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Advice from Ritchey-Chrétien owners please


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As the imaging season has now closed for me, there is a little devil on my shoulder telling me to collimate my StellaLyra 8" Ritchey-Chrétien. I have used it through 'Galaxy Season' and got some pretty good results, but I do know the collimation is slightly out. Looking though a Cheshire the secondary mirror is a long way off centre and I can see some tilt in the primary. Although when imaging the results are OK, the stars at the outer edges are slightly mis-shaped. There are also some gradients with the reducer. (The reduced seems to exaggerate the faults).

Now I have read the numerous threads regarding the problems with collimation of RC's. I refuse to pay £500 for collimation gear when the scope only cost £800, so with this in mind I purchased one of the REEGO type collimators to check the set up and it shows it is a long way out. (I suspect the optical and mechanical axis are quite out of alignment). As I also have a new focuser and tilt adjuster on the way it seems like the right time to collimate.

My plan is to collimate as accurately as I can with the REEGO, then use the DSI procedure to get the final result. Any guidance or advice before I start would be appreciated.

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This link is to a method for collimating an RC developed by Es Reid about four years ago when I was struggling to collimate my RC.

https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey Chretien with a plastic disc V2.pdf?dl=0

I had tried all the methods, including the very expensive Hotech laser collimator for SCTs, which worked well but is not a simple method for occasional use or rechecking. The limitation in its use, I found, is that the accuracy of the method is dependant on the stability of the surface you are working on. It worked well on a granite worktop, for example,  but produced variable results on a desktop due to the slight sag in the desk. And it is not cheap.

The GSO 8-inch RC is a [removed word] to collimate whatever badge it is wearing, due to its inherent mechanical design. Forget about tilt adjusters and using the focuser, you need to align the primary and secondary mirrors on their own before anything else. 

By the way the method described in the note is the same as Es uses. It is simple, cheap and it works.  

David

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Thanks David. Really useful.

I had seen this method some time ago - probably whilst searching through the forums looking for the best system. I think I was put off by the necessity to dismantle the scope at the start, given the first rule seems to be don't move the central screw, so removing everything does lead to a certain amount of fear! Obviously this is in part to keep the mirror distance correct.

I think my plan will be to try the TSRCKOLLI (or Reego) as I have bought it. Also, reading through the method here, the Reego theory is similar - if I can get it accurate enough. It is basically a method of aligning the two mirrors without accounting for any tilt in the focuser. If it works great, if it does not I am in no worse a position to try the Es Reid method.

I have got a suitable tilt adjuster for the focuser, but my plan was to optimise the collimation first using a suitable method - then us the adjuster to compensate for any error in the alignment with the camera. Interestingly, when I queried the focuser issue with FLO I was told that the focuser are normally aligned correctly and I should be able to collimate using a Cheshire. It remains to be seen. Interestingly, if the alignment is correct then I have wasted a load of money on a posh LED light and a tilt adjuster!

If in a couple of weeks time you see an RC8 for sale in the classifieds, you know where it came from🤣

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Clerkey,

I think you are approaching this the right way, getting the two mirrors parallel and with co-linear optical axes is the way to go. 

I have a tilt adjuster but don't use it. I also have a HG laser a Hotech laser and a Cheshire but don't use those either. So you will be in good company.

I once enquired of Teleskop Service how and when I should use their tilt adjuster. They were very clear, you don't use as part of the collimation process but only after you have collimated the primary and secondary mirrors. The adjuster compensates for tilt between the extension tubes (and focuser) and the primary mirror. Also, they said, the tilt adjuster should be mounted immediately behind the primary mirror, as close to the point were the original tilt error occurs. Interestingly, in that position, it blocks access to the primary mirror adjustment screws and so further adjustments of the primary are impossible once it's fitted there.

