Tools for collimating a Ritchey-Chrétien

[04Stefan07 (Stefan)]

I have a RCA 6 OTA and have been doing some research on how to collimate these telescopes. I wanted to ask what tools can be used.

I found guides on using Howie Glatter, Takahashi Collimation Tool and a simple Cheshire.

Recommendations?

[glend (Glen)]

What tools do you have now? Did you buy the scope new or a previous owner - ie it maybe close to collimated?
The online video by Stephen Kirk uses just the Cheshire and the Howie Glatter laser with the optioinal concentric ring attachment. I have both of thode and they can get you very close, however it is faster with the TAK collimation scope. Now if you don't have the Howie Glatter and the TAK scope, it will cost you about what your RC06 cost to buy them. Borrow them if you can, or rent them, once collimated the RC will hold it well. One piece of advice, do not touch the primary adjustment unless absolutely necessary and ony after the focuser and secondary are close to perfect - as you will have to do them again if the primary requires adjustment. Read everything you can find on RC collimation before you start.

[04Stefan07 (Stefan)]

Originally Posted by glend

What tools do you have now? Did you buy the scope new or a previous owner - ie it maybe close to collimated?
The online video by Stephen Kirk uses just the Cheshire and the Howie Glatter laser with the optioinal concentric ring attachment. I have both of thode and they can get you very close, however it is faster with the TAK collimation scope. Now if you don't have the Howie Glatter and the TAK scope, it will cost you about what your RC06 cost to buy them. Borrow them if you can, or rent them, once collimated the RC will hold it well. One piece of advice, do not touch the primary adjustment unless absolutely necessary and ony after the focuser and secondary are close to perfect - as you will have to do them again if the primary requires adjustment. Read everything you can find on RC collimation before you start.

Thanks for the reply.

I have downloaded a few guides that uses different techniques. One uses just a cheshire would might be more than enough to get a decent collimation as I can't afford to invest as much as the tube to get the expensive equipment.

Any particular Cheshire you recommend?

[barx1963 (Malcolm)]

Stefan
I have the exact same scope. I am waiting on a collimation ring at the moment but I have the Howie Glatter kit including the concentric ring attachment so I am hoping to get it pretty close with that and then think about the Tak.

I'll make you a deal. Come to Snake Valley in Nov and you can borrow the Glatter. Unfortunately I need it as my main tool on the dob so cannot lend it any other way.

Cheers

Malcolm

[barx1963 (Malcolm)]

There's a HG Collimator for sale here http://www.iceinspace.com.au/forum/s...d.php?t=139083

Cheers
Malcolm

[glend (Glen)]

Stefan just about any Cheshire will do the job, so don't spend alot on that. You will need a laser to align the focuser with the secondary centre spot, but you can actually get away with a fairly cheap narrow beam one like maybe the GSO one - but make sure the laser is properly collimated itself and not aiming all over the place when you rotate it on a target. I don't know if there is a collimation ring for the RC06 but certainly there is for the RC08 and 10. The collimation ring gives you the ability to adjust the alignment of the focuser to dead centre on the secondary, necessary for later secondary alignment. Your probably looking at $100 for the Collimation Ring and really GSO should equip each scope with one from the factory. People will chime in and tell you to replace the focuser but mine works fine with my DSLR. I believe the placement of the extension tubes and collimation ring determines how stiff the back end of the RC is going to be. I experimented with mine in various combinations and found that putting the extension tubes on first (from the rear of the scope, and then the collimation ring and the focuser mounted to that, gave me the stiffest structure. I found that collimation ring threads and bevel gave a better focuser lock than just the focuser on the extension tube.

Your scope maybe aligned pretty well out of the box, I'd suggest you put in on a mount and do a star test and see what you get before you touch anything. Pick a star field that has stars out to the corners, use a fairly wide angle EP, and infocus to the point the stars resemble donuts. If they are all perfectly round, and then out focus to pin point stars without tails other than the usual diffraction spikes - you may have a well setup scope. However, if they are oddly shaped in a corner of the field, or don't look like perfectly round donuts then you will need to go through the collimation routine.

