Orion OMC250 'sub aperture' Maksutov

[FLO]

Has anyone heard anything about Orion's OMC250 'sub aperture' Maksutov?

An interesting design...

From the website:

1. 250mm aperture with secondary mirror obstruction of 31%

2. Open, air cooled cell for rapid cooling. Rotatable vent cooled for easy covering when ambient temperature achieved.

3. No full aperture corrector plates at the open tube end. On a large aperture, doomed to dew and totally disrupted images. Sub aperture corrector plate allows cooling and stabilising of image up to 5 times faster than closed tube designs.

http://www.orionoptics.co.uk/acatalog/OMC_Maksutovs.html

[GazOC]

I asked aout this a while ago on SAA and Cloudynights, Steve. The general feeling was that the design was a 'compacted Newt'.

Apperently (and I can't remember why ) the sub-aperture corrector presents a new set of compromises. The reason that OO give (no dew) isn't the real reason that the corrector is sub-aperture, it can be thinner if further down the tube, if memory serves.

[John]

I think the TAL 200K and 150K, which are Klevzov-Cessegain designs, use this approach as well - the sales pitch for them is that it reduces cool-down time and enables a more accurate figure to be put on the smaller glass meniscus that sits between the secondary and the primary mirrors. I have read mixed reviews of them - I believe that collimation is a bit of a nightmare.

Is a mak's front element more susceptical to dewing that a schmidt-cass design then ? - Celestron and Meade don't seem to let this stop them going well over 10 inches in aperture.

John,

North Somerset

[GazOC]

Is a mak's front element more susceptical to dewing that a schmidt-cass design then ? - Celestron and Meade don't seem to let this stop them going well over 10 inches in aperture.

No, dew doesn't care where it forms! SCT corrector plates are a lot thinner than Mak correctors, 12" Maks are heeeaaavvy!!!

[FLO]

I think the TAL 200K and 150K, which are Klevzov-Cessegain designs, use this approach as well...

The Tals were on my mind when I asked the question.

Is a Maks front element more susceptible to dewing that a Schmidt-Cass design then ? - Celestron and Meade don't seem to let this stop them going well over 10 inches in aperture.

I am not aware of either Meade or Celestron producing large aperture Maks. Meade do have a 7" LX200GPS-SMT Maksutov which is stunning, a proper thoroughbred but not hugely popular because of its weight/cool-down time.

[moondog]

The OMC250 did appear to me to be similar in design to the TAL200 Klevsov. I was in two minds about buying a TAL200K but the collimation difficulty and cool down time put me off.

Reviews of the OMC140 & 200 seem to be pretty downbeat with little enthusiasm shown for them, perhaps the OMC250 is in a different league?

MD

[russ]

The OMC200 is a one trick pony really. With a focal ratio of f20 and focal length of 4000mm, it's never going to be anything more than a planetary scope. And the OMC140 has much the same problem at f14.3.

Not heard anything about the 250 other than it's pretty cheap compared to the 200.

The big SCT's seem to just suck up the dew. I found my 5" and 6" Maks held out a bit longer. But as Gaz says, dew doesn't care where it forms....Mak or SCT.

I fancied a Tal200K at one time but then the reviews were mixed and that put me off. Preferred the idea of a Vixen VC200L. Acedemic really as i can't afford either!

Russ

[GordonCopestake]

Or a mewlon

[The Warthog]

Just off the top of my head, I would think it loses the advantage of other SCTs and Maks in not having vanes in the optical path.