Ritchey-Chrétien

[GazzMeister]

Hello Everyone

I'm just asking out of interest (noseyness...) but, what the heck is a "Ritchey-Chrétien" telescope? I'm finding it hard to find info that isn't involved with Meade selling me something. How much difference is there between a "Ritchey-Chrétien" and a normal schmidt? Stupid question, I know but you've all been very tolerant in the past :-)

Any answers much appreciated.

Gareth

[g__day]

There are various ways of building mirror lens systems to focus light, parabolic mirrors, hyperbolic mirrors, spherical mirrors etc.

Each has trade-offs - such as costs to manufacture, and benefits - optical performance - lack of coma.

The RC is a particularly good but hard to manufacture, costly design. See here:

http://www.rcopticalsystems.com/

esigned principally as a photographic instrument by American George Ritchey and Frenchman Henri Chrétien, the Ritchey-Chrétien optical design is coma free. Coma is an aberration common in all reflecting telescope systems. The Ritchey-Chrétien design utilizes a hyperbolic primary mirror and secondary mirror. This design corrects for coma and results in a smaller spot size on and off axis

http://www.atscope.com.au/rcos.html

Meade Instruments has recently announced an "Advanced Ritchie Chrétien design" telescope. Sadly their advertising hyperbolae is very misleading about type of optical design these telescopes employ. They are clearly not Ritchie Chrétien's. The Ritchie Chrétien design ( inverted in 1910s by George Willis Ritchey and Henri Chrétien, their design is used in many, if not most of the largest professional astronomical telescopes ) uses two hyperbolic mirrors only. The Meade RCX design is a clear step ahead of the standard schmidt cassegrain telescope, however, any design that uses a correcting plate is a Catadioptric. As a consequence, their design has off-axis chromatic aberrations which are shown "spot diagrams" below. The polychromatic spot size matrix shown here clearly shows the chromatic error introduced by the addition of a corrector plate. The design employed in the Meade RCX has been well known to optical designers for over a decade, and is more correctly called an aplanatic schmidt cassegrain design. The design is not without merit. Though the spot sizes are significantly larger than a classical Ritchie Chretien, the off-axis performance is superior to a standard Schmidt Cassegrain.

[jase]

http://en.wikipedia.org/wiki/Ritchey-Chr%C3%A9tien_telescope

Not to be confused with Meade's version of the "Advanced Ritchey Chrétien" design. This is not a true RC.

For the real thing go here;
http://www.rcopticalsystems.com/
or
http://www.opticalguidancesystems.com/

Both the above manufacturers get the hyperbolic mirror sets (primary and secondary) from Star Instruments - http://www.star-instruments.com/

I feel sure Fred will chime in being the owner of a fine 10" RCOS. ;-)

[acropolite]

This article (http://www.rfroyce.com/cassegrains.htm)explains the differences quite well, as the others have said, the Meade RCX design isn't really a true RC as the primary mirror is sperical and a combination of corrector and hyperbolic secondary is used to achieve similar optical characteristics.

[Stephen65]

Meade's design is in no way a RC let alone an "advanced" RC. What it is is an improved SCT. The "advanced RC" and "Hubblie in your backyard" claims are marketing gumf. I believe there is litigation going on in the USA right now where Meade is being sued by a maker of real RC's over these misleading claims.

[Dennis]

And if that were not enough, my Takahashi Mewlon 180 Cassegrainian is a Dall-Kirkham design!

From Wiki:
"It (DK) uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis".

Cheers

Dennis

[ausastronomer]

A true Ritchey Chretien uses a hyperbolic primary and a hyperbolic secondary mirror, with "NO" corrector of any description. Importantly, the curves of both the primary and secondary mirrors must be perfectly matched to each other, to optimise optical performance.

As others have said Meade's claims as to an Advanced Ritchey Chretien telescope is pure advertising hyperbole. Only 1 of it's 3 main optical components match that of a true Ritchey Chretien. That being the hyperbolic secondary. That doesn't mean it isn't a good telescope, it means it isn't a true RC.

