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Equipment Discussions >> Classic Telescopes

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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5575517 - 12/17/12 10:58 AM Attachment (30 downloads)

Here is a picture of what the pitch lap would look like to figure the secondary. This one is much larger in diameter thou and was for a 6" convex surface I was working on. One for C-8 would only be 2" diameter.

- Dave


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5575725 - 12/17/12 01:26 PM

Wow, Dave; thanks! That's an impressive tome, something I can follow, but can not yet imagine doing myself. Here's my take on the situation:

It sounds as though if one has the tools and the skills, then not surprisingly, the optics of any telescope could be rebuilt. The one exception is corrector plates. Strictly speaking, they could be rebuilt, too, but it was Tom Johnson's genius in inventing a means to mass produce them that made Schmidt-Cassegrains practical. If the corrector is bad, we replace it. Mirrors, we (some of us!) can grind. (An aside: I've heard that the meniscus lens of a Maksutov is well within reach of a commited, determined amateur.)

The problem comes down to the time required to acquire the skills, the cash required to build an optical lab, or --Larry's approach-- the economics of working with what is affordable and available. Larry cleaned the corrector (he says his mild polish did not remove the coating, but only its top layer). He has since installed the mirrors from another scope (one with a broken corrector). Against an artificial star, he has rotated the components to optimize them to each other, however imperfect that may be. Next clear night, he'll star test it against a real star focused at infinity, and test it against known double stars to determine its wave fraction (I'm not sure how to call that; is it 1/2 wave, 1/4 wave, a lucky 1/8 wave, 1/1 wave (ugh!)). With any luck, we'll have a serviceable scope at an affordable price. If not, then we invent Plan B.

I like the idea of sending the original mirrors out to be stripped and assessed. Can anyone recommend a good lab?


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Masvingo
scholastic sledgehammer
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Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5575739 - 12/17/12 01:38 PM

Quote:

The one exception is corrector plates.




I did find this page on manufacturing Schmidt correctors amusing, especially the part on using the vacuum pan method where a sheet of plate glass is used as the lid of a vacuum chamber and is deformed under vacuum into a catenary curve and the top side ground flat* so that when released it has the correct profile for a corrector plate.

What tickled me was their comment "Although the process seems a bit involved, - Schmidt Correctors are surprisingly easy to make."

Although they do go on to say "Once you have a large polishing machine, a vacuum chamber and a fair bit of experience!"

James

* Edit - I'm getting confused withe the master block process, in the vacuum pan method a spherical curve is ground into the top surface whilst deformed.


Edited by Masvingo (12/17/12 06:43 PM)


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5576105 - 12/17/12 05:40 PM

Interesting page, James. It seems to show a different process from the one Celestron used. I thought Celestron sucked the glass hard against a machined steel mold, then ground the opposite side flat. That was Tom Johnson's innovation for making correctors inexpensively.

Just received a Japanese (!) Meade Series 4000 f/6.3 focal reducer won on eBay. No one was bidding on it. Cost about half of a new Chinese, and is better. The box shows it was originally purchased from Hands On Optics, and was once priced at $80. Came with original bolt case and both end caps. Any idea how old it may be?


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DAVIDG
Post Laureate
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Reged: 12/02/04

Loc: Hockessin, De
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5576136 - 12/17/12 06:00 PM

Schmdit corrector are actually easy to make. You don't need a vacuum chamber or anything like that. The tough part is finding someone who has a lathe to turn the vacuum pan which is nothing special. Just a thick piece of plate with a flat rim machined on it and a very slight depression machined in it. Once you have that it is fairly straight forward. The amount of vacuum required is very small. You can use your lungs to create it and one uses a spherometer to check the depth of the curve. The pan is filled with preboiled water to remove any gas from the water and you just suck out some of the water to pull a vacuum. You want to use water and not air because water is much less compressable so it supports the corrector while it is being fabricated. Also with water you don''t have to worry about changes in the air pressure changing the depth of the curve while your making it. Once the curve is at the correct depth you grind the corrector's surface with a premade convex tool which is made by classic mirror making methods until the complete surface of the corrector is fully ground. The surface is then polished. The vacuum released, the corrector is flipped over and rotated 90 degrees, vacuum pull again to set the correct depth and the back surface ground and polished. So 1/2 the correction is placed on each side of the corrector. This is done so one surface doesn't need to be optical flat or nearly so and also because the requirments for the quality of the corrector blank is greatly reduces because your going to work both surfaces. This is the classic vacuum pan method and it takes more time then what Celestron does with their Master block method but it will give the same results if not better, it just takes longer and in business time is money.
In the Celestron method the glass is sucked down against a glass disk that has the exact opposite Schmidt curve in it. The curve in the Master block is made by sucking it down against Schmidt corrector that was made using the classic vacuum pan method.


