Ritchey-Chretien collimation

[Karlzburg]

Hi all,

I'm going to do a write up for collimating a Ritchey-Chretien telescope, as you know they can be a beast to deal with ( a phrase taken from a convo today, thanks Alex)

I've read the net and watched the youtubes none have given anything of much use or explanation, I will be putting up pictures as well.
I've tried quite a few different ways shown on other forums and blogs with no real success, I'm mechanical minded and excellent problem solving skills to work out the issues behind doing them.

Cheers
Karl

[DJT]

Hi KarlAre you doing the write up because you have figured out how to do it? Sounds like not?

[Karlzburg]

Yes I am doing the write up as I have worked it out.

[Stonius]

Look forward to reading it :-)

[DJT]

Likewise!

[pmrid]

You can be certain of a wide audience when you do release this write-up.

The part which I would hope to see included is that dealing with collimating the primary. I have committed the cardinal sin of adjusting my GSO RC8 primary mirror - out of frustration combined with ignorance.

Currently I am working with a borrowed Tak collimating scope (thanks Rick) and believe I have managed to get it just about right. I've tried that method, the "hall of mirrors" method, a Howie Glatter projection system and others and none have been very satisfactory. No doubt this is a case of a poor tradesman blaming his tools. All of this is why I await your work with truly bated breath.

Peter

[ChrisV]

Jeez and I thought a newt was hard to collimate. It'll certainly provoked some discussion Karl

[Karlzburg]

Newts are a walk in the park compared to RC's. Though very fiddly and a cause for another couple of greys.
I haven't used a Tak scope, it's all been Howie with circle work.
I have my focuser squared with my secondary through full rotation of the focuser, I noticed that if I did move the focuser to allow a few more stars into a shot it there collimation off as it was set at one position, which made me think even moving the camera alone would still put one side off. So sharp stars across the sensor would be gone.

Hoping to have a clear night tonight for a star test.

[Karlzburg]

Adjusting primary will be in it too Peter as it's one part of 3. Feom what I can work out it's like the final fine adjustment to the lot.

[Marke]

Thats correct after everything else is in alignment you finish with primary adjustment :thumbsup:

[John K]

Also looking forward to this! About time someone from IIS took the lead on this as there are lots of CDK/RC scopes out now with forum members.

[Karlzburg]

I've noticed quite a few for sale lately and wondered if it was due to poor collimating? Like most things these days it's easier to get rid of and get something different than fix what you have. Very much a throw away society this day and age.

[Stonius]

I think it's the gnawing uncertainty - could it be better? Was it the seeing or my collimation? That kills people in the end, even if they're getting quite reasonable FWHM's

[glend]

Just about all the RC collimation drama threads have to do with the nightmare that are the GSO made small models 6"-10", which are the budget end of all RCs. I have never heard any complaints about Planewave RCs.
Perhaps the write up should be GSO specific.
Good luck.

[Stonius]

True, but ppl who can afford a planewave are probably more experienced

[RickS]

Planewave scopes aren't Ritchey–Chrétien. They are a corrected Dall-Kirkham. The difference is that the CDK has a spherical secondary and it doesn't matter if it isn't perfectly centred. This makes them somewhat easier to collimate.

Cheers,
Rick.

[glend]

Ok thanks. So the next question is, who, besides GSO makes RCs for the amateur market?

[RickS]

There are some high end suppliers like Alluna and Officina Stellare. GSO is the only low-cost vendor that I know of.

[Marke]

There is a huge leap from GSO to anything else but I think WO are working on something I have seen a 14" they did with a 12" on the way I believe.

[DJT]

Hi GlenThere is a huge gap between GSO and the other manufacturers who are all pretty much in same price range, around the 10k plus mark. As I understand it, the complexity is in the mirrors? But the cost ramps up as other materials are used along with better mechanical design.


There are no other non clone manufactures doing 8 inch mirrors from what I can see and that is the sweet spot for a lot of amateurs mounts. CFF has a 10” but that’s still going to cost 10k landed, 3x the cost of the GSO RC10 and would stretch most mounts capacity.


so whilst mechanically the GSO is a bit needy, there are plenty of astrophotographers out there using them successfully. Kudos to GSO for coming up with a way to produce the mirrors so they are affordable and quite good.


Fabiomax posted up a couple of really great images recently and he has the budget RC8 with QSI CCD setup.


Very much looking to see a simple Collimation write up. But I still think dropping $1500 for a decent focuser, t-ring and tak scope is well worth it for the images you can get from these scopes given focal length.

[DJT]

Hi GlenThere is a huge gap between GSO and the other manufacturers who are all pretty much in same price range, around the 10k plus mark. As I understand it, the complexity is in the mirrors? But the cost ramps up as other materials are used along with better mechanical design.


