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My take on GSO Ritchey-Chrétien collimation

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#1 TinySpeck

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Posted 04 August 2020 - 07:26 PM

I've been trying to collimate my GSO-built 8" Ritchey-Chrétien scope for five months now, and finally found something that works.  This method is easy and is done indoors, the measurement metrics are clear, and it doesn't require exotic equipment or procedures.

 

I came across probably a dozen different methods in my search and tried several of them with little or no success.  Some methods seemed unnecessarily difficult, requiring partial disassembly of the scope.  Some relied on imprecise judgments with fuzzy parameters.  Some simply couldn't have worked, since they didn't account for the way the GSO primary mirror is attached to the visual back.  When you adjust the tilt of the primary on this scope, you also re-aim the focuser.  Any method which aims the focuser and then adjusts primary tilt without correcting focuser aim can't work with a GSO.

 

Try putting light pressure on the focuser, similar to the weight of a camera assembly, during the procedure below.  I found that my laser dot would move slightly when I did this.  If you see this, it means that collimation will shift slightly depending on which part of the sky you point to.  The best you can do in this case is to collimate with the scope pointed within 20 degrees or so of vertical and accept that it's not going to be perfect everywhere.  There is probably some temperature effect too.

 

For this method you'll need an adjustable tilt on your focuser.  Some focusers provide this, or you can insert a tilting platform.  Note that if you use a Moonlite CS (like I do), adjusting the tilt locks the focuser rotation.  Make sure you have the spacers you'll use for imaging between your scope and focuser when you collimate, because you won't be able to rotate your focuser to compensate for different spacer threading afterward.

 

(continued)


Edited by TinySpeck, 04 August 2020 - 07:56 PM.

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#2 TinySpeck

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Posted 04 August 2020 - 07:27 PM

TOOLS REQUIRED

 

• Laser collimator, dot only (like a Howie Glatter)
• Headset magnifier (like an OptiVisor)
• Inspection mirror (angled mirror on a stick)
• Small diffuse or side-illuminating flashlight
• Cheshire
• Binoculars
• 2.5, 3, and 4 mm hex keys for the scope, also whatever you need for focuser tilt
• #2 Phillips screwdriver (to set focal length if necessary)

 

PROCEDURE

 

1. Set focal length (if necessary).  This scope is widely spec'd as 1625 mm focal length, but it is actually 1600 mm.  See post here: https://www.cloudyni...rs-for-gso-rc-8 .  The OPT website and some others have it correctly now.  When you set the focal length to 1600 you also get the spec'd back focus of 250 mm.

 

It is important to have the correct focal length for an RC scope.  This generally needs to be done under the stars with a plate solver.  If you decrease the mirror to mirror distance by x, you increase the focal length by at least 10x.  Note that increasing the focal length will increase the back-focus, too, and vice-versa.

 

Adjust focal length by moving the secondary incrementally in or out using the center screw while keeping the three tilt adjustment screws snug.  To move the secondary, the center screw is pull: CW pulls the secondary mirror away from the primary and reduces the focal length.  The three hex screws are push: CW on them pushes the secondary mirror toward the primary and increases the focal length.  Loosen the push before pulling and vice-versa.

 

2. Reset the primary (if necessary), maintaining focal length.  Do this if you know your primary is way out of whack.  Fully loosen all the small black primary screws, tighten the large silver screws all the way (counting 1/6 turns by flats on the hex keys), then loosen them the same number of turns each (the average of the tightening turn counts) and tighten the black screws.  This maintains the focal length you set in the previous step.

 

3. Reset the focuser tilt (if necessary).  For a Moonlite CS: loosen the focuser rotation lock thumbscrew and all four focus tilt setscrews.  The focuser is now loose on its flange.  Pulling the focuser away from the OTA, tighten the tilt adjust setscrews until they all just make contact with the same snugness.  Focuser rotation will drag slightly at this point.

