CORRECTOR LENS

I decided to try my hand at making a Maksutov corrector lens. I had another reason also. When making the secondary mirror for the Cassegrain Optics I was making 1 found that small diameter convex surfaces was a different game to long radius concave surfaces. How was I to know if the mirror was zone free. What 1 needed was a test plate to check them by interference. As I intended making a corrector lens later I wanted to use the same tooling. I therefore made the radius of the secondary the same as the radius of the convex surface on the corrector. I would make matching convex and concave test plates, the latter to be used to test the Cassegrain secondaries. I was not interested in making a Maksutov telescope, only in making the corrector lens.

Getting blanks for the corrector lens was going to be a problem. As it didn't have to be optical glass I decided to use plate. First I had to cut out four 150mm. diametes discs from a piece of 15mm. plate. My practice is to always make more than one. This allows for accidents and second tries. I thought of cutting the discs out with a biscuit cutter and carborundurn powder. But I found a place Jet-Cut that had a machine using high pressure water and garnet abrasive who could cut them out. This was quite fast as the finish on the outside was not fussy. I intended grinding this later. Most of the time went in programming the computor that controls the machine.

Next I cast and machined two sets of cast iron tools, concave and convex. one set for the concave survace and the other for the convex. The machining and channelling of one of these sets is described in "Cast Iron Tools". The other was machined to use as formers for slumping the plate glass to form a meniscus lens. Before risking one of the glass discs I obtained from The local branch of Pilkington a piece of square piece of glass 6" across corners and 1/2" thick. His is the thickest carried in Calms. I was given pamphlets on the float process of producing plate glass and was told that when the glass came off the tin bath it was at 600° C. and was in a plastic state. This I reckoned would be the temperature that the glass needed to be heated to to slump.

What follows could be consinered a case of "fools rushing in where angels fear to tread" or not enough though was put into it. Years ago I passed over my small foundry set-up to Peter Tolputt who has made my aluminium and bronze castings since. I took the glass and the formers round to Peter to cant' out the operation at his convenience. Fig.1. illustrates how we set it up in the furnace. The formers were raised up from the furnace bottom so that the flame would be directed under the formers. A steel band protected the glass from direct contact with the flame. A weight on top of the glass was to press the glass down as it softened. Peter waited until he finished pouring a pot of aluminium before placing the set-up in the furnace. This would give a gradual warm-up before turning on the burner. He did not have a pyrometer. We have been melting aluminium and bronze for thirty years and judge when the metal is hot enough to pour by looking at it. When he considred that heat had soaked through everything, he lit the bumer and ran it at its lowest. The lot came up to a red heat before the weight sunk down. He shut the burner off and put a cover over the too of the furnace to stop air passing through and causing too rapid cooling. He left it overnight. Next morning he lifted it out.....DISASTER!

We had got it too hot and had too much weight. The glass had melted and the glass fatened out to a 6" circle. Also it had fused to the iron former in places and when the formers were pulled apart the glass was shattered.

Now Debie Evans who manages my son's office makes dolls as a sideline and has a electric kiln. I asked her if she would have a go at it. She was reluctant at first in case she ruined my glass but as I had four pieces of glass and only needed two I assured her I could afford to loose a couple. Her furnace is computor controlled and the heating and cooling cycle is programmed in. The glass was placed in the furnace resting on the concave former, with no weigth on it. The temperature was set at 600° C switched on. It takes hours to come up to temperature and when it did. the glass hadn't moved and looked as if never would. Now they say "When everything else fails, read the Instruction Book". Referring to her instruction book she found a little about glass slumping and the program. A temperature of 1350° F. was required. It also said that when the glass slumped the lid of the furnace should be lifted to drop the temperature rapidly to 1000° F She entered this program and switched on. I went home. On the way I called on Peter to tell him of progress. Since I had last seen him he had been talking to Judy, a woman he knows and who does glass slumping. She said we had got it too hot in the gas furnace and should not have put a weight on it. Also we were going about it the wrong way. We should be slumping the glass over the convex tool, not into the concave former. She said that she had only slumper glass up to %." in thickness and had no experience with anything thicker. She said though that the lid of the furnace had to be lifted when the glass slumped to cool it down before the edges began to flow. Her furnace has the heating elements in the lid and the heat is directed down on the glass. Why didn't I asked her to do it for me? Because doing it oneself is the "Name of the Game". Judy makes her own glass and could if required mould a piece between my formers.

I On leaving Peter I phoned Debbie and told her to switch off the furnace. She picked up the convex former and took it home for another go. Before she went to bed that night she had a look at the kiln. The temperature was up to 1350°F. She lid for a look. The glass had slumped. I don't know if lifting the lid to have a look was the same thing as doing so to drop the temperature. but she closed the lid and left it in the hands of the computor control. On removing it from the kiln the next morning it was beautiful to see: no cracks. SUCCESS !.

I asked her to do another one and to bring them to work the next morning. I then phoned Peter to give him the good news. He said to wait `till we had the second one before celebrating. The first may have been a fluke. Next morning we had the second one. Debbie said she put it in the kiln, switched it on and left it to the computor. The cycle took seven hours. Should the credit go to the program. No! says Debbie: to her for being able to key it in. She could be right. Below is the program. It makes no sence to me. She was as excited as anyone at the success of the operation and when she said she would slump the other two pieces of glass I accepted. Fig.2. illustrates the glass slumped over the former. By the time the outside diameter is squared it will be down to 5-3/4". This will not matter.

The former had tool marks on it and these came out on the glass. Also it was black from being in the gas furnace. This was picked up on the glass also. So I polished the surface up to remove both. But the glass slumped over it on the third time had blemishes. It appeared as if on cooling the top surface hardened before the bottom surface and contraction pulled the glass up in places. See photo. As the depressed areas were only 1/16" deep they would come out with diamond milling.

We decided we would slump the forth piece of glass into the concave former to find out how it would go. The former was glass blasted to give a smooth surface. It slumped OK, but it had a deeper depression and in a neat circle about 2" in diameter. See photo. We could not explain this and destroys the above theory.

Fig. 1. shows the set up we tried for slumping in the crucible furnace.
Fig.2. & 3. shows how the glass disc was placed on the formers in the electric kiln.
Fig.4. shows the dimensions of the blank after slumping.

All the above has taken place over a period of time. It must be two years since I had the formers cast. Now there will be another pause. I have to build a machine to diamond mill the outside diameters.