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My Home Grown 32" F2.4 / F8 / F16 Nova-Search Telescope

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Below is a picture series that creates one of the largest telescopes an amateur would ever think about making.
 
Yes, larger is better, but larger just means more degrees of freedom for mirror and mount problems to creep-in.
 
This has not been an easy project.   It took a year to get the mirror blank.  It took a year to grind, polish and pre-figure the mirror surface.  It took about 6 years from 1972 to 1978 to build the mount and trailer transporter.
 
I did all the work, Grinding for 100 hours, Polishing for 200 hours, Figuring another 50 hours. Testing the mirror surface is especially tough when the mirror is F2.4. The mount and drives needed to be simple to create.
 
The Downey Rockwell Astronomical Club provided the incentive to try a large mirror because they had a large Draper Machine to handle the fabrication of the mirror.  The rest of the work was mostly done in a home garage. 
 
It took several years to accumulate the helpful equipment to work on this project, lathe, mill, welder, home made auto-grinder,  strait arm Draper table polisher, drills and a 20" vacuum coating bell jar system for aluminizing mirrors.
The primary steps in creating a large scope are the same as for any small scope.  The glass fabrication was done with the mirror face up.  
 
The purchase of the 80cm (32" as i call it) Duran 50 glass blank was from Schott Glass Works in Mainz, Germany.  This blank cost about 2000$ including shipping by boat in 1971 dollars.
 
At this size, one is buying by the volume of glass.  A 27" standard thickness glass blank was pressed into a 32" precurved F4 blank. The 32" blank is only 3.5" thick at the edge.  The mirror sagitta drop at the center is about 0.7".  Believe me this a deep bowl.

Large Draper Machine with Oval stroks
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Draper Machine for fabricating larger mirrors

The Rockwell Astronomy Club just happened to have build a large Draper mirror fabrication machine. The Draper is 4x8 feet in size with a 5 horse power motor. The design of the Draper  gave it oval strokes.  The oval strokes would require more polishing and figuring time than a strait stoke Draper.  if you ever build a Draper machine try to use the strait stoke design. 
Let's start with the grinding.  The mirror was pre-curved F4 which for a 32" is very deep.  The mirror weights about 75 lbs, so it is too heavy to lift or push by hand face down. I decided to grind, polish and figure face up. 
 
Grinding face up was done with small pre-curved steel tools.  A company in Long Beach, California makes steel pre-curved tools.  I used 6" and 14" diameter steel tools during grinding.
 
The grinding ran through the standard grits, 80 to 600 grit.  The number of man hours for grinding was about 100.  The raw grits came from Coburn Optical of Los Angeles, California. Late in the grinding the steel tool was dropped on the floor.  This warped the cast iron tool so that it no longer conformed to the glass surface.  So, it was decided to just start polishing from 600 pits!

32 inch Mirror on the Draper Grinding Machine
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Grinding was done face up witih a smaller steel tool

Testing the Mirror to keep the surface spherical
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A wide leg spherometer is used to monitor spherical while grinding

Let's continue with polishing the fine ground mirror surface.
 
The mirror was pre-polished using Cerium Oxide. The grit size for this polishing compound is rather large. It is a fast acting polishing agent.  When used on glass care must be taken to not polish too long, as the larger grains in the grit can cause sleeks or scratches.
 
One must change over to good old Red Rouge, which is finely filtered.  The Rouge is stored in plastic bottles of clean water.  One shakes the Rouge powder up inside the bottle, but must let it sit for 1 minute to allow any large grit grains to settle to the bottom before applying to the mirror and pitch lap.
 
I decided not to make a 32" pitch lap and polish face down. The mirror and pitch lap would be too heavy to manipulate.
 
Many pitch lap sizes were used from 6" to 15" diameter during the polishing.

Polishing the 32" mirror face up on the Draper
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Polishing face up has it chanllenges

32" Flipped up on its end to Foulcault test
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A tip-tilt jig was used to orientate the mirror for testing

The next step was to decide on the Cassegrain core hole size. I used the 100" Mt. Wilson telescope diameter ratio to its hole size, and decided on a 6" core hole.
 