 

4. Adjust focuser tilt so the laser dot points directly at the secondary donut.  Insert the laser snugly into the focuser.  You can clearly see the secondary donut by looking in from the OTA open end and using the headset magnifier, inspection mirror, and a diffuse or side-illuminating flashlight hung on a hook inside the OTA a few cm.  The magnifier lets you see the donut in good detail from close up.  The flashlight illuminates the secondary donut from inside the OTA so you can see it more easily, and it eliminates the visual dazzle of the laser dot.  The inspection mirror is an angled mirror on a stick so you can look back at the secondary mirror from the open tube end.

 

5. Adjust secondary tilt until the reflected laser dot is centered on the laser source.  Use binoculars from the OTA open end to see a reflection of the laser source face in good detail.  Yes, good old field glasses!  I was amazed that this worked, but the effective distance to the laser source image is far enough that you can use binoculars.  This brings the laser source image up close and you can make a good judgment on how well centered the reflected dot is.  The reflected "dot" is actually smeared out by the secondary with many diffraction rings visible, but you can center it well by eye this way.

 

6. Adjust the primary tilt using a Cheshire.  Install the Cheshire snugly in the focuser.  Look through the Cheshire "pinhole" and center the dot (the reflection of the pinhole) in the secondary mirror donut.  This is hard to explain but easy to do with precision once you see it.

 

7. Iterate steps 4 – 6 until converged. 

 

8. Fine tune the primary tilt with a star near zenith (if necessary).  I used this primary fine-tuning method from Vixen.  Place an out-of-focus bright star in the center of the view.  Move the scope so the star donut moves in the direction of the narrowest part of the donut.  Adjust the primary in the direction to bring the star back toward center.  This eliminates guesswork over which primary screw to adjust.  My scope only required about 1/16 turn on one primary screw.
 

trim.png

 

(continued)


Edited by TinySpeck, 05 August 2020 - 03:39 PM.

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#3 TinySpeck

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Posted 04 August 2020 - 07:28 PM

TEST RESULTS

 

My imaging camera is a ZWO ASI294MC, a one-shot color with a 19.1 x 13.0 mm sensor (on the small end of what's called APS-C).  For these tests I ran the scope at its native focal length with no flattener or reducer.

 

I took some random star field images in and out of focus, and some images of M13 (which was nearly straight overhead).  I didn't have guiding going and only "pretty good" polar alignment, so the images were single 40 – 60 second shots.  PixInsight did not find enough stars for a FWHM contour plot of the random star fields.  The images below have been debayered, converted to grayscale, noise-filtered with MMT (delete layer 1, -0.5 bias layer 2), and had automatic stretch applied via STF and HT.  The mosaics are from the AberrationInspector script.

 

Here is the full M13, no cropping:

 

M13 full r.jpg

 

(continued)


Edited by TinySpeck, 04 August 2020 - 07:48 PM.

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#4 TinySpeck

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Posted 04 August 2020 - 07:37 PM

Here is the M13 mosaic:

 

M13 Mosaic r.jpg

 

(continued)


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#5 TinySpeck

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Posted 04 August 2020 - 07:38 PM

And here is a mosaic of an out-of-focus random starfield in the southern sky:

 

Out of focus r.jpg

 

It looks to me like the star donuts are pretty good in the center tiles and symmetrically "pointy" toward the center around the periphery.  The M13 mosaic also looks this way, with slight star elongation toward the center around the periphery.  I think I can attribute this to RC field curvature.

 

Overall I'm really happy with this.  My autofocus HFRs in Sequence Generator (after putting my reducer back in) are lower than ever before, even when the scope was new, and I'm pleased with the star images over the whole field.  The process is straightforward, doesn't require disassembling the scope, relies on easily-verifiable tests, and uses gear that is readily available and isn't too costly.


Edited by TinySpeck, 05 August 2020 - 10:06 AM.