There are no other non clone manufactures doing 8 inch mirrors from what I can see and that is the sweet spot for a lot of amateurs mounts. CFF has a 10” but that’s still going to cost 10k landed, 3x the cost of the GSO RC10 and would stretch most mounts capacity.


so whilst mechanically the GSO is a bit needy, there are plenty of astrophotographers out there using them successfully. Kudos to GSO for coming up with a way to produce the mirrors so they are affordable and quite good.


Fabiomax posted up a couple of really great images recently and he has the budget RC8 with QSI CCD setup.


Very much looking to see a simple Collimation write up. But I still think dropping $1500 for a decent focuser, t-ring and tak scope is well worth it for the images you can get from these scopes given focal length.

[Paul Haese]

Hmmm, sorry I don't agree with that statement, it is generalised and does not account for fast Newtonians.

RC's are relatively simply due to the symmetrical nature of how errors are shown. Correcting the errors is easy to undertake with the right tools. Collimation becomes simple once you understand what is going on with them. Clearly, with an RC, you need to centre the focuser to the secondary (using Howie Glatter laser), then align the secondary and primary (using a Tak scope). Ken Crawford produced a very instructive video on how to use a Tak scope for collimating this type of scope and I suggest many go and take a look at it. I can generally collimate an RC now within 5-10 minutes. Finally, star testing to tweak collimation on the primary.

A fast Newtonian on the other hand are a much harder beast to deal with. I have owned quite a few Newtonians over the years but all of them except the latest ones were f4.5 and up. Yes those are a walk in the park for collimation and less demanding. f4 and below might as well be in the impossible range. The problem lies with weight, the way the secondary is supported and size of the secondary. Getting the secondary in the correct position to produce a flat coma free field is time consuming and frustrating. The faster you go the harder it gets.

Any mechanical errors in both types of scopes makes things difficult to sort collimation and hold collimation. GSO has produced a functional system if you don't use their focuser. All the rest is rigid and put together well. Certainly from the RC10 and upwards. Yes not an Alluna scope but still something that can produce great results.

[glend]

Paul, what focuser would you recommend as a replacement for the stock GSO?
:thanx:

[lazjen]

There's a few options - I went with the Moonlite for my RC10 - excellent unit.

[Karlzburg]

I have a moonlite on mine with a collimation ring.

[Marke]

I have rack and pinion Feather touch with motor it can lift a lot of mass and zero slip along with an adjuster plate.

[DJT]

Interesting...I heard that CFF purchased 10” GSO mirror sets that they will install in their OTA to help resolve the mechanics issue.There is an add on astromart.




https://astromart.com/classifieds/astromart-classifieds/telescope-ritchey-chretien/show/rc-250mm-f7-f8-stock-or-short-time-delivery

[RickS]

When I had a GSO RC10 I used a FLI Atlas focuser. It worked great but was possibly overkill :)

[Wavytone]

I'd agree with that having built a 12.5" f/3.7 and a smaller f/3.5 long ago when I had a workshop and the tools to make a decent secondary cell and spider. Not quite impossible, but certainly damn difficult.

The 12.5" was intended to be a classical Cassegrain - I got as far as figuring a secondary but never finished the scope when I realised how massive the mount would have to be. Its temporary configuration as a very fast altaz newtonian became permanent.

Never again. f/7 - or close to it - is nirvana IMHO for fast reflectors and refractors, apart from lunar and planetary which is where the larger maks from 7" and up have a whole game to themselves.

[Paul Haese]

There are a number which fit the bill Glen. The Feather Touch is the most affordable and robust but does take up a lot of back focus. I wouldn't use a standard Moonlite, they just don't have the strength to hold heavy loads still. Second on my list is the Atlas, great focuser but costs a fair bit and has a very shallow draw tube which can cause problems for adapter calculation. Then there is the Moonlite Nitecrawler; also a great focuser and has the added feature of rotation. That costs even more but if you need accurate rotation this is the focuser of preference in my opinion. Draw tube length is modest but longer than 10mm. Very happy with this focuser.

[DJT]

Just on that, I have used an STL6303e with FW8, and an MMOAG plus guide camera for the last 3 years on an 8” and on a 10” RC and it’s never slipped for me. This is the CLS 2.5 version.
All told that’s 2.5kg including filters.
That’s just on the limit quoted by Ron.

[Logieberra]

Add GSO RC 12”, 14” and 16” to that list.

[redtail]

cant wait to see the write up.

[brenchen]

Hey Karl! I'm feeling so much pain right now with collimating it. How's the write up going? Can't wait to read it :)

[Slawomir]

A good list with great explanations. I would also add Optec - these focusers seem to be even more robust that FTF, in particular in regards to the way these focusers are motorised, and there are a few options to choose from, from Optec LEO to Gemini.

[redtail]

I don't think this write up is ever coming.

[luka]

While waiting make sure you have a look at this thread (http://www.iceinspace.com.au/forum/showthread.php?t=174594) for a very nice and detailed guide to RC8 collimation.