Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Value of 12.5" Classic Ritchey–Chrétien with optical issues

  • Please log in to reply
31 replies to this topic

#1 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:31 AM

I'm trying to determine the value of our club's vintage 12.5" classic Ritchey–Chrétien.

 

This telescope was purchased in 1967 by the University of Maryland for $8k with an observatory-class mount.

 

The telescope was manufactured in 1967 by Optical Craftsman.

 

As delivered from the factory, the telescope never delivered the quality views that were hoped for. Hours were spent working on the telescope, and they determined that while the primary mirror was well formed, the center hole had been drilled slightly off center.  

The mount was moved to another telescope, and the OTA was eventually purchased by the Hopewell Observatory in VA.  It has been sitting for some time now in storage at the  Georgetown Mirror Making club.  

 

This was an observatory-class instrument as sold back in 1967.  There are tons of notes on tuning the telescope and optics. As a Ritchey–Chrétien it has hyperbolic mirrors.  There is no focuser at the rear of the telescope to hold eyepieces.

 

The OTA is massive, and heavily engineered. 

Attached Thumbnails

  • DSCN0932.JPG
  • DSCN0933.JPG


#2 siriusandthepup

siriusandthepup

    Viking 1

  • *****
  • Posts: 804
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 19 February 2019 - 10:34 AM

photos?


  • tim53 likes this

#3 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:37 AM

The telescope was designed to achieve focus through a mechanism at the front of the telescopethat moved the secondary mirror in and out. The secondary can be moved manually now.

 

Rotating the mechanism by hand moves the mirror in or out.  It may have been powered at one time, but is manual now.

 

The secondary mirror vanes are very thick  - this is built like a tank.

Attached Thumbnails

  • DSCN0934.JPG


#4 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:38 AM

The primary mirror cell is fully collimatable.

 

The glass center piece that sticks out from the primary in the back was apparently added as a means to hold an eyepiece or collimator.  It is not original.

Attached Thumbnails

  • DSCN0931.JPG


#5 tim53

tim53

    Voyager 1

  • *****
  • Posts: 13831
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 19 February 2019 - 10:39 AM

Photos would be very helpful.  Who was the manufacturer?

 

The central hole could be made centered by putting the mirror on a rotary table and boring a larger hole while slowly rotating the table.  You could use a micrometer gauge to make sure that the mirror is centered on the table and eliminating runout.  This way, the new, slightly larger hole would be concentric with the outside of the mirror.

 

-Tim.


  • siriusandthepup likes this

#6 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:41 AM

Any thoughts or insights are greatly appreciated.  It may be that the best option for fixing the issues are to drill a slightly larger center hole (would allow using 2" accessories").

 

Alternatively, replacing the tube with a modern truss design would let you set this up for 1.25" accessories and center the existing hole with the secondary by slightly shimming or off setting the primary. 


  • tim53 likes this

#7 tim53

tim53

    Voyager 1

  • *****
  • Posts: 13831
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 19 February 2019 - 10:42 AM

I would also consider a crayford focuser, and find the best separation of the mirrors and fix the secondary at that separation.


  • tubehead999 likes this

#8 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:44 AM

The telescope was manufactured in 1967 by Optical Craftsman and purchased by the Univeristy of Maryland.

 

At the time, Optical Craftsmen heavily marketed these as top of the line instruments. I believe the primary was tested and appeared to be well configured.  The secondary may have some issues (not sure on that).  Alignment issues are definite.


  • tim53 likes this

#9 pbealo

pbealo

    Viking 1

  • *****
  • Posts: 877
  • Joined: 12 Oct 2006
  • Loc: New Hampshire

Posted 19 February 2019 - 10:45 AM

How "off" is the central hole and what is its size. It looks small.

 

Who made this? Not Fecker for sure. 



#10 starman876

starman876

    Hubble

  • *****
  • Posts: 18503
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 19 February 2019 - 10:50 AM

Looks like a great scope for the right person.   Did not know that Optical Craftsman used to build scopes like this.    If I were to buy it I would correct the optical issues and put the glass in a lighter tube.   Still would take a substantial mount.


  • tim53 likes this

#11 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 10:52 AM

The central hole actually has a glass plug in it that has just been placed into the current hole as a quasi replacement for a focuser. The actual central hole is at least 2" - I can get a measurement tonight.