To cut the hole I used a 6" piece of pipe turned by a small motor. It took about 30 hours to grind the hole core through the mirror.  I ground about 90% of the core hole from the back of the mirror toward the front.  Then I flipped the mirror over and ground the last bit from the front.  I wanted to minimize chipping around the core hole. Duran 50 glass will chip eaisily.

Coring the 32" mirror took about 30 hours
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The 6" core that was ground out.

The 6" core pipe grinding into the back of the 32"
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A clay dam helped keep the 80 grit in the grinding hole

Let's continue with testing of the mirror polished surface.  The available tests for a mirror this size can be narrowed down to a few.  The Foucault and Ronchi tests are easy to set up and view. These tests are used to tell if the mirror surface is staying close to a spherical radius of curvature.  
 
For more preceission a two axis Caustic tester was designed and fabricated with small motors and digital readout. 
 
Lastly, I hoped to use a Null-Test when the final figure on the mirror surface was close to the desired values.
 
The Foulcault test was used primarily to maintain the 32" in a nearly spherical surface.  It was almost impossible to maintain a sphere, so it was decided to always deepen the mirror in the middle and polish less on the edge.
 
When grinding or polishing face up one must not let the tool overlap the mirror edge any more than necessary. Grinding and polishing worked mostly the center and mid zones of the mirror. 
 
Polishing with the Draper oval stroke caused many difficulties in maintaining the mirror surface.
 
Figuring was done by deepening the center zones and leaving the edge as a standard conic.  This avoided a turned or rolled mirror outer edge.
 
There are many conic surfaces that one can put on a mirror.  The final 32" design was to be a 107% parabola or Ritchey-Cretien type telescope.
 
However, an amateur doing a Ritchey-Cretien curve on an F2.4 surface with simple tools did not look easy to accomplish. 
 
I decided to try the easier 80% parabola or elliptical Dall-Kirkhim design.  The Dall-Kirkhim can be null tested using the two ellipse focii.
 
The first 6" diameter secondary will be easier to fabricate. It will be a convex sphere.  I ground a 6" pyrex mirror blank with about an f3 convex curve and tested the mirror by looking throught the back side.  The back side of the seondary 6" mirror was ground with about an f6 concave surface and polished out.  I could then do a null test on the front convex surface by looking through the rear surface and finding the front surface reflection.  I did not take into account he spherical astigmatism that occurs when you look through a thick lens, so the final spherical convex front is probably slightly parabolic.

Foulcault testing a 32" F2.4 primary
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It is near impossible to test and F2.4 with foulcault testing

All wood and fiberglass roll-around test cell
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The 32" out side for Null testing

This now takes us to the first telescope 32" mirror cell, and design of a roll-around mount to try star testing.
 
The design of the surrier truss test mount started early in the testing.  During all mirror fabrication the mirror was supported on an 18 point rocker system.  The 18 point cell was later changed to the 36 point cell for more zone support.

First Test Roller Mount of Plywood
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The 32" in its roll around testing mount

The construction of the first test box
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Surrier Truss was being tested as a design of the final mount

Here are some of the designed parts of the 32".  The 36 point cell with edge protection and a tube surrier truss mounting.
 
An 18 point rocker support system can be modified to a 36 point support by just adding two point rocker arms on each side of the triangle rockers.  This was a neat design.
 
The surrier truss is created by using Aluminum Rail-Fittings and Aluminum tube. 
 
The RA axis fork mount is made by welding 2" OD heavy wall steel tube into the surrier truss design.  The fork cage is bolted to a 30" diameter tank turret ball bearing cage.
 
The DEC axises were created using  3" air conditioner pillow block mounts.  The center of gravity of the mirror, mirror cell, and truss pieces falls just above the mirror cell. It was possible to mount a 3" steel tube right off the mirror cell and into the pillow blocks to create the DEC axises.

