Ritchey-Chrétien Questions

[Catanonia]

I have a RASA 8 on a EQ8-R which is great for F2 widefield paired with a ZWO 2600MC Pro.

It has its downsides with changing filters due to back focus and have been contemplating swapping it for a RASA 11. This is £4600 or more upgrade (yes will get some back on sale of the RASA8)

On top of that I would really need to upgrade to a full frame camera from the APC 2600 to get the full benefit from the 11, another 4k or more !!!!

What would I get from it all, the same really with the ability to change filters easier, but the same as the 8 nearly.

Well I don't have a long focal scope and so galaxy season is really out of the question for the Rasa

This has got me thinking, how about one of these below from FLO, keep the Rasa8 and have this as well for a fraction of the upgrade costs ???

 

I have never used one of these scopes before, I understand some of the principle, but have some questions to those in the know if you would be so kind.

• What are they like to collimate ?
• Do they produce flat fields, or is a corrector needed ?
• What drawbacks do they have ?

 

 

 

Edited February 10, 2022 by Catanonia

[vlaiv]

50 minutes ago, Catanonia said:

What are they like to collimate ?

Some people have very tough time collimating those. I managed to collimate mine without much issues (have 8" one).

51 minutes ago, Catanonia said:

Do they produce flat fields, or is a corrector needed ?

They produce flat field up to a point - after that you need some sort of corrector. On 8" version I use 4/3 sensor without issues. For APS-C sized - field flattener should probably be used (although it says corrected field up to 28mm).

52 minutes ago, Catanonia said:

What drawbacks do they have ?

They are back heavy - plan for counter balance on the tube itself (I use 1kg Baader that attaches to the vixen rail).

Stock focuser is not the best for AP (not sure which version comes with StellaLyra version - mine had simple 2" monorail with clamping attachment - I replaced that with 2.5" R&P and M68 threaded attachment).

Other than that - I've seen quite a few people not happy with theirs and selling it. I guess it is down to "ease of use". I'm quite happy with mine and don't find it particularly hard to use.

[Catanonia]

@vlaiv Many thanks, most useful and some topics for me to research.

[Clarkey]

Like Vlaiv I have the 8" and I was very reticent initially due to the collimation issue. It did take a bit if time to get right, but now I have a method that works for me and once dialled in, it does not move really.

I would agree that up to APS-c you can probably get away without a flattener. I've not got anything bigger so cannot comment.

Personally I am a fan, and this scope is a definite 'keeper' in the inventory. I think the focuser on the 10" is the same as the 8 so I expect you would want to change.

[kirkster501]

I never had any issues collimating my RC and I made some videos about doing so many moons ago.  You can do the collimation with a Cheshire cap just fine.  I ultimately sold my RC because I prefer refractors for any sort of DSO imaging and now use either my TEC140 or my FSQ85 for that purpose.

[Same old newbie alert]

4 hours ago, kirkster501 said:

I never had any issues collimating my RC and I made some videos about doing so many moons ago.  You can do the collimation with a Cheshire cap just fine.  I ultimately sold my RC because I prefer refractors for any sort of DSO imaging and now use either my TEC140 or my FSQ85 for that purpose.

Seen your videos, informative and a awesome setup

[Grifflin]

I got the RC 8" f8 last year but haven't used it in anger yet.  I did put it on the mount and have taken some images (M101, M51) just to work out balance , back focus etc.  Since then I have taken the plunge and finally got round to collimation which , worryingly, I found fairly straight forward....so I have probably done something wrong!  Haven't had a chance to use it since collimating but it will be on the mount for the next clear night.  Planning on imaging Mel 15 to compare with the images taken with the SW 80 ED.  I'll let you know how I get on.  Equipment as per signature.

Neil

[davies07]

I believe that the StellaLyra 10" RC that you mention is a fine telescope. It has the focuser and image train attached to the backplate of the scope as it should be. This contrasts with the 8", which has the image train hanging off the back of the primary mirror mechanism. The 8" can be a challenge to collimate and is best avoided, in my view.

I think that a full-frame camera on the RC would challenge the quality of the optics out to the edges of the frame.; you might see some astigmatism creeping in. I would have thought that you would need a flattener. Your ZWO 2600MC should work fine but the pixels are a bit on the small side (giving bloated star images) for the focal length so I would be inclined to install a X0.7 reducer.   The 071MC with its bigger pixels could be a better choice. As mentioned, you might also want to upgrade the focuser and you will need off-axis guiding. I suspect that you would be seeing limited at the focal length of the RC10 and would gain little in terms of resolvable detail over a, say a large refractor, which would be less hassle but cost more.

David