Why no large semi-prof. SCTs?

[nmoushon]

So I was drooling over the large semi-professional scopes and notices I didn't see any that were SCTs. I only mostly noticed RCs and DKs. I can understand the a SCT is not pratical at very large sizes because of the needed size for the corrector. But why not at some of the smaller large sizes like 20-24" or so? I would think that at that size it could be very possible. Would the size of the corrector be the only reason why they aren't used or do DKs and RCs have better quality optical design?

[Zakalwe]

Meade sold the 162 for a number of years. They also have a 20" monster available, though look at the mount that's needed

http://www.meade.com/max

[Alien 13]

I would think thats its the same reason you dont get big refractors the lens or corrector plate is only held at its edge so it will distort depending on its angle.

Alan

[Bodkin]

Celestron used to offer a 22 inch SCT back in the 70s.  It was a pretty professional looking beast, though I don't know whether they sold many of them.

[LukeSkywatcher]

IIRC, the corrector plate on SCT's is only about 3mm thick. So i am thinking that if you had a large SCT (larger then the biggest one available.........8"?), the likely-hood of the glass breaking grows with every inch of aperture.

Glass lens' are also are more expensive to produce then mirrors. Thats why reflector scopes come as big as you want.

[nmoushon]

I can only assume they would very expensive thus only the semi-professionals and universities would be able to afford them. Not the mention the mounts needed.

I would think that if they can make secondaries for professional scopes that are larger than most semi-professional scope that they could figure out something to hold the corrector in place properly. Especailly with new tech like carbon fiber or such light weight but strong materials. I would think that you just making the corrector thicker would solve and possible breaking or cracking. I know it would be difficult but with the size of mirror and lenses today I'm sure its not that much of a problem. Not that I know what I'm talking about. All I'm doing is guessing here and trying to solve this question that burning a hole in my head lol.

[Zakalwe]

I can only assume they would very expensive thus only the semi-professionals and universities would be able to afford them. Not the mention the mounts needed.

I would think that if they can make secondaries for professional scopes that are larger than most semi-professional scope that they could figure out something to hold the corrector in place properly. Especailly with new tech like carbon fiber or such light weight but strong materials. I would think that you just making the corrector thicker would solve and possible breaking or cracking. I know it would be difficult but with the size of mirror and lenses today I'm sure its not that much of a problem. Not that I know what I'm talking about. All I'm doing is guessing here and trying to solve this question that burning a hole in my head lol.

The problem with carbon fibre is that its not transparent   so you still only hold the corrector around the edges.

Its easier to make a large Newtonian or RC as the secondary can be held by a spider.

[nmoushon]

The problem with carbon fibre is that its not transparent   so you still only hold the corrector around the edges.

Its easier to make a large Newtonian or RC as the secondary can be held by a spider.

Thats true. I guess at that point of needing a spider the extra cost of the corrector isn't really worth it. Though with a spider you now have 2 extra sides to support a corrector inbetween the spider veins. Would this be benificial at all or would it same correction be done with a smaller lens near the focuser/focal point?

[DaveS]

I *think* that the largest Schmidt camera is the old 48" on Mt Palomar. It's not a SCT but a true Schmidt *camera* where the corrector plate is at the radius of curvature of the primary (72", oversize IIRC). The photographic plate has to be warped to fit the focal surface, which is a sphere with it's centre at the RC of the primary.

[ronin]

Meade do a 20" SCT, $35,000, the OTA is 190lbs.

The OTA, Mount and Weights come to 694lbs, puts it just outside the grab and go range.

They give you an Autostar handset with it.

[Peter Drew]

The cost of producing the Schmidt corrector plate was initially very high due to the hand figuring difficulty. Celestron eventually devised a method whereby a flat parallel plate was vacuum sucked to a correctly shaped former, one side was then made flat again and then when the vacuum was released the plate sprung into the correct shape for the telescope specification. Meade legitimately copied this process when the Celestron patent expired and this is largely the reason why such systems are now relatively inexpensive. To make the formers for much larger SCTs would probably not be economic in relation to the sales. 

[JamesF]

I wonder if there's any reason someone might actually want a very large aperture SCT?  Let's say for the sake of argument someone was interested in a 20" SCT.  Is it likely that they'd have the resources to house one (space, particularly) but not, say, a much cheaper to produce RC of the same focal length?  Obviously there is the issue of diffraction spikes, but would there be any other reasons to prefer the SCT?

James