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Arief
super member
Reged: 06/02/04
Posts: 439
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Here's the product description of one of russian klevtsov cassegrains:
"The TAL-250K provides an aperture of 250mm and focal length of 2130mm in a very compact tube length of only 450mm. Klevtsov's revolutionary catadioptric design employs a sub-aperture meniscus correcting lens (like the familiar Maksutov) combined with a Mangin mirror-lens secondary. Its fast focal ratio of f/8.5, flat field, and excellent correction of coma, spherical aberration, and astigmatism (14 micron star images at a distance of 10mm from the optical axis!) make the TAL-250K a superb instrument for astrophotography, far surpassing conventional Schmidt-Cassegrain and Ritchey-Chretien systems. Every optical component must pass stringent tests. Interferograms on three axes of each optical set are retained by the factory's quality control department."
The price is much cheaper than a 10" Mak, Dall-Kirkham, or Ritchey. So, if the performance is what they are stated, why are they not popular?
clear skies,
Arief
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Levente
sage
Reged: 01/20/04
Posts: 293
Loc: Budapest, Hungary
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Maybe because it is difficult to actually purchase them. The factory has ummm.... problems with keeping shipping dates ...
Btw, I compared the 6" and 8" Klevtov's to the same sized Celestron SC's and I was not amazed at all ...
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gatorengineer
professor emeritus
Reged: 02/28/05
Posts: 725
Loc: Hellertown, PA
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Owned a 6" TAL Klestov, very not impressed. Optics had a color tint to them, OTA was made out of steel, and was unbelievably heavy for its size. Focuser was poor even by russian mak standards.....
That being said, I am still very interested in someday looking through the Vixen implementation of the Klestov design.
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16" & 20" Dobs
4" Triplet Refractor
10" LX200R OTA
8"F20 Dall Kirkham
12.5" F3.5 Newt
CGE
Lots of binos---
Nikon 8x30, 10x35, Prostars,10W/18 x 70 astrolux; 10x50 Fujis. Collectors items 12x60 BLC, 10x80 Flaks, Gas Masks, US Mark 28, 30, 41, 43's
Telescopes Past - 8" Stf Mak, C8, Meade LX10-10", SN8, TAL150K, Orion 150MC, Jason 60mm refractor, ATM 6" F8, WO 110FLT, 92mm Off Axis Newt, Nikon 20x120 bino's, 15x110 Boarderguards, Kuhne Flaks
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studentofoptics
member
Reged: 06/25/06
Posts: 21
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Theoretically Klevtsov and other catadioptrics based on sub-aperture correctors are excellent and versatile designs. It is attractive from a fabrication standpoint, since all surfaces can be spherical.
The main problem is that in order to perform well, the optical surfaces, especially those of correctors, need to fabricated to high precision. The light bouncing off of the primary travels twice through the corrector before hitting the eyepiece. Also, the overall optical correction is quite sensitive to placement of correctors.
So, the bottom line is that the design is sound, it just needs to fabricated and built well enough to provide optimal performance.
BTW, the Klevtsov design is slightly different from Vixen VMC. While the Kletsov uses a mangin mirror (reflecting surface on the back of a correcting lens), the Vixen uses a traditional convex secondary with one or two element correctors in the front. Vixen VMC is essentially a Shafer-Maksutov design featured in Telescope Making magazine many years ago. I think there is a 20" Shafer-Mak in UK. Google Shafer-Maksutov and you will probably find that link.
Mike
Edited by studentofoptics (04/05/08 07:56 PM)
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dvmak
member
Reged: 05/11/06
Posts: 75
Loc: Moscow, Russia
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To be more exact, Klevtsov has not only Mangin secondary, bu also a lens just before it, that works in double pass. There are 6 passes through the glass-air surface, so the requirement for the quality of the polishing is high.
I don't have a lot of statistics, but some people have very good results in Moon imaging with them, so I decided to get such scope, and I have got it this week.
Later I shall make the startesting, and, maybe, interferometer test.
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Clear skies for all!