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#6 scottsdalejohn

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Posted 04 August 2020 - 07:39 PM

Looking forward to the remainder of the series, but in the interim a question: based on your observation that a laser dot would move with light pressure on the camera - have you considered locking-down the camera on an extended [vixen style] dovetail?


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#7 TinySpeck

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Posted 05 August 2020 - 09:32 AM

@scottsdalejohn - I'm not familiar with that, but I'll look into it.  Thanks!



#8 JWST

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Posted 17 November 2020 - 05:14 PM

Greetings - I was about to purchase the same telescope (my first and probably last), but reading this post and related posts has given me "cause for concern" to put it politely.

If you had to do it over again with your knowledge and experience now, would you purchase this telescope again?

cheers,

Tim



#9 TinySpeck

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Posted 17 November 2020 - 05:27 PM

Greetings - I was about to purchase the same telescope (my first and probably last), but reading this post and related posts has given me "cause for concern" to put it politely.

If you had to do it over again with your knowledge and experience now, would you purchase this telescope again?

cheers,

Tim

Welcome to Cloudy Nights!

 

The hard part was figuring out how to get a good collimation.  You'd think that would be a simple, standard procedure, but that's not what I found.  The procedure here is not difficult and shouldn't steer you away from an RC.  Overall I've been happy with the scope and would certainly do it again.  Be sure to put whatever scope you do get on a good mount, though, if you're going to do astrophotography.

 

Any scope is a compromise, they all require care and maintenance of one form or another, and you have to choose what balance of positives and negatives works for you.



#10 celegroz

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Posted 22 November 2020 - 10:07 AM

I bought the GSO RC6 about a month back and have been devouring anything I can find on collimating and using these telescopes. I immediately bought a Moonlite as it was clear the stock focuser was garbage for astrophotography. But, now I am stuck as I have no collimation tools (Howie Glatter or Cheshire). It is becoming clear that to get these inexpensive RC scopes working for AP requires these tools and careful and tedious collimation. I've been sitting here for several days now trying to decide whether to just cut my losses and sell the scope or to put another $300-$500 more into this "project". The current collimation on my scope is totally off and unusable from the factory so it will have to be collimated before any imaging can be done with my 2600MC (heavy camera). I guess I'm just looking for perspective from others who've been in my situation. I clearly didn't do enough research on these scopes before buying and here I am. I really want the extra focal length these scopes provide beyond my 80mm APO (my only other telescope). But, reading the process here for collimation seems really daunting to me. Feedback, suggestions, ridicule, whatever, is all welcome. :)



#11 TinySpeck

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Posted 22 November 2020 - 12:49 PM

Ridicule...!  lol.gif  Not from me.  I've been ridiculous myself too many times.

 

The RC is known to be fussy about its collimation, but I didn't find this method to be difficult.  Maybe I'd been so daunted by all the methods I'd read about and tried that this method seemed straightforward.  smile.gif  It does require a laser and Cheshire though, assuming you already have some binoculars, which adds cost to your whole system.  Maybe you can justify that as tools you can use with any scope, and which will be used many times over the lifetime of your equipment.  I can't find what I paid for my Cheshire, but I think it was only about $30.  The Glatter laser is pretty spendy at about $300, but others are available for much less.  Check quality, and maybe look for used gear also.

 

It stinks that your scope needs collimation out of the box, but you do need to collimate RCs now and then so you might as well gear up for it.  I'd encourage you not to give up yet.  Astrophotography is kind of a fussy hobby anyway, and this is just a little more fuss.  



#12 celegroz

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Posted 22 November 2020 - 02:24 PM

 The Glatter laser is pretty spendy at about $300, but others are available for much less.  Check quality, and maybe look for used gear also.

 

It stinks that your scope needs collimation out of the box, but you do need to collimate RCs now and then so you might as well gear up for it.  I'd encourage you not to give up yet.  Astrophotography is kind of a fussy hobby anyway, and this is just a little more fuss.  

Do you think this would work in lieu of dropping $300 on the HG?

 

https://www.highpoin...-cheshire-fp216



#13 TinySpeck

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Posted 22 November 2020 - 02:31 PM

I don't have any experience with that, but it has all rave reviews there on High Point.  I'd check around for more opinions here on Cloudy Nights, and that may be a much more economical alternative.  I see it comes with a Cheshire too, so you may be in the collimating business with only a $132 outlay.



#14 Paul Sweeney

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Posted 26 November 2020 - 09:44 AM

I have a VC200L, which is basically the same scope. I tried using a laser, but I found that the laser caused more problems than it solved. So here is my method for collimating using a cheshire.

First, make sure that the secondary is marked for orientation.

To align the focuser, take out the secondary. Make sure that the focuser is flush with the backplate of the tube. These are machined surfaces, so they should be close to the same plane. Put in the cheshire and align its cross hairs with the vanes of the spider. You should be looking right through the secondary bolt hole. You can do this with a well collimated laser by putting a piece of scotch tape over the bolt hole so you can see where the laser hits. When the focuser is aligned, replace the secondary.

Make sure that any spacers that were behind the secondary get put back in. Snug the secondary bolt up and look through the cheshire. Like with a newtonian, you want to see your eye looking right back at yourself with everything concentric. This can be done with a laser, but if it is out of collimation, you will be off. The curve of the secondary is the problem because it magnifies any errors. No matter how you do this, it will not be perfect, si don't worry for now.

The primary collimation can be done on an artificial star or on a real star of about mag 2. Polaris is good. Start with about 100x. As noted above, defocus until you see the dark and light rings. If the dark point is not centered, adjust the primary to center it. Now, refocus and slowly defocus. The star image will expand as a ring from the star. If the ring is not concentric, adjust the secondary to make it so. Now defocus until the dark and light rings are visible. Still centered? If not, adjust the primary again. Now check the break out again, and adjust the secondary. As you can tell, this is simply an iterative process, with the adjustments getting smaller each time. When all is good, go to 200x and redo.

This sounds like a lot, but that is only if you take the scope apart. Normally, you only need to tweek it now and then.

#15 TinySpeck

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Posted 26 November 2020 - 02:19 PM

I don't see anything here about getting the correct focal length first.  Are you assuming that has already been done?

 

... When the focuser is aligned ...

Do you mean adjusting the tilt of the focuser?

 

...
Make sure that any spacers that were behind the secondary get put back in. Snug the secondary bolt up and look through the cheshire. Like with a newtonian, you want to see your eye looking right back at yourself with everything concentric. This can be done with a laser, but if it is out of collimation, you will be off. The curve of the secondary is the problem because it magnifies any errors. No matter how you do this, it will not be perfect, si don't worry for now.

...

I don't see an adjustment of secondary tilt in your procedure, only what looks like a claim here not to worry about it.  Secondary tilt adjustment is a crucial part of collimation.

 

...
The primary collimation can be done on an artificial star or on a real star of about mag 2. ...

When you adjust primary tilt your focuser will no longer be aimed at the middle of the secondary, at least with GSO type RCs.  Your primary collimation step will nullify your focuser aiming step, so some sort of iteration between the two is required.

 

Also, collimation affects the whole field.  If you adjust something so that a star in the center of the field looks good that doesn't mean that collimation is correct elsewhere.



#16 celegroz

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Posted 26 November 2020 - 03:05 PM

To align the focuser, take out the secondary.

Ugh. I shudder to think about doing this on the GSO scope. How would I figure out if the focal length is correct after reinstalling? Why not just adjust the focuser to hit the center spot on the secondary? What is gained by removing it?



#17 jgraham

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Posted 26 November 2020 - 11:14 PM

I have been imaging with the Astro-Tech branded GSO RC8 for several months now and it has been an imaging machine! I bought this scope used and the colimation was whacko off at first. Fortunately, it was just the secondary which was easy to adjust. Once set, I have never had to adjust it. I also replaced the focuser with a Moonlite and I added a Baader Mk III MPCC.

