Will Ritchey-Chretiens change the world of astrophotography?

Siguy · #1 (**permalink**) 27-April-2008, 02:36 PM

Yesterday at the NorthEast Astronomy Forum I saw something that might be revolutionary. It was a prototype 8'' RC made by GSO and sold by Astronomics under the Astro-Tech label. The finished model with a carbon fiber tube and better optics will be $2995. There will also be a 6'' for $1295 and a 10'' for $4995. Sure, they're twice the price of Meade ACF, but they are true Ritchey-Chretiens and are still much less than half the price of the RCs on the market right now. Some might cringe at the thought of a "made in China" RC, but this could be the beginning of affordable (I'm using "affordable" somewhat loosely) flat-field astrographs!

mahesh · #2 (**permalink**) 28-April-2008, 11:17 AM

Why cringe, Siguy?

I recall Dave Mitsky has commented on the subject of RCs, on this board.

ok, I had a look. Back in december 2006. I'll try to link it.

mahesh · #3 (**permalink**) 28-April-2008, 11:46 AM

here we are...more info....
i can't link it though...

need educating.....

Dave's comment December 2006:

Quote:

Originally Posted by Dave Mitsky

From the Meade home page:

http://www.meade.com/rcx400/index.html

"The dream of owning the ultimate Advanced Ritchey-Chrétien telescope is finally here."

http://www.meade.com/lx200r/index.html

"Meade’s all new LX200R brings Advanced Ritchey-Chrétien optics within reach of aspiring astronomers everywhere. Nearly every observatory reflector in the world is a Ritchey-Chrétien, including NASA’s Hubble Space Telescope. Now you can own what the professionals own."

Meade's cavalier and improper use of the term Ritchey-Chrétien* has resulted in litigation.

http://www.rcopticalsystems.com/FactsofCase.html

http://www.rcopticalsystems.com/FactsofCase01.html

Is this a case of instant karma or what? (Meade sued Celestron not all that long ago.)

When all is said and done, the fact remains that R-C Cassegrains have only two optical surfaces and do not have corrector plates.

Dave Mitsky

From http://www.astronomy.com/asy/default.aspx?c=ss&id=9

Ritchey-Chrétien

This telescope design was developed jointly by American optician George Willis Ritchey (1864 – 1945) and French optical designer Henri Chrétien (1879 – 1956) in the first decade of the 20th century. These two telescope designers found that the lower the amplification factor of the secondary mirror, the flatter the field. The Ritchey-Chrétien system has a secondary mirror that magnifies 2.7x, whereas the Schmidt-Cassegrain design has a 5x secondary. The Ritchey-Chrétien design is coma-free, whereas the Schmidt-Cassegrain is not. Ritchey-Chrétien telescopes have hyperbolic primaries and secondaries that correct for coma; production-type Schmidt-Cassegrains use a spherical primary and secondary and do not correct for coma. Finally, the Ritchey-Chrétien design has two optical surfaces. All Schmidt-Cassegrains have four. Why are Schmidt-Cassegrain telescopes more popular (by far) than Ritchey-Chrétiens? One word: price. A Ritchey-Chrétien telescope is quite costly to produce and, therefore, expensive to buy.

* http://www.rfroyce.com/cassegrains.htm

* http://en.wikipedia.org/wiki/Ritchey...tien_telescope

Are the prices really coming down so steeply?! Nice!

I reckon , about an RC...affordablity is always going to be painful. Owning / sharing one, a pleasure!
Using one, simply Heavenly!

RickJ · #4 (**permalink**) 28-April-2008, 03:22 PM

Note that while RC scopes have far less coma and a somewhat flatter field than standard cassegrains the field isn't totally coma free as there's 5th order coma. Also there's an astimatism problem that can crop up. For large fields you do need a flattener. It has to be specific to the scope as well. See: http://www.rcopticalsystems.com/redcorr.html

They are easier to correct than a standard cassegrain or newtonian or even Meade's so called RC like system. With say an ST-10 there's no need for any correction but with today's super large CCD's or medium format film you'd want a corrector.

I find nothing on this new line on Astronomic's website. Most chinese optics I've seen are good but not great. Most mirrors seem to have a zonal problem that while small would scatter light for astrophotographic purposes looking like poorer seeing than there really was. Mechanics are highly important in RC systems and part of the high cost. I'd worry those aren't up to snuff as well. I suspect they'll be about the overall quality of Meade's modified SCT's rather than RC Optical System's or Star instrument's quality. Good but not great. The lack of a corrector plate and its blue chromatic abberation would give it a slight advantage over Meade's scope if the rest of it is up to snuff.

I'd say "interesting but show me" is my attitude for now.

Rick

Siguy · #5 (**permalink**) 28-April-2008, 07:32 PM

Well, the mechanics seemed pretty "up to snuff" since it used a nice GSO Crayford focuser rather than moving the mirror. It also seems like they will have carbon fiber tubes (great feature in a scope of their price) and maybe cooling fans.

Edit: I just saw some specs. The final production model will have a Feather Touch focuser, CF tube, and dialectric coated quartz optics that are at least 1/12 wave or better. Sounds like a sweet deal for somebody who has this kind of money...

RickJ · #6 (**permalink**) 29-April-2008, 05:45 AM

RC scopes are best focused by moving the secondary. Even Meade does this with their with their LX-400 line. A fixed camera position is a great advantage. You can add a lot of gear without worry the focuser won't handle it. I have 2 CCDs, a web cam and an eyepiece position on the back of my LX-200R. It took a very expensive 3" focuser to handle the weight without sagging or slipping. Also focus is so critical with an RC you need electric focus for best results. Is that an option? It should be standard. It is on American RC scopes.

I've seen so many claims for wave accuracy that are just hot air I doubt the 12th wave optics claim will really pan out. True 12th wave is very difficult to achieve! Smoothness is more important for this system than some simple wave calculation by a method not specified.

I hope I'm wrong. In any case it should be at least as good and possibly better than Meade's LX-200 line without the disadvantages caused by the corrector but I'm not convinced it will equal RC Optical System's scopes. Once they are on the market we'll know more. If they've done the carbon tube correctly that will be a great help as well. But how well it will hold collimation and how easy it will be to collimate -- very critical in an RC, will the focuser carry the weight of today's large CCD's without sagging or slipping are questions still to be answered. They are filling in the under 10" size which is good strategy.

Rick

RickJ · #7 (**permalink**) 29-April-2008, 06:55 AM

I see there was an American made RC introduced at NEAF in the same price range. It does have the features I was mentioning. Field flattener for large CCDs, moving secondary focuser, secondary heater, etc. Optics by Star Instruments so you know the quality will be there. Only in 10" size with 14" coming. Price is $5000 without corrector and $6000 with. Though the instrument controller at $300 is a necessary addition so raise those by $300. Not covered is how well it holds collimation and ease of collimation. Only user reports will likely cover this.
http://www.deepskyinstruments.com/products.htm

Rick

skintigh · #8 (**permalink**) 02-May-2008, 02:51 PM

These posts make the Meade LX200 ACF models sound clearly superior to Celestron optics, but then I read the opposite elsewhere. Are there any sites that actually do scientific reviews, or is all just personal experience, and do you have to be a guru to even notice the difference?

Thanks

RickJ · #9 (**permalink**) 03-May-2008, 03:39 AM

Quote:

Originally Posted by skintigh

These posts make the Meade LX200 ACF models sound clearly superior to Celestron optics, but then I read the opposite elsewhere. Are there any sites that actually do scientific reviews, or is all just personal experience, and do you have to be a guru to even notice the difference?

Thanks

Depends on what you are doing.

For deep sky photography the Meade is superior because it has far less distortion to the field of view and you can lock the mirror. While Celestron has limited mirror flop it is still an issue from what I've heard. If you use a small CCD with a flattener then the difference is smaller but with added elements you have more color issues and reflection issues.

For visual use and planetary photography the Celestron has the advantage of a slightly smaller secondary. I find current optics to be about equal except for that. Meade's quality control is a bit poorer from my experience and Celestron's 9.15" tends to be superior to anything either turns out. My mirror lock arrived not working. I found the fix on line so it wasn't a problem for me but could be for others. Meade LX200 series being designed for deep sky work is more massive than the Celestron.

Basically I'd go Celestron for non deep sky work. But since that is my main interest I got the 14" LX200R where I could use the full field of view without a compressor and therefore have higher resolution.

I'd never buy any RC or RC like scope for anything but deep sky photography. Though the images in my 14" LX200R are far superior to an older C14 I used for many years. But back then neither company turned out optics of the quality they do today.

Rick

dazastar · #10 (**permalink**) 05-May-2008, 09:57 PM

OK, lets not bag chinese and asian optics, Celestron....there Synta - a chinese company and GSO make some of both Celestron and Meads gear as well. also, Meads APO refractors are chinese made maxvisions re-badged. In fact most of the optics are made by either synta or gso for both these guys.

Here's another exsample, Williams Optics are re-badged Long Perng as to a few other brands.

Kokusai Kohki in japan make University Optics ortho's, Mead ortho's, Antes orthos, Celestron orthos etc etc which these brands sell from $60 though to over $130 for the same thing Yet you can buy them direct for under $40.

Siguy · #11 (**permalink**) 06-May-2008, 08:59 PM

You can already preorder them from Astronomics. Too bad the 6'' doesn't have the features of the 8''.

http://www.astronomics.com/main/cate...F9DX9A0/Page/1

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