Dmitry Makolkin
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1558
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Quote:
Here's the product description of one of russian klevtsov cassegrains:
"The TAL-250K provides an aperture of 250mm and focal length of 2130mm in a very compact tube length of only 450mm. Klevtsov's revolutionary catadioptric design employs a sub-aperture meniscus correcting lens (like the familiar Maksutov) combined with a Mangin mirror-lens secondary. Its fast focal ratio of f/8.5, flat field, and excellent correction of coma, spherical aberration, and astigmatism (14 micron star images at a distance of 10mm from the optical axis!) make the TAL-250K a superb instrument for astrophotography, far surpassing conventional Schmidt-Cassegrain and Ritchey-Chretien systems. Every optical component must pass stringent tests. Interferograms on three axes of each optical set are retained by the factory's quality control department."
The price is much cheaper than a 10" Mak, Dall-Kirkham, or Ritchey. So, if the performance is what they are stated, why are they not popular?
clear skies,
Arief
Why (I'm a-talking about the Tals here)?
1. They present images, when properly collimated, that are similar to an SCT, which is lighter, better finished, and, in the U.S., often cheaper.
2. They got a bad reputation, initially, because the manual instructed users to collimate by adjusting the secondary instead of the primary. Monkeying around with the KCT secondary is a recipe for disaster.
They are OK scopes; I kinda like 'em, and on the continent and in the UK where Meade and Celestron SCTs are insanely overpriced they are a valid choice.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using the New CATs--coming soon!
Edited by rmollise (08/06/08 07:46 AM)
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Jeff Young
Post Laureate
Reged: 08/04/05
Posts: 3146
Loc: Ireland
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Quote:
.... BTW, the Klevtsov design is slightly different from Vixen VMC. While the Kletsov uses a mangin mirror (reflecting surface on the back of a correcting lens), the Vixen uses a traditional convex secondary with one or two element correctors in the front. Vixen VMC is essentially a Shafer-Maksutov design featured in Telescope Making magazine many years ago. I think there is a 20" Shafer-Mak in UK. Google Shafer-Maksutov and you will probably find that link.
Mike
Mike --
Very interesting. What about the Orion Optics (UK) sub-aperture Maks? Do you know if they're Shafer or Klevtsov?
Cheers,
-- Jeff.
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Nikon 18x70s / UA Millennium Colorado:
Solarscope SF70 / TV Pronto / AP400QMD Coronado SolarMax40 DS / Bogen 055+3130
APM MC1610 / Tak FC-100 / AP1200GTO Tak Mewlon 250 / AP600EGTO
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dvmak
member
Reged: 05/11/06
Posts: 75
Loc: Moscow, Russia
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Quote:
...
1. They present images, when properly collimated, that are similar to an SCT, which is lighter, better finished, and, in the U.S., often cheaper.
...
No, Klevtsov is coma-corrected design, and a useful field of view is bigger than in Scmidt-Cassegrain, that is not coma-corrected.
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Clear skies for all!
Dmitry Makolkin
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1558
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Quote:
Quote:
...
1. They present images, when properly collimated, that are similar to an SCT, which is lighter, better finished, and, in the U.S., often cheaper.
...
No, Klevtsov is coma-corrected design, and a useful field of view is bigger than in Scmidt-Cassegrain, that is not coma-corrected.
I stand by what I said about the image quality of the TAL scopes. That said, though, they are not IMHO bad telescopes. Especially now that they are being sold without the shaky, CLUNKY, substandard mount they originally shipped on. Frankly, though, the evidence is that the TAL scopes as produced and shipped into the U.S were NOT able to produce images _as good_ as the average C8. For an excellent review, see this:
http://cloudynights.com/item.php?item_id=448
That said, I like the TAL and wouldn't mind seeing it in the U.S. again. It does have advantages, and if the price were right, and the bugs of the original were worked out, I might even take a chance. 
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using the New CATs--coming soon!
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1723
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For visual use, you have a big challange... The scope has a very large central obstruction, but to add insult to injury, you also have a large cast iron spider. So while the secondary obstruction is listed as 12.5%, the TOTAL obstruction is somewhat higher, and if you figure that the spiders degrade the image maybe 2% by themselves, you are looking with a telescope with equivilent to maybe a 38% central obstruction..
