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Equipment Discussions >> ATM, Optics and DIY Forum

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perdrix
journeyman


Reged: 09/28/12

Building a Ritchey-Chrétien? new
      #5632670 - 01/20/13 06:30 AM

I'm considering building a telescope. Is there any reason not to aim for an RC type if I'm able to source a good mirror set (I'm not sure I'd be able to figure a decent parabola, let alone an accurate hyperbola).

If I were to do this, what sort of aperture should I aim for? Or put another way, at what aperture does the job start to become hard?

Thanks
Dave


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Ajohn
sage
*****

Reged: 12/03/07

Re: Building a Ritchey-Chrétien? new [Re: perdrix]
      #5637541 - 01/22/13 06:52 PM

I think your best bet on deciding on that would be to ray trace various set ups and see what you gain. I think you will find it comes into it's own at larger diameters and faster F ratio's with relatively low 2ndry mirror magnifications. The larger diameter being to make up for the relatively large central obstruction needed to give a wide field angle. I looked at a commercial design recently and had to wonder what it offered in relationship to a newtonian with a coma corrector.

Making the mirrors for the real thing can be rather difficult. One professional approach is to look at the amount of glass that has to be removed during figuring - if more than 0.001in grind most of it away locally. This means polishing to a sphere as usual and then grinding and polishing another sphere into that and blending in. In the extreme it may mean grinding and polishing more than 2 spheres. This sort of thing is probably easier on larger mirrors.

Ray tracing will also give you some idea what to expect. Aberrations grow with increasing field angles but the design can give smaller circles of confusion so has better photographic efficiency than the classic curves.

John
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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Building a Ritchey-Chrétien? new [Re: Ajohn]
      #5638093 - 01/23/13 12:19 AM

Guess I'd first ask, why a RC in particular? Do you plan any instrumentation that needs or makes use of a wide coma-free but non-flat field? Of the three common Cass systems (Dall-Kirkham, Classical and RC), the RC has the strongest field curvature and tightest alignment requirements. The aspherics are also the strongest, making (good) RC optics the most expensive of the three. This is of course without auxiliary refractive correctors in the path. The ellipsoidal primary and spherical secondary of the Dall-Kirkham make its mirrors the least difficult to make, plus a DK has the loosest alignment tolerances of the three. The DK has the flattest field but worst coma of the three, but if you're mostly going to use it visually or for narrow-FOV photography, the increased coma isn't a major driver for image sharpness.

Just my 20 millidollars,
Mike


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Building a Ritchey-Chrétien? new [Re: Mike I. Jones]
      #5638150 - 01/23/13 01:12 AM

hi Mike,

I didn't know the DKs are the easiest to fabricate... but if you say they have the strongest coma - is it as simple as slapping in an MPCC?


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Ajohn
sage
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Reged: 12/03/07

Re: Building a Ritchey-Chrétien? new [Re: orlyandico]
      #5638315 - 01/23/13 07:00 AM

I was asking the same question as Mike really but feel it's best to get an idea of what various designs can do for yourself.

Really the choice depends on what you want from the scope. Roughly speaking

Dall - Kirkham - narrow field planetary type use
RC - "wide field" astrographs for photography.
Classic Cass - A bit of both really depending on design.

I put wide field in quotes as I suspect a Cass could do a similar job at amateur sizes and F ratio's or even a straight Newtonian. Taking an 8in F8 GSO for instance an 8in F8 newtonian doesn't produce too bad a field angle of the same size and could presumably be cleaned up even more with a coma corrector.

One problem with the DK seldom mentioned is the need for the spherical mirror to be accurately matched to the primary. I've never toleranced one but it's bound to be rather tight. The classic Cass doesn't have this problem as variations in the 2ndry just alters the final F ratio - providing the curve is correct.

All of these have Gregorian variants. Usually just dismissed as a no no because it results in a longer scope. For home construction personally I don't think that this is a major problem really compared with the advantage of 2 concave mirrors.

John
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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Building a Ritchey-Chrétien? [Re: Ajohn]
      #5638726 - 01/23/13 11:28 AM

Actually, the DK secondary radius can vary quite a bit relative to a finished primary if the mirror spacing and BWD are optimized to match. No time now, but soon I'll put up a graph of optimized OPD, spacing and BFL vs. DK secondary radius. I'll use a 12.5" f/4 primary, optimized for a DK having 10" BWD, 1.5" FOV and f/15 overall, if you want to do the same analysis in parallel.
Mike


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oldtimer
Pooh-Bah
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Reged: 11/13/08

Loc: Lake County Illinois
Re: Building a Ritchey-Chrétien? new [Re: perdrix]
      #5652354 - 01/30/13 01:40 PM

About 10 year ago I fabricated a Rithie using Star optics. I unfortunitely did not understand how critical alignment would be on this optical design. I used commerically avaialble lens cells for the primary and secondary. They were not up to the task of keeping colimation. I could never keep it colimated and finally broke it down and sold the optics. If you persue this project I hope yo know a real good machinist.

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Ed Jones
Carpal Tunnel
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: Building a Ritchey-Chrétien? new [Re: perdrix]
      #5652477 - 01/30/13 02:39 PM

I'm not a big fan of Cassegrains but take a look at post #5650793. It's got a parabolic primary so you can always convert it into a Newt as well.

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JohnH
Carpal Tunnel
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Reged: 10/04/05

Loc: Squamish BC Moved!!!!!
Re: Building a Ritchey-Chrétien? new [Re: Ed Jones]
      #5652786 - 01/30/13 05:08 PM

I have to agree there are a number of trade offs when considering a build of an RC telescope.

For a DIY telescope, the RC is head and shoulders more difficult than most scopes.

The primary can be hard to figure and hard to test. The secondary being a convex surface is very hard to test as you either have the polish the rear so you can test it through the back surface or make a test plate.


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Ajohn
sage
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Reged: 12/03/07

Re: Building a Ritchey-Chrétien? new [Re: JohnH]
      #5653978 - 01/31/13 10:05 AM

Testing 2ndry's.
Seem to remember that a Hindle sphere can be used to test the secondary? May have the name a little wrong. Where I have seen professional do 2ndrys it via the mirror to be used and auto collimation. Also mention of artificial stars and that a flat with a few waves of error doesn't seriously change the result if it's form is spherical to closer limits. All of these could be ray traced to check.

With the RC and Cass I think there is a strong need to focus via moving a mirror. I would suspect that it wouldn't be a bad idea on a DK too where the main interest is the central diffraction spot area.

John
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