32" Aluminum 36 rocker points mirror cell
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This mirror cell minimizes the thin mirror deforamtion

Close up of a 36 point rocker system
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Each of the 3 rockers holds 12 rocker arms

The upper section of the 32"f2.4 Surrier Truss
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The whole telescope is Aluminum parts

The final assembly of the 32" on the Trailer
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The main RA gear is a tank turret ball bearing

The final 32" telescope was assembled on a welded steel sled. The sled can be mounted on top of a 21' flat bed trailer for transportability.  The idea was that I could lift the whole sled plus telescope off of the trailer when a final observatory site is found.  This site might be California City, California where we still have some dark skys.
 
Another idea for the telescope sled was so the sled could be jacked up off the trailer to minimize vibration from people walking around the trailer during viewing.
 
The RA axis can be tilted in elevation using a 1.5" all-thread bolt.  This allows adjustment for the elevation angle to the north pole star where ever you take it for viewing.
 
RA and DEC drivers were designed to use pulley drives.  A large pulley on each axis would have several strands of steel fishing line warpped around the pulleys and run down to a gear reducer drive motor.

32"F16 Fully Mounted on the Travel Trailer
32intrailer.jpg
The Forks are steel tube, the telescope is aluminum. Total weight about 600 lbs

The telescope was designed for traveling.  Using the hand cranked gear widget, one can lower the RA axis tell the mirror cell sits vertically on a rubber tire.  This dappens the trailer vibration going into the telescope.
 
The trailer empty weights about 700 lbs.  The fork mount, sled, and RA axis bearing weight about 300 lbs and the telescope mirror, cell, surrier truss and secondary weights about 400 lbs.
                                     Aluminizing the 32"
 
This is a lucky day in august 1972.  I could not find any of the professional aluminizing companies that could handle a  32" mirror.  I thought about building a small bell jar to aluminize the surface myself.  At this time I was in college and my funds and places to build heavy bulky equipment were limited.
 
I decided to drive up the big astronomy department at Pasadena, California.  Yep! Cal Tech.  Why not give them a  try. They can do 100" mirrors so a little 32" might just fit in any bell jar they have that is still working.
 
I walked into the current office of the astronomy and optical department director.  I politely asked if I could arrange with his master optical man to coat my mirror.  He replied that it was not the policy of Cal-Tech to do such functions for private people.  I don't know if the no answer was political, or economic, or insurance related.  This did not seem a thing large universities do for public persons.  But, he did suggest that I go visit his master optician at the time.  I walked over to the optical building and found a Mr. Goff (deceased 2002).  I explained my problem and that I had the mirror in the car. What can we work out.  He said come back at 6pm and we will go to dinner, of which I will buy of course!  He cleaned the mirror with proper liquids and powders, and we put it in the original 50" bell jar. He started the vacuum pump motors. After dinner and some 4 hours later the vacuum was ready.  To aluminum a mirror you melt pure 99.999 aluminum pellets on wicks inside the vacuum jar.  When the pellet melt and then vaporize inside the bell jar the aluminum vapor coats everthing in its path.
 
The mirror was not over coated with any clear coatings.  I did not know how many years the aluminum would last.
 
I will show you here the original mirror coating of 1972, and now after 30 years.

Fresh aluminum coating on a 32" glass mirror
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Aluminized in the Cal-Tech 50" bell jar of old times

Personal List of helpers during the 32" grinding, polishing and figuring.
 
Mr. Stan Truitt master optical Tech of Rockwell in the late 60's.
Mr. Larry Harden master optical Tech of Rockwell in the last 60's.
Mr. Brian O'connel, student, helped make many a telescope.
Mr. A J Kiss, draftsman helped make many a telescope.
Mr. Jim Kissick (deceaced 2004) past president of the club.
Mr. Bob Haney ran the Rockwell Downey Club many a year.
Mr. Larry Huges graphic artists, helped run the news bulletins.
Mr. Manny Machado Engineer who brought steel pipe mounts to life.
Many others i will try and remember and add.