- Dave


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: DAVIDG]
      #5576170 - 12/17/12 06:18 PM

Pre-boiled water as the fluid for making the vacuum -- brilliant! I think of "air" with vacuums, and had not seen how air could have supported the grinding tool.

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Masvingo
scholastic sledgehammer
*****

Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5576191 - 12/17/12 06:28 PM

Hi Joe

Tom Johnson's patent applications give a summary of the 'prior art' to his inventions and state that there were two existing methods for producing correctors:

1. The classical approach which was to grind the Schmidt curve directly into the face of a glass blank of sufficient thickness that it would not deform or bend under the temperature or pressure of the grinding operation. Progressively finer grits are used until the plate is ready for polishing. It is during the fine grinding phase that the figure is checked optically and those zones which are identified as being too high are worked down, this cycle of testing and reworking being repeated until an acceptable figure is achieved. These cycles of testing and reworking make the classical approach impractical for volume production.

2. The vacuum deformation technique, which was attributable to Bernhard Schmidt, who is credited with inventing the Schmidt corrector plate in 1931. Johnson's patent application was rather scathing of this method and suggested that it would be difficult to produce quality correctors from it due to the lack of any solid support for the plate whilst being ground and that any inhomogeneity in the glass would result in non-uniform bending. Again, this method was not suitable for mass production.

Johnson's master block patent solved these problems and, once the master block had been produced, enabled rapid, consistent production of plates. One of Johnson's patent applications, #3889431, suggests that high quality corrector plates can be made at a rate of 1 plate every 4 hours for each machine in use and that a single operator can easily handle as many as 12 machines after he has developed some skill.

The patent applications also mention an article, 'Study on the Fabrication of Aspherical Surfaces' by Sakurai and Shishido in Applied Optics in November 1963, which suggested that Schmidt corrector plates could be made by bending a thin glass plate over a mould having the inverse of the Schmidt curve and then grinding and polishing flat the other side of the blank. Johnson goes on to note that the article did not indicate how to bend the glass plate over the mould nor did it deal with other practicalities of implementation. It was these issues which Johnson solved and which, in part, lead to him being awarded the David Richardson Medal by the Optical Society of America in 1978, Johnson being one of only a few non-Ph.D. optical engineers to ever receive the honour.

Congratulations on the f6.3 reducer, sounds like a good buy. I'm sorry but I don't have any idea of the age, it would be interesting to know when they switched from Japan to China.

James


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Masvingo]
      #5576273 - 12/17/12 07:24 PM

Quote:

The classical approach... was to grind the Schmidt curve directly into the face of a glass blank




Hard to picture how that is done. You're saying they took the optical equivalent of a router or Dremmel tool, and carved a circular rise into the glass? The process sounds similar to my whittling a corrector out of a block of wood, albeit with a lens grinding gizmo instead of a jack knife. Was the tool of significantly smaller diameter than the blank?


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: orion61]
      #5576537 - 12/17/12 10:24 PM Attachment (27 downloads)

Lots of interesting chatter here, but we all know Cloudy Nighters love pictures, so let's return to the core of our epic tale. Larry had quite a battle with the salt-encrusted, steel-corroded-to-aluminum screws holding the focuser, but he emerged victorious. In his own words:

"I finally got the focus cap screws busted loose on it. I worked about 2 hrs getting the screws and the focus assembly out. There is an end screw that must not be twisted out because of the location and depth a regular tap will not work, we would have had to replace the whole focuser those cost $38.00! BUT, I got it.

"The screws I ordered that were suggested to me were the wrong size, so I had to reorder a new tap. Those C8s take 4/40 screws. I will be replacing them with Phillips style. The older ones I have seen all had that kind, not Allen. My Super sharp C8 has Phillips, and it is a year or so newer than yours. I think they just used whatever they had. The Hex screws are about 12 dollars shipped; the Phillips are 1/4 of that price!! If you really want hex, I will put those in for you.

"I really love working on these things."

Indeed, it shows, Larry.

Now, about those screws at four times the price: Hex, schmex. We're going with Phillips.

Edited by Joe Cepleur (12/17/12 10:34 PM)


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5576553 - 12/17/12 10:30 PM Attachment (24 downloads)

Another image of the focuser upon removal. See all that salt! Looks like it got tossed into the end of the production line at a pretzel factory. Amazing to see it removed intact. Larry has the necessary skills, patience, and passion for this sort of work. It's play for him!

