Meade 8 inch SCT with no Corrector Plate

toc (Tim) · #1 23-03-2006, 10:25 PM

Hi everyone

I have in my cupboard, a blue Meade 8 inch SCT with a missing corrector plate. Both the Primary and secondary are still with it, although they are probably in need of a re-coat.

Is this scope a complete writeoff, or is it worth getting fixed? I already have a Celestron C8 that I use...

Astroman (Andrew Wall) · #2 23-03-2006, 10:45 PM

If you can pick up a corrector it would be worth keeping, or if you dont want to bother sellit off either on IIS or some other forum, I am sure someone who wants a cheap ATM SCT project would jump at the chance

mick pinner · #3 23-03-2006, 10:49 PM

corrector plates are matched to the primary mirror so you would have to get the whole optical system refigured if you got a new corrector.

toc (Tim) · #4 23-03-2006, 10:55 PM

Quote:

Originally Posted by mick pinner

corrector plates are matched to the primary mirror so you would have to get the whole optical system refigured if you got a new corrector.

Ive heard conflicting views on that one, at least as far as Meade is concerned. Ive been trying to obtain a corrector for a long time, but they are as rare as hens teeth

Astroman (Andrew Wall) · #5 24-03-2006, 06:15 AM

Just offtopic slightly, hope you don't mind. But can an SCT be used without a corrector? Then it would be just a CT?

toc (Tim) · #6 24-03-2006, 08:11 AM

Quote:

Originally Posted by Astroman

Just offtopic slightly, hope you don't mind. But can an SCT be used without a corrector? Then it would be just a CT?

Well - When I first got this scope, some bright spark had replaced the original corrector plate with window glass

What tipped me off to this fact was that when I first looked through the scope there was massive spherical aberation going on - I mean really, really bad.

So yes it can be used without the corrector, but it dont work to good that way

Roger Davis · #7 24-03-2006, 08:14 AM

Aaaah! So we like spherical aberration do we?
Having said that, there is a thing called a "lensless Schmidt".
You won't have coma, but you will have the spherical aberration. Rutten and van Venrooij discuss this in their book "Telescope Optics", page 82. Trouble is that the focal length of the commercial SCT primary is wrong. Taking a 200mm aperture system a focal length of 1600mm is needed to give a maximum blur of 0.25mm when in focus. System is f/8 which would make the tube 3.2m long as the entrance pupil must be positioned at the centre of curvature of the mirror which is 2R.