful.

The guys obviously know what they are doing but they cannot express themselves clearly.

 

Below is an image of defocused stars.

Any suggestions on how to fix this?

Thank you very much!

 

M_101-stars-collim_Light_40_secs_006-St.jpg


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#2 pyrasanth

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Posted 24 June 2022 - 02:51 AM

This looks more like tilt rather than a collimation issue. A first check would be to confirm that all your adapters are screwed orthagonaly with camera into the imaging train. If you post an image of the stars in focus and the image scale I can run the field through CCD inspector for you and get a report if it is collimation or tilt or even both.

 

You've not broken anything permanently- its just than some models of RC don't hold the optics in alignment very well so you often spend  time tweaking to keep things spot on- this exercise your going through whilst worrying is a good learning curve for you when you learn how to fix the issue- so look for the good in the situation even if currently it makes you feel a bit uneasy.


Edited by pyrasanth, 24 June 2022 - 02:54 AM.

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#3 maxsid

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Posted 24 June 2022 - 03:31 AM

I played with my RC6 a little. The primary is not easy to move without loosening the screws.

So, the primary is most likely OK.

Everything is screwed in orthogonally.

Actually, two things happened.

-- I crudely mounted the scope on the mount. The focuser actually moved while I was doing that (which it shouldn't as it was held by EAF).

-- I installed/inserted a baffle extension from Buckeyestargazer..

 

Since then, I tried to collimate the secondary and removed the baffle extension.

Didn't help.

 

The stars look elongated and even double.

 

Left-up edge.

m101-40s-stars-left_up_edge.jpg

 

 



#4 Tapio

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Posted 24 June 2022 - 04:11 AM

It could still be tilt.

What exposure is in this last exposure ?



#5 maxsid

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Posted 24 June 2022 - 04:26 AM

40 sec



#6 Tapio

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Posted 24 June 2022 - 04:36 AM

A shorter exposure like 5 sec would tell more about optic issues.


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#7 maxsid

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Posted 24 June 2022 - 04:52 AM

I am not sure why a shorter exposure would make a deference. It depends on gain, etc.

I think, my first image does tell the story. I just don't understand why such a big misalignment after not too much action.

 

A shorter exposure like 5 sec would tell more about optic issues.


Edited by maxsid, 24 June 2022 - 04:54 AM.


#8 osbourne one-nil

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Posted 24 June 2022 - 05:28 AM

Moved from Reflectors



#9 happylimpet

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Posted 24 June 2022 - 05:37 AM

Hi All,

 

By accident, I handled my RC6 pretty roughly while mounting it on my newish CEM40 (I hate its saddle).

Now the scope is completely out of collimation.

I tried to collimate the secondary mirror, the stars would look OK in the middle but at the edges they are a mess.

I think I need to adjust the primary mirror.

This scares me. I watched a bunch of videos. They are not that helpful.

The guys obviously know what they are doing but they cannot express themselves clearly.

 

Below is an image of defocused stars.

Any suggestions on how to fix this?

Thank you very much!

 

attachicon.gifM_101-stars-collim_Light_40_secs_006-St.jpg

The edges just look weird because of slight vignetting of the light cone, which i expect is normal - i get exactly the same thing with my newt out of focus. This just looks like collimation is off. No reason at all to invoke or suspect tilt.

 

I am not sure why a shorter exposure would make a deference. It depends on gain, etc.

I think, my first image does tell the story. I just don't understand why such a big misalignment after not too much action.

short exposure eliminates tracking errors which can confuse things. Its a very good idea.

 

does seem odd, but theres no question youve a collimation issue.

 

It'll be fine. Be cool!


Edited by happylimpet, 24 June 2022 - 05:38 AM.

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#10 Tapio

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Posted 24 June 2022 - 05:43 AM

I am not sure why a shorter exposure would make a deference. It depends on gain, etc.

I think, my first image does tell the story. I just don't understand why such a big misalignment after not too much action.

40 second exposure could exhibit tracking issue (if no guiding).

 

First image tells more, yes.



#11 Paradoxdb3

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Posted 24 June 2022 - 07:06 AM

I know nothing of RCs, but are they supposed to be completely flat and coma corrected? Because the stars in your image look normal for most mirrors. But like I said, I know nothing of RCs.

#12 VincenzoZito

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Posted 24 June 2022 - 07:26 AM

Sorry for the triviality your RC is out of collimation, this shows your first image. The stars are not in place in the middle, only after that do you worry about possible tilt.



