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Value of 12.5" Classic Ritchey–Chrétien with optical issues

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#26 junkbum35

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Posted 20 February 2019 - 05:48 PM

I think DaveG is right: optics should be tested. Some are great, some suck, some are fair. Ya never know until you try them.

 

One cool feature is that there is a handwritten notebook/log of notes, starting in 1967 at the University of Md and then when it passed over to National Capital Astronomers and the Hopewell Astronomoical Society. It has dated and initialed entries with sometimes very funny but useful remarks on breakdowns, repairs, upkeep, and collimation, both for the mount (which we have kept in operation; it can carry a tank) and for the telescope. We also have some typewritten letters to and from Prof. Michael A'Hearn of UMd about the mount and telescope, as well as some receipts for the transactions I mentioned. The last UMd Observatory entry is a poem about them making the decision to sell the Ealing mount and scope and buying something more modern.

 

Anybody want me to reprint the poem? It's clever and funny. It also rhymes and scans.

 

Funny thing is, having read the entire notebook, I think I'm the first person who ever put the primary mirror on a bench for a Foucault/Couder zonal test. (I also did an autocollimation test, and a cloudy-night eyeball test on local radio towers).

 

I am pleased that my deduction that it was really an R-C was confirmed by Paul Valleli, and that my conclusions about the optical quality overall were confirmed by everything else I've read on this thread and on the other one.

 

That being said, I think that figuring convex surfaces is extremely difficult for most of us mortals - including me. 

So I'm not going to refigure it. It's a project, not a completed, well-functioning telescope. The glass itself, already cored, is worth quite a bit, and could be refigured into a short-FL f/4 paraboloid if you wanted to. 

 

Or, if you have super optical powers like DaveG, then you could merely refigure the secondary in autocollimation and then you'd have an actual Ritchey-Chretien worth many thousands of dollars!

 

BTW I recently re-coated the mirror nyself - bare aluminum, no overcoating. It looks quite nice IMHO.


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#27 Steve Allison

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Posted 21 February 2019 - 05:52 PM

Peter Ceravolo could probably re-figure this in his sleep...



#28 dhferguson

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Posted 22 February 2019 - 02:52 AM

Cheers,

 

Yep, the mirror(s) may need to be refigured. Please do not under any circumstances rebore the central hole unless you are ready to pretty much start over with the primary. You will almost certainly change the stress near the hole and cause further mirror distortion. However, should you choose to regrind (fine), repolish, then refigure the primary, you may then first rebore the central hole. Hint: hire an experienced optician who has access to an interferometric tester.

 

Happy observing always,

 

Don



#29 photoracer18

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Posted 26 February 2019 - 04:56 PM

1967 was during the time Dick Brandt headed their optical shop and I can't believe someone of his reputation would send something like this out (the period when their output was rated the best). Be interesting to find out during what years they were made as opposed to what years they were sold. Maybe they sat around waiting for orders, or like another said maybe the parts were not paired back together. Are there any serial numbers on the optics, Ben? The Dick Brandt 12.5" primary I have has them on the back like a lot of Cave's. Serial number starts with the last digit of the year, then either a 3 digit number or the month followed by a 2 digit number (mine is #6221). I don't have enough samples to pin it down.


Edited by photoracer18, 26 February 2019 - 04:57 PM.

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#30 junkbum35

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Posted 27 February 2019 - 11:09 PM

I don't recall seeing any serial numbers, and I don't really feel like taking this scope apart again just to figure out what month it was made.



#31 tim53

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Posted 28 February 2019 - 10:20 AM

Optical Craftsmen were notorious for not putting serial numbers or dates on their mirrors.  Of the 5 i've got in my shop, only 2 are dated.



#32 DAVIDG

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Posted 28 February 2019 - 10:24 AM

 What people don't understand is small telescope making businesses in 40's -70's had people that understood how to make one certain type of optics. Companies like Cave, and Optical Craftsman specialized in Newtonians ie making parabolic primaries.  If you have never made optics you don't understand that it takes a different skill set to make other types and it also requires a different types of test equipment. So many time what would happen is someone with enough money, be it a college or the government who would want a specialized telescope, would approach a company like Optical Craftsman if they were interested.  They would take job but really did not have the skills to make the optics correctly. You also have to understand, most don't have the skills and test equipment to check optics so how is   someone going to prove that the optics were poorly figured ? 

   Just like today many  people assume that the optics are figured exactly as spec'd  and  when  an image in a  scope doesn't look as good as it is believed it should, people look to blame the collimation, the seeing, the eyepiece, the optics were switched, the figure was altered when coated etc but don't believe it is the optics because they were suppose to be made to "1/10" wave. 

   This telescope seems to also be an  example of that in that it was used for years and people tried many things to improve the image quality. It  doesn't look like until now that the quality of optics were critically examined as the real reason why the image was poor.  The simple answer to the bad image quality is that the optics were not correctly figured because the people that made them didn't have the skills and/or the time  to do it right.

 

                         - Dave 


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