Subjects
 
Optical Shop
Polishing Bowl
Edging Machine
Null Tester
Tube Fittings
Worm Drives
Optical Workshop Equipment
Spindle Nose & Adaptors
Slip Ring Seting Circle
Mirror Cell
Diagonals
Odds and Ends
 













 

ODDS AND ENDS.

Laps.

Years ago, polishing laps were made of pitch. I bought it from the local hardware store. It was used by boat builders for caulking. Some of it in my opinion was synthetis rubber. A lap made of it could not be pressed : it would just spring back. A block of it could be left on the shelf and it never moved. I did find one brand. De Mericks that was OK and I used that. Later I obtained some Sweedish optical pitch which was better. Recently when I went back to mirror making I found the hardware stores did not carry pitch, so I bought a lump of pitch from the local battery factory. I left it on the shelf and observed it. It never moved. After four years it is still in the original shape. I phoned Astro Optical Supplies in Sydney and was told oitch was no longer used. They could supply me with Tropic Lap'. I purchased enough for two eight inch laps. On receiving it I found I was supplied with resin, wood flower and castor oil. As I wanted a quantity that would last me for years I decided to mane my own. Years ago I used resin for other purposes and purchased it from the hardware store, but when I asked for it they had never heard of it. Through a Government Department that renders assistance to industry I was put in touch with a Rodeo supply company operated by and old couple. They had some but it was so long since they purchased it they didn't remember where they got it. It is used to rub into lasso ropes to hold them in a stiff circle when throwing. They didn't have a great quantity and I purchased the lot. Next was the wood flower. I bought a length of 3" wide softwood and cut it into 3" lengths. Using a coarse disc I sanded them into wood dust. The side grain is presented to the sander. not the end grain. A vacuum cleader was used to draw off the wood dust from the sander. in a f ew minutes I had made enough wood flour to last years. Castor oil was no problem. I used the quantities I had purchased for the mixing ratio. I found that this was not very critical.

Low Profile Focussers.
It was suggested to me that I fit low profile helical focussing units to my telescopes to allow a smaller diagonal with less obstruction to be used. I made helical focussers but didn't like them so designed a fairly low ,profile Crayford type unit. Detals of these are elsewhere. But then I did some calculations. If the diagonal minor axis is reduced from 1-1/4" to 1-3/16" The difference in area is .119 sq.ins. or .004 of the mirror diameter. What difference does so small a gain make? It's just hair splitting.

Setting Circles.
I fitted setting circles on all the telescopes I made, but personally never used them. I asked a local chap who has been actively viewing for over thirty years if he had and he replied he wouldn't know how to. As I understand a calculation has to be made which takes into consideration local time and Grenwicn Mean Time, by the time it is done it is out of date. Of course today a calculator could be programmed to do this in seconds. But it has to be done for every new sighting. On the De-Luxe mountings that I made with motor drives I fitted Slip Ring R.A. Setting Circles. The circle is carried on the worm wheel and when the motor is switched on is driven continuously. A Friction clutch drive from worm to the Polar Axis allows the telescope to be directed where wanted without having to unlock anything. The circle is held on the worm wheel by three screws that locate in a groove in the worm wheel. The heads of the screws provide a finger grip for turning the circle. Grease provides the drag to hold it in place. The telescope should be set up accurately with the Polar Axis at the local Latitude and ,pointing at the Celestial Pole. How accurately this can be done with a [portable mounting is questionable. However I understand setting circles are only needed to get the object in the finder scope. In operation the motor is switched on and a bright star located and centred in the eyepiece. The slip ring is then turned so that the indicator which is fixed to the axis reads off the RA for that star. The telescope is then set for the night as long as the motor is not switched off. To find an object swing the telescope to its RA readind on the circle; locate its declintion on the declination circle, and you have it. As simple as that. No calculations. Below is a design suggestion.

 

Astigmatism.
When I returned to telescope making after a thirty year break, I made up five 6" equatorials; grinding, polishing and figuring the mirrors. These were aluminised. I assembled the first one and that night tested it on a star. The out of focus image was astigmatic. No, I thought: not that. I knew that astigmatism is difficult to detect on the test bench. I put another mirror in the telescope. Still the same- I rotated the mirror. No different. I put in another diagonal. Still no change. I rotated the eyepiece. No change. The only thing left to rotate was the eye. I always stood on the left side of the tube and observed with my right eye. I moved to the other side and looked again. Lo and behold. the astigmatism had turned 90 degrees. The fault was in my eye. A couple of months later when getting a new pair of glasses. I asked he optometrist if astigmatism had been detected in my eyes in the tests they make. Oh! Your eyes have been astigmatic for years was the reply. A couple of years later I could not get a sharp focus on a telescope I was viewing with. I couldn't understand it as the optics checked OK on the test bench. I don't know why now but I decided to try my left eye- The image was sharp and dear. I then tried dosing one eye and then the other when reading. My right eye had gone suddenly blury. I was referred to an eye specialist and after a range of tests and examination was told that nothing wrong could be found with it. Later when getting a new pair of glasses I mentioned it to the optician. Old age, he said. There was nothing could be done about it. 1 accepted this.

Scribing Setting Circles.
To do this I built a scribing machine using a circular milling table. This had a r'2: 1 worm reduction: one turn of the handle equal to five degrees. I made up and fitted to this a five position Geneva indexing attachmeent. One turn of the handle indexed one degree with a holding period during which the line was scribed.

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