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Ritchey-Chretien Cassegrain

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#1 Ticondrius


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Posted 18 August 2013 - 03:52 PM

I'm looking for someone to help me work out the exact optics for scope I'd like to build. My specs are:

240mm mirror with a -1.00230 conic constant, and a 576mm focal length.

Yes, those are Hubble's optics, but at 1/10 scale.

I have experience making Newtonians, I've made an 8", 10" and 14", but I wanted to make a Cass for my 4th scope, and I figured I'd go all out and make a scope that as much a detailed scale model as it is a telescope. I'm just not sure what things to look for, optically, that is different in the design.

#2 Jeff Phinney

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Posted 18 August 2013 - 04:12 PM

There's a full size duplicate Hubble mirror hangin' in the Smithsonian and it's figured to the correct conic. Maybe they'll let that go for cheap.

Which begs the question, which conic are you going for, the intended one or the one it launched with? :)

And what about those solar panels?

#3 Ticondrius


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Posted 18 August 2013 - 06:20 PM

Hehe, The intended conic. As for the panels, I intend to use them for power. Optics first, everything else second though.

#4 MitchAlsup


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Posted 18 August 2013 - 06:46 PM

Also note, Hubble is designed with the image plane immediately behind the primary. Thus you would be putting the camera at the same position as the mirror cell.

Whereas the typical Ritchey pushes the image plane out a significant distance. Thus the curve of both the primary and secondary are slightly different for a usable scope.

But in any event, you can puruse:
and use:

To figure out the question.

#5 MKV



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Posted 18 August 2013 - 10:43 PM

You need to provide more information. What's the final focal ratio? Where is the image plane supposed to be? How are you going to measure your conic to four decimal places (!) accurately? How do you plan to make the aspheric secondary? In an RC the optics are demanding and so is the tube assembly and the mount. Optical centering and alignment are critical and require precisely machined parts, baffles and spacing.

Besides, the mirror at cc = -1.0023 is a praboloid. With that you can only get a Classic Cassegrain solution, not a Ritchey-Chretien Cassegrain.


#6 wh48gs


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Posted 19 August 2013 - 07:52 AM

I'm not sure what is it that you are looking for, but these are the specs for HCT scaled down to 1/10 the aperture:


1 -1.1040e+03 -490.000000 120.000000 REFLECT -1.0023

2 -137.140000 647.000000 15.616303 REFLECT -1.497

Primary is a very mild hyperboloid, and the secondary is still moderate. At 25mm off axis it is just diffraction limited over its best field, but the field curvature is very strong: -65mm.


#7 kfrederick



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Posted 19 August 2013 - 08:23 AM

Oslo Edu has the spec for HST

#8 Ticondrius


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Posted 19 August 2013 - 10:47 AM

Thanks wh48gs. That's what I needed. All I had, as you saw, was some numbers from the HST website and wikipedia.

As to how I'm going to do it? I'm not sure yet. Probably trial and error a few times first.

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