Newtonian to Classic Cassegrain mod

[koputai (Jason)]

Just say someone had a spare 8" f6 newt, and a spare secondary from an 8" SCT.
Just say that person placed the SCT secondary somewhere along inside their Newt, and moved the Newt secondary down close to the primary mirror and reversed it.
He could then place a focuser on the OTA down near the primary and check the result.
What would he be likely to see? Would the SCT secondary work the same as a classic Cassegrain secondary?
Would he just get some smudgy smeary result?

Cheers,
Jason.
Who may have the parts to give it a try.

[Merlin66 (Ken)]

The SCT secondary is designed to work with a f2 primary mirror and give roughly an amplification of x5 ( efr then f10)
Working at f6 it will not be corrected properly and give carp images...
I think in ATM book two, there's a chapter headed " How to make a Cassegrainian - and why not to!!" Worth reading.....

[Wavytone]

Details on my website:

http://web.me.com/nicholas.loveday/S...Telescope.html

With modern high-reflectivity coatings that are also durable, this is not as crazy as it seems. A primary at F/6 is too slow though, even for a classical cassegrain. You need a parabolic primary in the range f/3.5 to f/5 to start with, f/4 would be ideal.

[scopemankit (Chris)]

You could try an 8" lensless Scmidt

[Merlin66 (Ken)]

Chris,
Now that IS a good idea!!
Just like the old Coulter design; it would probably work with todays CCD's.

[koputai (Jason)]

Originally Posted by scopemankit

You could try an 8" lensless Scmidt

Pardon my ignorance, but doesn't a lensless Schmidt use a spherical primary and
a flat or no secondary? I have a parabolic primary and a convex secondary, so
it's not really an option. Please correct me if I'm wrong.
Or do you mean I should use the parabolic primary, convex secondary etc, and
include an apperture stop at the primary's curve?

Cheers,
Jason.

[Merlin66 (Ken)]

Normally uses, as you say a spherical primary and a diaphragmed aperture, no secondary etc the camera is located in the middle of the tube. So there's only the main mirror and camera.

[Satchmo]

A lemsless Schmidt still needs a curved film holder as far as I know.

I have a real Celestron 5" F1.65 Schmidt for sale , but of course you need to use film with it.

Mark

[Wavytone]

Originally Posted by koputai

Just say someone had a spare 8" f6 newt, and a spare secondary from an 8" SCT...

I guess the real question is can you do anything else with a spare f/6 Newtonian primary.

For a Schmidt the figure is wrong - it needs to be spherical, so it would have to be refigured.

In its present parabolic form, it's too slow to be any use as a Cassegrain or Gregorian, it really needs to be f/4 or less.

As for other designs... from memory I think there is in theory a viable design involving a moderately concave (yes, concave !) secondary that will result in a fast optical system - about f/3 in this case - but the focal plane is in the middle of the scope. If you had say a 16" or 20" primary mirror, it could work with a small CCD head stuck in there, but for an 8" the head is going to be a huge obstruction in the lightpath - and hence not really viable in this case.

This design is an old one - no-one ever built it to my knowledge because the focal plane was inaccessible - there's no way it could be used visually, and it was too challenging for photographic plates.

I've forgotten the name of this, will scratch around - maybe Satchmo knows.

[koputai (Jason)]

Thanks for the comments guys.
What I'm really after is a use for the parts of an 8" SCT I have, complete except for a smashed corrector plate. I might give the lensless Schmidt idea a try. It's a Celestron, so I suppose the primary is f2 and the secondary is 5x.
So the aperture stop would be 800mm from the primary? Any idea what size the stop should be?

Cheers,
Jason.

[Wavytone]

OK thanks, now we understand. It's not much good for anything - except making an SCT - if you can find a new corrector, or perhaps it could do duty as a solar collector for a science experiment.

[Rod]

Hi Jason,

One option would be to sell the parts to someone who makes their own instruments. The mirror would be useful for testing some cassegrain secondaries and could probably be used to build a null tester for paraboloids.

I might be interested in the primary if you want to part with it. I'd need to do some calculations first.

Rod.

[Wavytone]

.. could it take a new OTA ?

Or do you want to sell that ? Some might like it, depending what it is.