True Ritchey Chretiens 5, Meade 0, but who's counting :)

Cheers,
John B

[Bassnut]

Thanks for putting me on the block Jase ;-).

Im no tech guru, I just irritate ppl ;-). Ive got an LX and an RC (and have had an SCT), I like them both, but they give different image results, the LX is better than the SCT, and, despite what meade say, the RC is significantly better than the LX (given the rest of the rig is of a standard to show the difference). I cant give you fancy figures, suffice to say, stars are smaller and focus is tight as on the RC. But this difference BTW is not just due to the RC primary mirror. Meade also has a correcting lens which they tell me makes a difference, and since RCs are more expensive to make, RCOS for instance spend big on other aspects too which also make a huge difference. ie Ceramic mirror, carbon tube (reduces focus drift), secondary focuser, controllable cooling fans, inbuilt dew heater, I was amazed at the level of sophistication just in the OTA electronics.

In short, its not just the optics that seperate the "RC clone" LX from an RCOS RC, its the level of engineering generally, it all counts in final image quality.

Having said all that, I am more than happy with the LX200, for the price.

Its hard to make an objective evaluation on "real" RCs or wannabies based purley on optical specification, when there are so many other technical factors to consider that affect image quality, and the HUGE gap in cost. I think the "real RC or not" arguement is a bit over cooked for the average imager (unless money is no object). With carefull set up and image processing, the Meade LX series can produce awesome images. In fact I think youd be hard pressed to tell the difference (just on optical confurguration) unless all other factors were were at an optimum, including a very good mount and carefull image processing.

[Shawn]

Well Said Fred, I for one am very happy with my Advanced R C....:P


:D

[Shawn]

Although the mount ,,,,hmmm not good enough for the 14er Im afraid, not for its optics to be taken seriously anyhow....:(

[Bassnut]

Shawn. I messed some details, I mean the LX200"R", and then just the OTA. I dont have experience with the LX200R mounts (my OTA was on a G11), but from what ive heard, yes, the Meade mounts do let the OTA down for long exposure astrophotography.

[Gama]

Bottom line is, look thru one, if your happy with its views, or it does what you need or want, then buy it.
For some, its a magnificent viewing scope. For others, its not what they expected. Test driving anything you buy is a must these days.

I am very happy with my 14"RCX400, it produces images that i expect for its class of scope.
Anyone who digs out the money and expects it to be a RC in terms of the optical train, then they have more money then sense. I investigated for many months about what i was buying before laying out $18K for a RCX as apposed to $50K for its legit cousin.
There are so many optical variants, each with its pro's and con's, that its really left up to the individual and for what purpose he intends to use it in.

Theo.

[avandonk]

I agree with you Gama. Get the best you can in your budget and use it to your and its full potential. Gee I have to put up with really lousy optics with my 14" compared to the Hubble.

Next step put my backyard in orbit?

Bert

[Bassnut]

Too right Gama, thats the truth, You cant beat an LX200R 14", or in your case the RCX400, for the money, nothing else comes close, the RCX has many inovative features. I got the RCOS RC/ME for commercial gain, because these brands are derigure for those who pay by the hour (for them, hell, why not ;-) , im happy to go back to the 12" LXR/G11 for my own use. (You long FL big aperture guys might consider a better mount in the future tho ;-).

[Bassnut]

I have to say, in case I get trashed by Gurus, top gear is certainly an exquisite luxury, no mucking around, no fiddling, it does exactly what its supposed to do. Its another world, and the ME is really the only robotic mount suitable for totally remote imaging. Horses for courses.

[Gama]

One thing many people overlook, and thats a Newtonian with some good WYnne correcting optics.
It can produce images as good if not better than an RC, and cost so much less.
F ratios are much shorter for these types of scopes, normally around f3.5 or so, but you could afford to go bigger in aperature to get your image scale up a little.

Theo.

[Satchmo]

I would think also that a good F3.5 Newt with a 2" 2X Barlow ( so working at F7 ) should give quite acceptably coma free images across the field at similar to F# of an RC for a fraction of the price.