Good luck, they can be frustrating until you understand what your doing.

[Paul Haese]

Just going to "chime" in here.

First up collimation via star test should always be done with the focus racked out. Not in. Out of focus collimation is the most accurate method.

Next, forget Cheshires etc. Not sure how many times I have to repeat this but a Cheshire is not useful at all for collimating an RC. The figure of the two mirrors is such that it demands a very high degree of collimation to produce sharp images. I have a Howie Glatter laser for an RC and even that gives very inaccurate results. I have tried all the methods suggested and only one works every single time. It is hard to perfect it but it works! You can get very good collimation in only minutes, but perfection needs careful attention to detail. Search my posts on the matter and how to collimate using a Tak scope.

The best method for collimating an RC by far is a Takahashi collimation scope. It you own an RC or BRC these scopes are a must. It will give you an accurate collimation, so much so you will not need to do a final star test, if you are very particular. I have done collimation it so often with working on GSO RC's I could almost do it blind folded. I have owned GSO RC scopes since they first came to Australia. I have had several RC's in pieces and built them back together, and collimated each and everyone of them to a high standard. If you own a folded scope sooner or later you have to clean the mirror and then you will need to touch the primary adjustment screws anyway. You cannot do a full collimation with these scopes without touching the rear screws. So advice saying that not to touch them is not useful. Owning a folded scope that has primary collimation screws means inevitably you will need to adjust those screws. Fast Newtonians are the same and require adjusting of the rear collimation screws from time to time. Transit from Taiwan to Australia will upset the collimation and sure as eggs you need to collimate the scope.

That said, if you want to go and try the other methods, feel free to knock yourself out.

Jumping in the deep end and doing a full collimation will help you learn. If you get stuck and cannot get the scope back into properly collimation after trying, you can send it to me and I will collimate it and send it back. I will charge you for the return freight only. However, I doubt that you will need to do this. Collimation sounds daunting but everyone starts from the same position and has to learn.

[loc46south (Geoffrey)]

I will second Paul on the Tak Collimation Scope - I had a 12inch R/C that I ended up assembling and disassembling a regular basis over a period of some months. Once you became familiar with the procedure using the Tak Collimation scope, the scope could be collimated in a matter of minutes.

Geof

[jwoody (Jeremy)]

Tak collimation scope. It just works and is dead easy to use.
Saying that, I haven't tried any other method.
Jeremy

[glend (Glen)]

Stefan said below that he could not afford to invest the cost of his RC06 on additional equipment to collimate it - which is why I made the suggestions I did.

[Howie Glatter]

Paul said: "I have a Howie Glatter laser for an RC and even that gives very inaccurate results."

Hi Paul. I have a few questions about this. Do you mean inaccurate results or imprecise results. If inaccurate, is that because the collimator is inaccurate, or some other reason, or both.

The laser collimation procedure for Cassegrains, which theoretically should have all the optical elements centered and made square with a single line, is pretty simple, and relies only on the law of reflection.
I understand that mechanical difficulties of a particular telescope design may make the collimation problematical, but that is hardly due to the reference tools used.

[Paul Haese]

Originally Posted by Howie Glatter

Paul said: "I have a Howie Glatter laser for an RC and even that gives very inaccurate results."

Hi Paul. I have a few questions about this. Do you mean inaccurate results or imprecise results. If inaccurate, is that because the collimator is inaccurate, or some other reason, or both.

The laser collimation procedure for Cassegrains, which theoretically should have all the optical elements centered and made square with a single line, is pretty simple, and relies only on the law of reflection.
I understand that mechanical difficulties of a particular telescope design may make the collimation problematical, but that is hardly due to the reference tools used.