Wonderful scope!
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#18 TinySpeck

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Posted 27 November 2020 - 01:17 PM

The way I image is to wheel my scope out from the garage on a homemade dolly, around to the back patio on a paver walkway.  I recently built "Dolly 2.0" with nice cushy pneumatic tires, which is much easier on my collimation than the harder office-chair casters I was using before.  I think my collimation will last a good long time now.



#19 celegroz

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Posted 29 November 2020 - 03:40 PM

I don't have any experience with that, but it has all rave reviews there on High Point.  I'd check around for more opinions here on Cloudy Nights, and that may be a much more economical alternative.  I see it comes with a Cheshire too, so you may be in the collimating business with only a $132 outlay.

Update! Just got my Cheshire and Farpoint laser. OMG! My focuser alignment is way off! The laser dot is all the way up against the edge of the secondary centering dot - not even close. I even tried it with no extensions installed and just the Moonlite connected directly to the OTA. So, this explains why my stars are so misshapen with the camera attached. My focus tilt adjustment plate arrives tomorrow so hopefully I can try your approach and see what happens. I'll report back on progress. 



#20 TinySpeck

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Posted 29 November 2020 - 04:07 PM

Update! Just got my Cheshire and Farpoint laser. OMG! My focuser alignment is way off! The laser dot is all the way up against the edge of the secondary centering dot - not even close. I even tried it with no extensions installed and just the Moonlite connected directly to the OTA. So, this explains why my stars are so misshapen with the camera attached. My focus tilt adjustment plate arrives tomorrow so hopefully I can try your approach and see what happens. I'll report back on progress. 

Nice!  That's the kind of bad news you want to get: the fixable kind that explains the problem.  The Moonlite CS focuser has adjustable tilt built in though -- did you not want to use that?

 

Looking forward to hearing the good news that you've got your collimation spot-on.  smile.gif



#21 celegroz

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Posted 29 November 2020 - 04:10 PM

The Moonlite CS focuser has adjustable tilt built in though -- did you not want to use that?

 

 

I realized that but then assumed that it would render me unable to use the rotation function which I really want to be able to do.


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#22 TinySpeck

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Posted 29 November 2020 - 04:20 PM

I realized that but then assumed that it would render me unable to use the rotation function which I really want to be able to do.

Yes, that's true and it is a bit limiting.  Just wanted to make sure you knew about it.



#23 pwarborg

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Posted 29 November 2020 - 04:22 PM

I'm just wondering if the tilt plate (or focuser collimation) is generally accepted for issues with flex. I can see the value when you have kind of a mechanical kink in the optical train, but I would not want to have to adjust the tilt plate every time you point to a new target (maybe even during a long imaging run?) or every time after you did a meridian flip. Any thoughts?

Pat



#24 celegroz

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Posted 29 November 2020 - 04:40 PM

I'm just wondering if the tilt plate (or focuser collimation) is generally accepted for issues with flex. I can see the value when you have kind of a mechanical kink in the optical train, but I would not want to have to adjust the tilt plate every time you point to a new target (maybe even during a long imaging run?) or every time after you did a meridian flip. Any thoughts?

Pat

Definitely a concern I have. I've just lowered my expectations that I won't be able to collimate and have perfect mechanical alignment with the scope in all positions. The back focus on my RC6 scope is like 250mm. A scope engineered to experience zero flex over this great a distance would be much more expensive than the $400 I paid. :)

 

I'm not sure what the experience has been of the OP as his scope is the RC8. 


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#25 jgraham

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Posted 29 November 2020 - 06:50 PM

Just a quick thought... check the laser spot while either rotating the laser in the focuser or with the laser in different rotational positions. My laser collimator is a tad out out of alignment and this procedure helps to null that out.


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