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Masvingo
scholastic sledgehammer
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Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5577367 - 12/18/12 02:03 PM

Quote:

Hard to picture how that is done. You're saying they took the optical equivalent of a router or Dremmel tool, and carved a circular rise into the glass? The process sounds similar to my whittling a corrector out of a block of wood, albeit with a lens grinding gizmo instead of a jack knife. Was the tool of significantly smaller diameter than the blank?




Well, maybe not quite as dramatic. Conventional techniques as used for producing mirrors, using laps and progressively finer abrasive pastes would be applied but concentrating on the areas where glass is required to be removed. I would guess that generally speaking a lap some degree smaller than the plate would be used. From what I've read it is important that most of this work is done with the glass blank that will be the corrector being rotated about its centre to keep the glass plate a perfect figure of revolution.

I guess that this would also be the method used to produce the master block in Johnson's method which would have an inverse of the desired Schmidt curve ground into it.

James


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Masvingo
scholastic sledgehammer
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Reged: 01/10/12

Loc: Ayrshire, Scotland
Re: Restoring a C8 After Purchase -- Update new [Re: Joe Cepleur]
      #5577375 - 12/18/12 02:06 PM

Quote:

Another image of the focuser upon removal. See all that salt! Looks like it got tossed into the end of the production line at a pretzel factory. Amazing to see it removed intact. Larry has the necessary skills, patience, and passion for this sort of work. It's play for him!




Yikes! Salt everywhere. Kudos to Larry for having the patience to get things free. I invariably get impatient, use too much force and then shear screws or worse!


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Masvingo]
      #5578494 - 12/19/12 07:28 AM Attachment (21 downloads)

This innocent looking picture --if I understand correctly, which I may not-- hides the clue to what we anticipate as the only snag in the project. Here we're looking at the fork's cross arm, where it rotates about the mount's base. Larry reports that the bearings inside are broken. Best he can figure, after they froze from the corrosion, The Incredible Hulk tried to free them with brute force. The forks refused to rotate, but the base was willing to break. Note the corrosion in the axle:

Edited by Joe Cepleur (12/19/12 02:43 PM)


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5578498 - 12/19/12 07:33 AM Attachment (19 downloads)

The break was not visible until Larry dissected the base for repair. There, lurking between the two motors, hidden inside the housing shown below, were bearings, once upon a time. Now, there's just rust! Too bad. If they hadn't been broken, this would have been nothing a nice, relaxing bath in penetrating oil wouldn't have relieved.

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5578503 - 12/19/12 07:38 AM Attachment (18 downloads)

With the story being presented somewhat out of order, this sounds strange. Didn't the motors run already? Didn't the forks turn? "Yes" and "yes," but apparently there is a difference between the forks spinning unburdened on the test bench and their actually carrying the weight of the tube, along with attached cameras, eyepieces, finders, and counterweights. It had all looked so close to pristine at first, as the image below shows:

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5578515 - 12/19/12 08:02 AM Attachment (21 downloads)

One can hardly feign surprise, upon reconsidering the condition of the inside of this casting. No problem! Larry sourced the necessary replacement parts. He knew just whom to ask, after repairing scopes for so many years. (When I find his mention of his source in my email, I'll be glad to edit this to credit it. Cloudy Nighters will want to know where to call for parts!)

With the magic of PayPal, I slipped Larry a Ulysses S. Grant to cover the cost of the parts, and the project jumped forward again. While we always hate to see a scope unnecessarily parted out, it is comforting to know that parts are readily available for these old C8s. Margarita's original base will now add to the worldwide supply! It contains good gears, good motors, good castings... Larry will be free to swap in the best parts to prepare Margarita for her next forty years!


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5580351 - 12/20/12 10:12 AM Attachment (14 downloads)

Look what I found! Even better pictures of the bad bearings, or at least the housings around the bad bearings. A little too much salt on this Margarita, but Larry fixed it!

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5580357 - 12/20/12 10:14 AM Attachment (13 downloads)

A little blurry, but one can see the rust in the base's central hole.

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Brian RisleyModerator
Carpal Tunnel
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Reged: 09/04/06

Loc: SW Florida
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Joe Cepleur]
      #5582508 - 12/21/12 01:22 PM

Is that the metal or nylon RA Lock Plate? Looks like the metal version to me.
Brian


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Restoring a C8 After Purchase -- Broken Base! new [Re: Brian Risley]
      #5583739 - 12/22/12 09:06 AM

It is metal. Joe has a couple pics from through the scope
put together, it has been to cold and I have been too busy to get it completely finished I think some questiones will be answered by the shots.


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