Howie,

I mean inaccurate because the method employed with the rings provides inaccurate results in more ways that one. The degree of mis-collimation can still be significant even when the rings indicate that you have good collimation. The tool cannot be considered good if the collimation cannot be achieved to a very high degree. RC's with hyperbolic mirrors require very high degrees of accuracy to collimate them. A traditional cassegrain by comparison is so much more forgiving in regard to collimation. Hence my opinion. The collimator will get you close but not close enough. I am not into selling any particular product. I have no financial interest; just telling people what works the best with an RC. My imaging results give veracity to my statements here.

[Howie Glatter]

Hi Paul,

"The degree of mis-collimation can still be significant even when the rings indicate that you have good collimation."

If you are referring to the ring pattern projected in front of the scope, it is true that the ring pattern may be centered around the secondary baffle shadow, and the scope still be out of collimation. This is because laser collimation proceeds sequentially, element by element from the focal plane, to the secondary, and then to the primary.The collimation of any element can only be as good as the collimation of the elements that preceeded it - a chain is only as strong as its weakest link. However, misalignments of one or more elements can be compensated for in the ring pattern projected from the scope by tipping another element, so a scope that shows a centered ring pattern projected in front is not necessarily collimated because the aberrations don't cancel.
And also, thanks for your many wonderful images.

[barx1963 (Malcolm)]

This is an interesting discussion. I have the exact same scope as the OP. As I do not currently own a TAK collimating scope, I am proposing to have a crack at collimating it using the other methods. I will start with the Cheshire and then use the HG laser method. I expect this will get me close. As I have taken several images using the scope as it arrived (and I know it is quote out collimation wise) I think this will enable me to make a reasonable, although maybe not very technical, assessment of the value of those methods.
Once my bank balance recovers a bit from the recent purchase of mount, scope and bits and pieces, I will get a TAK and see how that goes.
Hopefully be able to provide a bit of a report!!

Cheers

Malcolm

[DJT (David)]

Tak collimating scope, Als Collimating software for final tweaks, 5 minute sub on a good starfield to validate corners.

I started with a Cheshire but my personal downfall was moving from a refractor to an RC and not really understanding what I was looking at. Tak nailed it for me.

Nothing wrong with over capitalising a GSO RC as once it's right ( or pragmatically close) it's a great scope and excellent bang for yer buck.

Enjoy

[barx1963 (Malcolm)]

Well I have had a bit more of a play with the RC. Conclusion is that yes you can improve collimation using the methods outlined in Stephen Kirks videos and it will yield better images. The improvement means is from absolutely horrible to just plain awful.
The second and third steps, just cannot be done accurately enough to yield worthwhile results IMHO.
So I am going to have to look at a Tak Scope!!
Anyone have one I can borrow for a few days??

Malcolm

[DJT (David)]

Hi Malcolm
Pm me and you can have a lend of mine. There is also a precise parts adapter for a moonlight focuser 2.5 COS, and the tak

I setup a la portable tonight for the first time and took the opportunity to nail the collimation ghosts through using a tilt tip ring from Bintel. After 2 years of tweaking framing to compensate it's now all done and dusted

So my tools of choose are

1) a better focuser
2) a Tak collimating scope
3) also collimation aid ( search this forum for details)
4) precise parts adapter between the focuser and the Tak
5) tilt tip ring from Those nice people at Bintel

My process was..

1) attach all adapters and rack the focuser all,the way out
2) collimate the dot on the secondary so it's in the centre of the ring when seen through the tak scope
3) adjust the tilt tip ring so the outer annulus is concentric
4) a quick iterative tweak of the secondary again per step 2
5) star test using als collimation aide, search this forum.

Easy...

[glend (Glen)]

The discussion on this thread highlights there are different ways to achieve RC collimation, dependent upon your budget in both cash and time. Sure if money is not an issue the TAK Collimation is a nice piece of gear to have, but you can reach perfect collimation without it. I have found even with a TAK Scope showing perfect bench collimation the RC still required collimation tuning through my camera. In my opinion the knowledge of RC optical alignment, what your actually seeing with the various tools and how to adjust from there is more important than the actual tool choice. Hopefully the OP has made some progress within his budget constraints.