Ritchey-Chrétien vs Classical Cassegrain for AP

[Jonny_H]

Hi all,

I hope everyone is doing well.

I am potentially in the game for a new scope which will be used mostly for planetary AP or extreme close-up lunar shots.

What are the pros and cons of RC's and CC's for AP?

To give two examples - I have been eyeing-up the StellaLyra RC 8" f/8 and the CC 8" f/12. The latter has a very impressive focal length but the RC seems to be a popular choice due to no coma, chromatic abberation etc...

Any views or pointers would be appreciated.

 

Thanks,

Jonny

[johninderby]

The CC keeps collimation and usually doesn’t need collimating when received although it is also easy to collimate if needed. Uses a Hyperbolic secondary mirror and parabolic primary mirror. The longer focal length and smaller central obstruction would make it ideal for lunar / planetary use.

The RC is more suited to DSO use. It is harder to collimate than the CC due to the Hyperbolic primary and secondary mirrors. The larger central obstruction doesn’t help with lunar / planetary in mind.but not a problem for DSOs.

The CC also doesn’t suffer from coma or CA either. 

BTW the photos on FLOs site for the CC are wrong and show the front of the RC with it’s large central obstruction.

Edited June 28, 2021 by johninderby

[Nik271]

I don't think either RC or CC have chromatic abberations because they are reflectors.

For DSO astrophotography the RC is better because it's better corrected and has faster focal ratio. For planets you only need the central parts of the field so CC has an advantage with longer focal length.

Edited June 28, 2021 by Nik271
typos

[Jonny_H]

28 minutes ago, Nik271 said:

I don't think either RC or CC have chromatic abberations because they are reflectors.

This is a very valid point. Evidently I got a little carried away when typing!

[CraigT82]

For planetary and hi res lunar I'd go with the the CC due to the extra focal length.

With a 2.4um camera (178) you don't even need a barlow

[johninderby]

For lunar / planetary the smaller the central obstruction the better which gives the CC the edge.

[Quetzalcoatl72]

I like the RC so far for dso, not tried it on planets yet but i'm hoping it's better than my celestron c5. Here's an example with my 600d dslr, it's not great because I don't do many subs, still a beginner. But i hope it gives you some idea

[Jonny_H]

Thanks all for your comments.

 Very helpful as always.

Now to pluck up the courage to add to the eye-watering amount of £ I've already spent on AP! 😲

[Guest]

14 hours ago, johninderby said:

The CC keeps collimation and usually doesn’t need collimating when received although it is also easy to collimate if needed. Uses a Hyperbolic secondary mirror and parabolic primary mirror. The longer focal length and smaller central obstruction would make it ideal for lunar / planetary use.

The RC is more suited to DSO use. It is harder to collimate than the CC due to the Hyperbolic primary and secondary mirrors. The larger central obstruction doesn’t help with lunar / planetary in mind.but not a problem for DSOs.

The CC also doesn’t suffer from coma or CA either. TW the photos on FLOs site for the CC are wrong and show the front of the RC with it’s large central obstruction.

 

 

I'm interested in this too but according to one reference I found the CC does suffer from COMA which is something that the RC fixes.

http://slittlefair.staff.shef.ac.uk/teaching/phy217/lectures/telescopes/L07/index.html

 

[johninderby]

Never noticed any coma with the CC although it is used for lunar / planetary. 

The RC design does suffer from astigmatism though so no design is perfect. 

Edited June 29, 2021 by johninderby

[vlaiv]

Probably best scope for planetary AP is very slow CC.

StellaLyra one is rather fast. You really want something like F/15 or slower (I've seen F/24 models with tiny secondary), but these are very expensive instruments (not because of F/ratio - but simply because they are made by companies that produce expensive instruments). For example:

https://cfftelescopes.eu/product/classic-cassegrain-250mm

8" F/8 newtonian with optimized secondary for planetary lunar will probably be better instrument - but very cumbersome and not easy to mount and use.

Above StellaLyra 8" is really best choice / compromise in terms of cost, size, performance ...

[johninderby]

The CC isn’t perfect but for performance vs cost is a great buy. You would need to spend a fair bit more to better it so yes it’s a great comprimise. 👍🏻

One upgrade I would recommend though is fitting a Baader Steeltrack focuser. The stock focuser is usable but for AP an upgrade is a good idea.

Build quality (other than the stock focuser) is excellent. Had someone who owns a Tak Mewlon take a look at my CC and he was impressed by the build quality. 

Edited June 29, 2021 by johninderby

[nfotis]

For planetary/lunar details, you need a long focal distance. For me, the Skymax 180 is a very capable "planet killer", and the CC8 should be near it regarding focal distance (a bit more aperture). I dislike spider vanes myself (and open tube construction), so I went with the Skymax 180.

The RC would be more suitable for DSO objects, if you manage collimating it (their design is quite sensitive to small diversions from ideal). The RC6 would offer a quite wider field of view.

N.F.

 

[Jonny_H]

Hi all,

Apologies for the delayed response. I have been up in Norfolk visiting family for the past few days.

Thank you for your comments so far. I think I have set my mind on the CC over the RC. Primarily as it will be used solely for planetary work (a little close up lunar also). I have another scope for DSO work.

One question I have now....do I go down the 2x Barlow route and carry on using my 533mc pro? Or would it be best to get something like a 224mc? This based on getting the 8" f/12 as previously mentioned.

Comments welcome

 

Thanks,

Jon

[johninderby]

 

The TS version of the CC8” states the following. An additonal correction lens isn´t needed, because the image field is well corrected up to APS-C sensor format.

[Jonny_H]

24 minutes ago, johninderby said:

 

The TS version of the CC8” states the following. An additonal correction lens isn´t needed, because the image field is well corrected up to APS-C sensor format.

Thanks John.

So if I have read (and understood) that correctly - it wouldn't be wise to use a Barlow but rather invest in another camera to get me the results I am after?

[johninderby]

I’m sure someone will be along shortly to recommend what camera would be best. 

[Jonny_H]

22 hours ago, johninderby said:

I’m sure someone will be along shortly to recommend what camera would be best. 

Thanks John.

Yes it would be good to hear everyone's thoughts.

I'm my simplistic mind - if I were to go down the Barlow route, I may be better off just sticking with my 6" f/5 newt and slapping a 2xBarlow on it. That coupled with something like a 224MC. Appreciating of course that the StellaLyra is more naturally suited to planetary without the need for Barlow and the like.

Hmm.....another food for thought.

 

Regards,

Jon

Edited July 5, 2021 by Jonny_H