I would assure you that dismantling the telescope should be perfectly Ok if you proceed with care and mark everything with tape or spots of paint before taking them apart. Give yourself plenty of space and light - I use the kitchen worktop which gives me a good flat surface. Photograph everything you do, so that you can refer back. Count the number of turns of the centre screw of the secondary, don't touch the adjustment screws and they will give you a reference depth.

By the way, if you think the secondary is the wrong distance from the primary, you can check that too using a Ronchi eyepiece or grating.  PM me if you would like a copy of a note I have on how to do that.

Properly adjusted, the RC will give you fabulous images. I love mine. With just two mirrors and a weak flattener or reducer, you'll get true colours and excellent image scale with your cameras. It'll be a step up from your 200P and in a more compact package. I would get a dew shield; not so much for dew protection but to prevent stray light getting into the tube.  

Keep it; master it; enjoy it.

David

      

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Just to add my support to David's suggestions, I collimated my RC6 using the plastic disc method, very happy with the results. I did final tweaks using real stars and metaguide, which is very easy to use if your camera is one of the ones that works with metaguide (my ZWOs do, you just need the video driver). 

Paul

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Thanks for the input gents. I am 'home alone' this weekend so I will have a go with the collimation without any distractions.

8 hours ago, davies07 said:

It'll be a step up from your 200P and in a more compact package.

I have not actually imaged with the 200p (although I do have the kit to do it), only the ED80 and the RC8. I have managed some pretty good images, but knowing the collimation is slightly out, I know they should / could be better. The slight error is also magnified when using the reducer. I think the inter-mirror distance is pretty close as it plate solves only a few mm out. (I know the inaccuracies of the mirror may make the required distance slightly different in reality - but it is a good enough starting point).

This is an image I got earlier in the year - before short nights took over. Do you think I expecting too much and should just leave it alone?

 

Whirlpool ST AP (1).jpg

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That image looks pretty good to me, but I think it is worth checking the collimation when you have the opportunity. PM me if you have any queries. 

By the way, it was Es who did the first Ronchi test on my scope and although I had spent weeks getting my scope to give me the specified 1624mm focal length, Es declared it 'very overcorrected' with a barrel-shaped Ronchi pattern inside focus and pin-cushion pattern outside focus. The mirrors were too far apart. Bringing the scope to a corrected state required winding the secondary mirror inwards. Once we had correct parallel Ronchi lines - no spherical aberration - the scope had a focal length of 1660mm, somewhat longer than the spec. Such are the variations in the figures of the mirrors, I guess. 

By the way, if you are interested in doing a Ronchi test, here is a link to my note.

https://www.dropbox.com/s/i68gibodxszoga6/Testing with Ronchi eyepiece.pdf?dl=0

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I'm interested how you get on @Clarkey  I have a StellaLyra RC6 and am fairly certain my collimation is far from perfect (a couple of images here - M51 and NGC 3718 - and on AstroBin will probably show a more experienced user the tell tale signs).  Primarily I see odd shaped stars - almost triangular in nature, but when stacked, it's not so bad.

I've mainly just eyeballed collimation with a Cheshire but I realise that's not the best way, especially given some of the excellent advice & guides that @davies07 has provided here and on another great thread about RC scopes.  It probably sounds silly but I was having issues getting an accurate disc I could put into the opening (with focuser and tubes off) to align both mirrors... in the end I just put the focuser back on and used the Cheshire again 🙄

Mine is also solving for a FL that's off the manufacturer spec so need to look at that.  

Learning as I go, I've taken the pragmatic approach and have been happy to perform tasks on the bench, but am not comfortable sacrificing the rare clear UK skies if I have something that's "not awful" !

Edited by geeklee
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Well I have tried the Reego (TSRCKOLLI) and it has certainly worked to a point. I used a camera on live view with a zoom lens to get the secondary collimated. I did have slightly more of a problem with the primary as the line round the baffles I am supposed to look for (according to TS optics video) did not really exist.