#13 jgraham

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Posted 24 June 2022 - 08:14 AM

RCs are coma corrected, but not flat. Ritchey used curved plates on his RC. I use a Baader Mk III MPCC on my RC8 and it does a great job cleaning up the little bit of field curvature. That may not be exactly what it is doing, but my field looks fantastic with a full frame camera. As for the above... it doesn't look too bad. The secondary is a tad out of collimation and I'd fix that first. Also, carefully reach in and make sure that your baffle is snug. Mine was loose and wouldn't hold collimation. Now all is well. Once the secondary is zeroed in check the field and see if you can live with any residuals. Perfection can be a slippery slope.

Enjoy!
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#14 fewayne

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Posted 24 June 2022 - 12:57 PM

There are probably as many ways to collimate RCs as there are RC owners...

 

Don't be afraid to adjust your primary if you have reason to believe that it's out of whack. Best way to determine that is with a star test. If a star in/near the center of the field is showing coma, that's indicative of primary misalignment. There are a couple of ways to determine that -- the simplest is to put in an eyepiece and look. There are also software aids like Metaguide (free) and SkyWave (expensive) if you have a Windows machine you can connect to the scope.

 

You can always un-adjust what you've adjusted; you may want to keep a log. It rarely takes very much.

 

Many of the collimation techniques you'll see online use mechanical alignment, e.g. the wall of the tube as a guideline. But the optical figuring and the mechanical bits do not necessarily coincide or align. For example, the donut at the center of the secondary mirror may be precisely in the middle of the circle, but the optical center of the mirror's curvature might not be! So first-order indicators like on-axis coma and off-axis astigmatism are more reliable. This is a classic, and relies solely on the optics.

 

Two complicating factors that drive many folks batty: Adjusting the primary may throw off your secondary (and vice versa), so the process is necessarily iterative, but hopefully it converges so that you're making tinier and tinier adjustments each time around. The other issue is that the GSO scopes that are all but ubiquitous at the lower end of the market move the primary and the focuser as one assembly when you adjust the former. So as you reduce coma, you can introduce tilt. If you haven't bent anything, and didn't have tilt problems before, you should be able to restore the former state of affairs, yah? If not, a tilt plate is not expensive (my 8" was $108), and you can get the collimation done first, then take out the tilt by eye or by using ASTAP (which I actually installed on my Pi -- slow, but it works).

 

Another aid of which I've become very fond is the Tri-Bahtinov mask, I use it all the time now to check up on (and touch up) the secondary on my AT8RC. That and a paper donut with one-third of its circumference cut out really does make it super-easy. Focus on one set of spikes. If the others aren't centered, cover up the first set and one of the others, then adjust the secondary until the single set showing is correct. Move to the second one that was out, tune it up. Remove the donut, voila, all three spikes centered, you're collimated. (Note: Do get rid of any tilt before relying on this though!)



#15 anismo

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Posted 24 June 2022 - 01:27 PM

The collimation is actually not far off. Get the center shadow corrected and it should fine.

What you are seeing on the edge stars are simply vignetting and not miscollimation (basically light drop off because it is getting cut off by the tube). 



#16 maxsid

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Posted 24 June 2022 - 11:19 PM

Thank you guys for ideas!

I will try to collimate again tonight.

Thanks!



#17 maxsid

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Posted 25 June 2022 - 01:07 AM

My astro camera is already busy with another (smaller) scope shooting nebulae.

Tried to star-collimate with a DSLR and no computer. Didn't work.

Will wait a couple of weeks until the Moon is big again and no real shooting is possible.



#18 Starman1

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Posted 26 June 2022 - 04:20 PM

The collimation is actually not far off. Get the center shadow corrected and it should fine.

What you are seeing on the edge stars are simply vignetting and not miscollimation (basically light drop off because it is getting cut off by the tube). 

Cut off is probably from the secondary rather than the tube.

 

There is a slight miscollimation of the star in the center, so the secondary is probably off.

The online tutorials on collimating an SCT will work for that.


Edited by Starman1, 26 June 2022 - 04:20 PM.


#19 maxsid

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Posted 27 June 2022 - 02:34 AM

I ordered Bob's Knobs - should make adjusting the secondary a bit easier.

Will give them a try in a week or so.

Right now I am shooting nebulae with a smaller scope.

Don't want to waste the New Moon week.



#20 Old Speckled Hen

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Posted 27 June 2022 - 08:46 AM

I ordered Bob's Knobs - should make adjusting the secondary a bit easier.

 

DO NOT DO THAT!

 

Fix Your Issues First.




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