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A SOLID 30 mm F/10 Schmidt Cassegrain

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#1 rik ter horst

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Posted 04 February 2013 - 09:05 AM

About fifteen years ago I wanted to make a very small Schmidt-Cassegrain out of one solid piece of BK7, just to see if it could be done. So it became a 30 mm F/10 instrument, just small enough to fit ín an eyepiece! Thought it would be nice to share here...

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!

Cheers
Rik

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#2 rik ter horst

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Posted 04 February 2013 - 09:05 AM

Here's another one

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#3 rik ter horst

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Posted 04 February 2013 - 09:06 AM

and the photoshop pic

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#4 ed_turco

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Posted 04 February 2013 - 10:07 AM

OH MY GOSH!!! Some folks have incredible talents and you surely are one of them. I could never even imagine your device, never mind build it.

Congratulations! :bow:

#5 highfnum

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Posted 04 February 2013 - 10:15 AM

one of a kind
does it work ?
what is pct central obstruction?

so cute!

#6 lukasik

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Posted 04 February 2013 - 10:39 AM

Rik,

that is one of the coolest things I've ever seen - Outstanding!!!

Best Regards,

Bob

#7 GlennLeDrew

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Posted 04 February 2013 - 10:48 AM

Bloody awesome!

#8 rik ter horst

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Posted 04 February 2013 - 11:10 AM

Thanks a lot!
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.

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#9 tim53

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Posted 04 February 2013 - 11:11 AM

Now that would put an end to tube currents, some thin' fierce! :grin:

#10 rik ter horst

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Posted 04 February 2013 - 11:13 AM

When made a bit longer, you could obtain better color correction and a slightly flatter field, but then it wouldn't fit in an eyepiece anymore ;-)
Small right?

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#11 siriusandthepup

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Posted 04 February 2013 - 01:28 PM

ooooo - I want one!

That is a telescope junkie piece if ever I saw one.

How does it do on the moon?

#12 Astrojensen

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Posted 04 February 2013 - 01:40 PM

Words fail to describe how insanely awesome this is! A 30mm f/10 telescope that could fit inside a 10mm eyepiece, making a 30mm scope with 30x magnification and around 1.5° field.

If you made it a Gregorian Maksutov instead, it would have an erect image! It would also be slightly longer.


Clear skies!
Thomas, Denmark

#13 ASTERON

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Posted 04 February 2013 - 02:20 PM

Very out of the box thinking, pretty cool !
I love it !
How fast does it cool down ?
What Glass is it made off ?
Thanks for sharing

#14 rik ter horst

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Posted 04 February 2013 - 03:08 PM

Thank you all for your kind words!

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...

#15 TVG

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Posted 04 February 2013 - 03:37 PM

Very cool! I want one for my key chain.
Now you can just scale everything up from millimeters to inches. :smirk:

Todd

#16 zjc26138

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Posted 04 February 2013 - 06:22 PM

That's pretty cool and a great idea!

#17 ahopp

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Posted 04 February 2013 - 08:06 PM

I want one please.

Tony

#18 Datapanic

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Posted 05 February 2013 - 01:27 AM

Can't wait to see the mount! Hopefully, it's a fork equitorial with motor drives on both axis and setting circles. Very Nice X5!

#19 meade4ever

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Posted 05 February 2013 - 08:13 AM

Incroyable

#20 Gord

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Posted 05 February 2013 - 09:20 AM

Hmmmm... could you attach an EP to the end and use it as a finder? It is *way* cool! :waytogo:

Clear skies,

#21 JohnH

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Posted 05 February 2013 - 10:22 AM

This is a variation of the "Solid Cat" Vivitar 800mm f/11 which used the same principle, except a larger scale.

Its advantage was it would never fall out of collimation, not without a force large enough to shatter the glass or metal housing.

#22 Stipe Vladova

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Posted 05 February 2013 - 10:33 AM

Take some photos thru the scope! :bow: :jump:

#23 rik ter horst

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Posted 05 February 2013 - 02:54 PM

Thanks all for your enthusiasm! :o

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.

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#24 ahopp

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Posted 05 February 2013 - 03:33 PM

When I said earlier that "I want one" I was not just saying it:

"I WANT ONE, please'

Tony

#25 rik ter horst

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Posted 05 February 2013 - 03:47 PM

Sorry Tony! So you're actually asking me to make one for you? Ehh.... I'm very flattered! Let me think about it for a while, and I'll answer you by PM. Is that ok for you?






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