Collimating a Ritchey Chretien telescope with the TSRCKOLLI tool - Bing video

I visually got it as good as I could. With hindsight I should have highlighted the baffle edge- maybe using some white tape or something similar. If I am doing it again I will certainly try this. Anyway, despite the general level of cynicism (from me and others) the Reego does seem to work fairly well and good enough that a final star test should yield good results.

As yet I have not managed a proper star test, however I can see the headlights on the M6 three miles away which works quite well. The collimation on the primary was slightly out (8th of a turn) which I have adjusted out. I have not done a full field check but so far so good. Once I get a real star test done I can start looking at optimizing the FL......

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  • 4 weeks later...

I have now spent the last two nights outside trying to finish the collimation on the RC8. Reego got it pretty close, but the star test was still a bit uneven. I have been using the DSI method which has been recommended from a number of sources, but despite 5 hours of adjustment I still cannot get it quite right. As it is not dark enough for imaging at least I do not feel I am wasting precious clear skies.

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

I can get the central area spot on by the primary adjustments, but the perimeter stars are slightly mis-shaped but balanced across the image. (All the outer stars are pointing slightly towards the edge of the image, as per image 3 on page 16 of the procedure). Does this mean I need to move the secondary mirror inwards slightly to extend the FL? I have increased the FL slightly, but I do not want to go too far in case I am doing the wrong thing.

Just to add - the star shapes in the OAG are poor which I would assume would also be linked to the collimation.

Any guidance greatly appreciated.

Edited by Clarkey
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  • 1 month later...
On 23/07/2021 at 14:10, Clarkey said:

I have now spent the last two nights outside trying to finish the collimation on the RC8. Reego got it pretty close, but the star test was still a bit uneven. I have been using the DSI method which has been recommended from a number of sources, but despite 5 hours of adjustment I still cannot get it quite right. As it is not dark enough for imaging at least I do not feel I am wasting precious clear skies.

A Procedure for Collimating Ritchey-Chrétien and Other Cassegrain Telescopes (deepskyinstruments.com)

I can get the central area spot on by the primary adjustments, but the perimeter stars are slightly mis-shaped but balanced across the image. (All the outer stars are pointing slightly towards the edge of the image, as per image 3 on page 16 of the procedure). Does this mean I need to move the secondary mirror inwards slightly to extend the FL? I have increased the FL slightly, but I do not want to go too far in case I am doing the wrong thing.

Just to add - the star shapes in the OAG are poor which I would assume would also be linked to the collimation.

Any guidance greatly appreciated.

Hi,

 

Did you manage to get your RC sorted?

I have spent a lot of hours trying to sort mine out now. 

I feel like I am just chasing my own tail though, it never really gets really good.

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Eventually yes. I sort of developed my own method. Technically it might not be perfect but it worked for me. Basically I collimated the secondary first using the TS RC collimator. Then I used a star test (or in my case car headlights on a road a few miles away) to get the primary right as per the DSI method. I then went back to the secondary with the collimator and repeated these steps until it was right. I might still get the Ronchi grating to confirm the FL is correct for the mirrors (currently it plate solves at 1626mm), but I am not unhappy with the result so far.

This is a single sub - the stars look OK to my eye.

 

Single Sub.jpg

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Technically you can do it with a star test. However, I did this with the TS RC collimator and an artificial star as described above. I was not paying £500 for the collimator! I did buy an M90 focuser tilt adaptor but it is not being used. Collimation is a bit of a pain - but it does not seem to move once 'dialled in'.

TS 2" Led RC Collimator | First Light Optics

 

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1 hour ago, Clarkey said:

Technically you can do it with a star test. However, I did this with the TS RC collimator and an artificial star as described above. I was not paying £500 for the collimator! I did buy an M90 focuser tilt adaptor but it is not being used. Collimation is a bit of a pain - but it does not seem to move once 'dialled in'.

TS 2" Led RC Collimator | First Light Optics

 

Thanks, yes I've seen those on the TS site, seems abit more reasonably priced 

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