#12 ngc7319_20

ngc7319_20

    Ranger 4

  • *****
  • Posts: 336
  • Joined: 25 Oct 2015
  • Loc: MD

Posted 19 February 2019 - 11:01 AM

New 12" R-C, carbon fiber truss, etc., can be had for about $4k.  Used one with problems / unknown optical quailty...  Maybe $500 - $1000?



#13 siriusandthepup

siriusandthepup

    Viking 1

  • *****
  • Posts: 804
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 19 February 2019 - 11:08 AM

How much does this beast weigh?

 

This will be key in determining its usefulness to anyone. Assuming that the optical issues can be solved (not trivial on a RC), ya gotta be able to mount and use it in a reasonable fashion.

 

These days RC often means carbon fiber and light weight.

 

A last question - has this scope ever produced a decent image?

 

 

Maybe $500 - $1000?

Closer to the lower number maybe... Not trying to be harsh, just the reality of today's advances and availability.


Edited by siriusandthepup, 19 February 2019 - 11:13 AM.


#14 junkbum35

junkbum35

    Sputnik

  • *****
  • Posts: 28
  • Joined: 25 Aug 2012

Posted 19 February 2019 - 11:20 AM

Ben Bartine is correct about the history.

 

Back in 1967, according to the original documentation, which I have, the scope and its massive Ealing mount cost $8,800.00 in 1967 dollars, or about $60K in 2019 dollars. Its actual value today? I have no idea.

 

When I tested the optics, I thought it would be a classical Cassegrain but was surprised to find that the primary was hyperbolic with a conic constant of about -1.2 (roughly; I'm not looking at test documents). Paul Valleli, a veteran optical engineer and craftsman, wrote that he had worked on these telescopes (made for the college educational market at the time) and that they indeed were intended to be of the Ritchey-Chretien design with a hyperbolic primary (instead of parabolic). 

 

It's quite heavy. Not sure exactly what the weight is, but I'm guessing between 40 and 70 pounds. Even with the full-thickness mirror removed, it's not light.

 

None of its owners were pleased with its optical performance. Some have suggested refiguring the convex secondary, which is way beyond my skill set. The primary could fairly easily be refigured to a paraboloid, to make an RFT.



#15 siriusandthepup

siriusandthepup

    Viking 1

  • *****
  • Posts: 804
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 19 February 2019 - 11:40 AM

70 lbs is not terrible for a 12.5" OTA. Many mounts can carry that.

 

Refiguring an RC (either mirror) is for super heroes only. lol

 

Value of the 12.5" mirror is then blank value minus hassle of eliminating RC curvature, call it roughly $200.


  • tim53 likes this

#16 tim53

tim53

    Voyager 1

  • *****
  • Posts: 13831
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 19 February 2019 - 11:53 AM

An optical craftsmen RC has to be pretty rare. I’d be inclined to fix the problems and restore the ota. 70 pounds for that size scope isn’t all that heavy, provided it’s semi permanently mounted. 



#17 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 12:15 PM

The focuser hole is 1.25" ID. The original hole is about 3" diameter.

The plastic (not glass) plug is easily taken out by hand - merely friction fit.

 

As for how much this is out of center - Not very much, maybe 1/10 inch, but R-C's are very unforgiving.


  • pbealo likes this

#18 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 8518
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 19 February 2019 - 12:34 PM

 Is this the same scope that was discussed here by Guy Brandenburg and on his blog https://guysmathastr...lescope-making/ and here https://www.cloudynights.com/topic/590934-classical-cassegrain-telescope-from-ealing-and-competition-testing-of/ 

   If so, here is the problem, you have a  set of optics that won't produce a good image even thou a number of people have tried over 50 years.  So in  reality what you most likely have are optics that are  misfigured.  What that  means is you have two pieces of polished glass and not a correctly figured set optics.  So to make it right both surfaces will need to be refigured as well the hole in the primary correctly bored.  That requires the proper test equipment has in a Hindle sphere to figure the secondary and high quality Ross lens to  figure the primary. The cold hard truth is that  the value of this scope is the cost of  a 12.5" and 3" mirror blank at best.