As mentined, because the designg is passing light back and forth throgh the sub-aperture lenses, tolerances have to be excellent (but that is not hard to do on such a small lens group really, is it?).
Since most big telescopes are really limited by seeing much of the time, these factors might not really mean that much though, and the flatter field as compared to an SCT might be a good compromise..
Oh.. And it weighs 16KG for the OTA... OUCH!.. And this appears to be without a Losmandy standard dovetail and radius blocks.. Figure 17KG when you add that... A C14 only weighs a bit more than this..
If you were imaging though, I would think that this would indeed be a VERY attractive alternative, or maybe even PREFERRED over an SCT.
Vixen used to sell a sub aperture CAT and while it was never popular here, in Japan, they were VERY popular.
I say you should go for it... LOL.. I am always quick to help other people spend their money...
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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sang33ta
professor emeritus
Reged: 07/08/08
Posts: 539
Loc: UK
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1) The veins holding the secondary may show up in images. The 3 curved veins in the Tals' are probably best.
2) Being open dust gets in so you may have to clean the optics more but you'll have less issues with cooldown.
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Hioptic 152mm f12.5 Maksutov
Celestron Advanced CG5-GT Mount (Mr Noisy!)
Meade 4000 Super Plossl Set
Casio QV-2900UX
Got fed up of waiting for Meade ETX-150 so put this together for £500/$1000 
Edited by sang33ta (09/20/08 11:30 AM)
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outlash
member
Reged: 06/16/07
Posts: 74
Loc: Cambridgeshire, England
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A guy I know had the 8" version for a while and what I saw through looked pretty good to me. Alas, his financial situation dictated that he had to sell it before he could really get the best out of it  .
Quote:
1) The veins holding the secondary may show up in images. The 3 curved veins in the Tals' are probably best.
Curved vanes don't show diffraction spikes. It does affect the image though,
Tony..
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Resident of cloudy Cambridgeshire, UK.
Celestron C8N
William Optics Zenithstars 66 & 110
Intes MK66
EQ6, AZ3, Hyperion EP's and a load of other bits & bobs .
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1558
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Quote:
If you were imaging though, I would think that this would indeed be a VERY attractive alternative, or maybe even PREFERRED over an SCT.
Vixen used to sell a sub aperture CAT and while it was never popular here, in Japan, they were VERY popular.
I say you should go for it... LOL.. I am always quick to help other people spend their money...
I agree, mostly. Cupla quibbles. Vixen actually sells subaperture scopes, still, the VC and VMCs. I wouldn't say they have exactly "caught on," but the VC, especially is gaining a lot more attention.
As for imaging with the KCT? The fly in that ointment is the focal ratio. Unless you can find a focal reducer that works well with this configuration (I've no idea how a standard reducer performs with this design), you'll be pretty unhappy, I reckon.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using the New CATs--coming soon!
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Eddgie
Carpal Tunnel
Reged: 02/01/06
Posts: 1723
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The Vixens have been popular in Japan for a very long time.
The scope is clearly positioned as an imaging scope.. That is what it says in the press.
While you can't get f/6.3, the focal ratio is only a little larger, and that might be beneficial to people that want to image targets like Globulars.
And the better off axis performance means that the focal reducer/corrector isn't NEEDED to still get good images.
So, that is kind of a neutral, but still a valid point.
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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)
The night sky is my mistress. She seduces me away from all other lovers.
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1558
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Quote:
The Vixens have been popular in Japan for a very long time.
The scope is clearly positioned as an imaging scope.. That is what it says in the press.
While you can't get f/6.3, the focal ratio is only a little larger, and that might be beneficial to people that want to image targets like Globulars.
And the better off axis performance means that the focal reducer/corrector isn't NEEDED to still get good images.
So, that is kind of a neutral, but still a valid point.
The VC is positioned as an imaging scope...the VMC is more of a visual instrument.
I've often thought about the VC, but have yet to work up the desire to do something about it. I've just finished an f/6.3 Newtonian, and if I don't like that for imaging, I may very well look at the VMC...
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using the New CATs--coming soon!
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