So why exactly are Cats closed tubes?

[Peter Drew]

 As a retired manufacturer of Maksutov and Schmidt-Cassegrain telescopes, may I eliminate some of the misconceptions about the optical characteristics of these instruments that have arisen in this thread.

Both designs employ spherical or near spherical primarys, the duty of the corrector plates is to introduce into the system spherical aberration equal to and of opposite sign to that of the primary. To achieve this the Schmidt-Cassegrain has a relatively thin flat looking plate whic has a varying degree of correction applied from centre to edge. The Maksutov employs a thick deeply curved relatively parallel meniscus corrector, the steepness of the curve, thickness and refractive index of the glass all form an important part of the correction. Each type of corrector has a very long focal length and is not image forming, if properly made, particularly with regard to wedge, no chromatic aberration is introduced.

A paraboloid at F2 would be very difficult to make,have horrendous coma and would require very expensive correction to compete with the SC and MC. Full tubes are the norm, easy to make, protects the optics and minimises reflections. 

[Stu]

Thank you Peter, I got it the wrong way round!!

Thinking about it now you have said it, I remember the thick corrector plate on maks being one reason why they are harder or mare expensive to make in larger apertures, and SCTs are more practical in the larger size.

I still don't quite get why there is now benefit to a dam blowing across the mirror to speed cooling and help break down the boundary layer?

[HD-AP]

The only piece of this I disagree with, and it is from experience, is that the mirror and corrector won't dew up inside. The issue is, I'm order to cool the scope you have to run the fans. This can draw moist air into the otherwise closed tube so the mirror and corrector can dew up inside.

It happened to my mak quite dramatically at PSP. Mind you, we were trying to observe from inside a cloud at the time!! ;-)

Hi

The dewing of the mirrors and  inside of the corrector plate was on the hypothetical open tube designs ,of cats proposed in the thread  not actual closed tube telescopes like your mak ,agree with your other points though.

[michael.h.f.wilkinson]

I think there is a misconception (not by all) about what defines a catadioptric design. It is just any design that uses a combination of reflection and refraction to form an image. A Boren-Simon astrograph is a catadioptric design, therefore. Dall-Krikham, Klevtzov, and the VMC design are also cats. RC by contrast are pure reflectors. As pointed out, there is no optical reason why an SCT or Mak-Cas need be a tube design, apart from reducing stray light (even the big 48" Palomar Schmidt camera was enclosed in a tube).

[Moonshane]

The only piece of this I disagree with, and it is from experience, is that the mirror and corrector won't dew up inside. The issue is, I'm order to cool the scope you have to run the fans. This can draw moist air into the otherwise closed tube so the mirror and corrector can dew up inside.

It happened to my mak quite dramatically at PSP. Mind you, we were trying to observe from inside a cloud at the time!! ;-)

I remember it well Stu, the only time my 16" primary on the dob has ever dewed up completely too.

[kev]

Cats and the like have eyepeice ends that face the sky. Surely by enclosing the tube you help reduce the extraneous light reaching the eyepeice.

[F15Rules]

I always thought it was just a matter of keeping good optical alignment.

The corrector plate is a precision part (try getting a quote to replace one!)

As such its a key part of the optical train so to have it properly fixed/aligned in the factory makes sense..its enough of a pain trying to collimate the secondary! I had two Intes M603s and was glad both were sealed tubes..just a pity Intes didn't suck out all the dust from inside the tube before sealing it!

Of course, there view is that a little dust is invisible in the views, but when you've spent a lot of money to buy one you just like it to liik pristine. Great scopes though.

BTW I found that cool down was faster if I pointed the scope straight down towards the ground and removed the lens cap and diagonal for an hour outside before observing..hot air rising etc and ni dust landing on corrector as it was pointing down..

Dave

[Stu]

I always thought it was just a matter of keeping good optical alignment.

The corrector plate is a precision part (try getting a quote to replace one!)

As such its a key part of the optical train so to have it properly fixed/aligned in the factory makes sense..its enough of a pain trying to collimate the secondary! I had two Intes M603s and was glad both were sealed tubes..just a pity Intes didn't suck out all the dust from inside the tube before sealing it!

Of course, there view is that a little dust is invisible in the views, but when you've spent a lot of money to buy one you just like it to liik pristine. Great scopes though.

BTW I found that cool down was faster if I pointed the scope straight down towards the ground and removed the lens cap and diagonal for an hour outside before observing..hot air rising etc and ni dust landing on corrector as it was pointing down..

Dave

Good points Dave. I think one of the biggest problems is clearing the warm air from the baffle tube. I used to use a fan in the eyepiece holder to draw it out, but your method works just as well I'm sure.

[michael.h.f.wilkinson]

I just keep the OTA in an unheated garage, with lots of little silica-gel packets scrounged from any package containing them. Cool down time is cut down dramatically. I just put it outside and keep all caps on so no dust can reach the optics while cooling

[Peter Drew]

A Dall-Kirkham has just two reflecting mirrors, a 70% corrected primary and a spherical secondary so does not qualify as a catadioptric. However, the Dall-Kirkham-Dall incorporates a transfer relay lens as part of the imaging system so that could arguably be classed as catadioptric. The relay lens which Horace Dall added to the system provides an erect image and completely eliminates stray light. A tube would not need blackening internally and some of Dall's instruments had no tube, just a spine to hold the optics in a collimated position, his smallest ones folded up and could be placed in the pocket.   

[michael.h.f.wilkinson]

A Dall-Kirkham has just two reflecting mirrors, a 70% corrected primary and a spherical secondary so does not qualify as a catadioptric. However, the Dall-Kirkham-Dall incorporates a transfer relay lens as part of the imaging system so that could arguably be classed as catadioptric. The relay lens which Horace Dall added to the system provides an erect image and completely eliminates stray light. A tube would not need blackening internally and some of Dall's instruments had no tube, just a spine to hold the optics in a collimated position, his smallest ones folded up and could be placed in the pocket.   

You are right, the original Dall-Kirkham has an elliptical primary and spherical secondary. The scopes labelled as Dall-Kirkham I have seen all had some corrector lens somewhere, but these should be labelled modified Dall Kirkham.