 

                     - Dave 


  • TOM KIEHL and figurate like this

#19 Stephen Kennedy

Stephen Kennedy

    Apollo

  • *****
  • Posts: 1476
  • Joined: 03 Jul 2014
  • Loc: California

Posted 19 February 2019 - 12:59 PM

Before going to Medical School I received my BS in Physics and Astronomy from the University of Maryland College Park and I think I remember this telescope and they are not fond memories.
  • bremms likes this

#20 bartine

bartine

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1867
  • Joined: 03 Oct 2007
  • Loc: Silver Spring, MD

Posted 19 February 2019 - 03:46 PM

 Is this the same scope that was discussed here by Guy Brandenburg and on his blog https://guysmathastr...lescope-making/ and here https://www.cloudynights.com/topic/590934-classical-cassegrain-telescope-from-ealing-and-competition-testing-of/ 

   If so, here is the problem, you have a  set of optics that won't produce a good image even thou a number of people have tried over 50 years.  So in  reality what you most likely have are optics that are  misfigured.  What that  means is you have two pieces of polished glass and not a correctly figured set optics.  So to make it right both surfaces will need to be refigured as well the hole in the primary correctly bored.  That requires the proper test equipment has in a Hindle sphere to figure the secondary and high quality Ross lens to  figure the primary. The cold hard truth is that  the value of this scope is the cost of  a 12.5" and 3" mirror blank at best.

 

                     - Dave 

This is the same telescope that Guy was discussing.  Thanks for bringing this "into focus" David.

 

Guy actually responded to this discussion above.  It does sound as if the optics here are limited as to their usability as is, and that most of the issues are directly related to the secondary mirror not matching the primary.  



#21 Boom

Boom

    Mariner 2

  • *****
  • Posts: 261
  • Joined: 11 Jan 2008

Posted 19 February 2019 - 05:01 PM

I'm curious how an 8000 1967 dollar telescope made using old world USA labor was delivered with such defects? and we give the far East a hard time about QC.
Then again look at Hubble, I guess anything can happen.
  • Stephen Kennedy likes this

#22 ngc7319_20

ngc7319_20

    Ranger 4

  • *****
  • Posts: 336
  • Joined: 25 Oct 2015
  • Loc: MD

Posted 19 February 2019 - 05:22 PM

I'm curious how an 8000 1967 dollar telescope made using old world USA labor was delivered with such defects? and we give the far East a hard time about QC.
Then again look at Hubble, I guess anything can happen.

Optical manufacturing has improved a bit in 50+ years...



#23 clamchip

clamchip

    Cosmos

  • *****
  • Posts: 9447
  • Joined: 09 Aug 2008
  • Loc: Seattle

Posted 19 February 2019 - 06:40 PM

I wonder if in the last fifty years the mirrors got sent in for a re-coat and instead of getting Jack and Jill

back maybe Jack and Wendy.

I'm sorry to make a joke but really its possible. I can't see The Optical Craftsmen turning out something

like that. I think Mr. Nelson's own personal telescope was a big trailer mounted Cassegrain.

The right person could make a fine telescope out of it.

 

Robert


  • tim53 likes this

#24 Boom

Boom

    Mariner 2

  • *****
  • Posts: 261
  • Joined: 11 Jan 2008

Posted 19 February 2019 - 06:43 PM

Optical manufacturing has improved a bit in 50+ years...


Ah!! It's no wonder ETXs walk all over Questars

#25 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 8518
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 20 February 2019 - 11:04 AM

 Poor optics happen all the time. That is what  I have been trying  to show people for years and why I keep telling people test their optics. The optics in the telescope is the most variable part and very few telescope have optics are better then 1/4 wave. 

  One also needs to understand that many of the telescope companies of 40's thru the 70's were making Newtonians, ie one "parabolic" mirror which they sort of knew how to make. You don't have to a get radius dead on the money with a one optical surface system. Plus or minus an inch is good enough.  Then they get an order for Cassegrain and which is two mirror system that required far more accuracy to  it get it to work. You have two radius that need to be close in tolerance and two optical figures that need to much more accurate and if done right tested as a system and the figures tuned to cancel errors. It also requires addition testing equipment and skills in making aspheric convex surfaces.  So when you put these Cassegrains on the test bench you see that that were "winging" it and really didn't have the skills needed to make them right.  I personally refigured a 24" Tinsley cassegrain that the sale ordered stated"1/10" wave optics and when  tested was 2 waves!

   So when it comes to this Ritchey -Cretien, the simple answer is that Optical Craftsman just didn't have the skills needed to make this system. They winged it. Cave had the same problem with their cassegrains as well. 

 

                       - Dave 


  • tim53 and Boom like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics