A SOLID 30 mm F/10 Schmidt Cassegrain
Posted 04 February 2013 - 09:05 AM
Here's what I did:
I drilled a cylinder of 30 mm diameter and made one end convex spherical (the primary mirror) and the other end flat (the corrector side). In this side a small concave sphere was made, being the secondary mirror.
I calculated the needed aphericity of the Schmidt-corrector (21 microns!),ground and polished this deformation into the surface and made corrections using flexible polishing tools. In the middle of the spherical primary I made a concave sphere to bring the focus far enough outside the 'telescope' to be used in combination with a CCD or eyepiece. To prevent straylight I drilled cylinders around the secondary and into the primary mirror acting as light baffles. Then the mirrors were coated with aluminum. Although it was meant to be used ínside an eyepiece I never actually made a barrel where it could fit in. However, I have made two larger ones, with 40 mm diameter and I kept this little one as a nice memory.... Unfortunately I have no pics of these large ones.
The pictures below show you the size compared with an 1 1/4 inch eyepiece and I did some Photoshop to give you a realistic idea of what it would have looked like ;-)
Thanks for watching!
Posted 04 February 2013 - 09:05 AM
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Posted 04 February 2013 - 10:07 AM
Posted 04 February 2013 - 10:15 AM
does it work ?
what is pct central obstruction?
Posted 04 February 2013 - 10:39 AM
that is one of the coolest things I've ever seen - Outstanding!!!
Posted 04 February 2013 - 10:48 AM
Posted 04 February 2013 - 11:10 AM
It does work within its limitations: Field is rather small, 10 mm diameter and it is curved. Linear obstruction is about 40%, and color correction is surprisingly good. Below the spot diagrams of the design.
Posted 04 February 2013 - 11:11 AM
Posted 04 February 2013 - 11:13 AM
Posted 04 February 2013 - 01:28 PM
That is a telescope junkie piece if ever I saw one.
How does it do on the moon?
Posted 04 February 2013 - 01:40 PM
If you made it a Gregorian Maksutov instead, it would have an erect image! It would also be slightly longer.
Posted 04 February 2013 - 02:20 PM
I love it !
How fast does it cool down ?
What Glass is it made off ?
Thanks for sharing
Posted 04 February 2013 - 03:08 PM
Asteron, cool down isn't an issue, unless you cool it to cryogenic temperatures in a really short time :-)
Material is 'ordinary' BK7, surface accuracy of the mirror surfaces was tested with polished spheres. Even at this scale, coma correction was done by aspherizing the secondary. By examining the fringe pattern on the test sphere you could see the aphericity easily.
Ed, I''ll make an image of the moon with it as soon as I get this thing in front of my camera...
Posted 04 February 2013 - 03:37 PM
Now you can just scale everything up from millimeters to inches.
Posted 04 February 2013 - 06:22 PM
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Posted 05 February 2013 - 01:27 AM
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Posted 05 February 2013 - 10:22 AM
Its advantage was it would never fall out of collimation, not without a force large enough to shatter the glass or metal housing.
Posted 05 February 2013 - 10:33 AM
Posted 05 February 2013 - 02:54 PM
Gord, it actually fits INSIDE an eyepiece and it might be used as a finder. The field of view is limited however, so it will not make a perfect finder. For that purpose a larger diameter would be required.
John, thanks for this this interesting comment! I didn't know this Vivitar lens so after some googling I found that the optical configuration of the Vivitar consists of several (bonded) components with different refractive indices and with some air gaps. For comparison below the layout of the 30mm Solid SCT which is made from just one piece of BK7, no bonding whatsoever...
The baffles are not displayed in the plot.
Vladova, hopefully I can do some 'astronomical imaging' during the next weekend!
If I have something to show I'll post it here.
Posted 05 February 2013 - 03:33 PM
"I WANT ONE, please'
Posted 05 February 2013 - 03:47 PM