ODDS AND ENDS.

Spin Casting Epoxy Mirrors.
Sack in the early fifties when I first became interesred in building a telescope I read an article about spin casting Epoxy mirrors for searchlights. The mirrors were spun and aluminised. The following formulae was given.

Now it so happened that I was experimenting with making epoxy patterns for castings. In those days the resin was not available in Australia and an Import Licence was necessary to get it from America. Epoxys were new in those days and a bit tricky to use. I already had a ball bearing mounted heavy slow rotating cast iron turntable which ran vibration free. I decided to make one of these mirrors. A mould was made and centred on the table. The resin was mixed and poured in . It was left running for twenty four hours. However when set it had an orange peel surface. I don't know the cause of this. I sent it down to Keith Murphy in Melbourne and he polished it,. It came up well he said. However he said the focal length was half what it was supposed to be. This made me wonder if the formulae was for the radius of curvature and not the focal length. I don't know what became of this mirror. Today they are spin carsing telescope mirrors.

Care of Mirrors.
To maintain the mirror surface in good operating condition it is essential to prevent contamination with foreign matter. The surface must not be touched. Airborn dust etc. should be blown off with a blower brush available from camera stores. When storing the telescope the tube should lie horizontal. The mirror surface is then vertical and dust cannot fall on it or condensation form on it. The open end of the tube should not be covered with plastic as the inside will sweat. Cover it with a linen cloth so that it can breathe.

Technical. ( I copied this from somewhere; I can't remember now.)

Raleighs Limit:- 1/4 wavelength of light. Experience shows that the same degree of perfection is obtained if limit does not exceed twice this figure, e.g. 1/2 wavelength.

Wavelength of Yellow Green Light. 1 /45000 or .000022"
Departure of mirror surface from a true paraboloid must not exceed 1/4 wave in order to meet the above 1/2 wave limit. Difference between a Spheroid and Paraboloid must be reduced to .0000055"
A 6" dia. f/8 mirror must be corrected to within 55/114 or approximately .045" of the value r^/R.
The actual seperation between the surfaces of a spheroid and a paraboloid of equal focal length, at any zone and optical axis coincident, is equal to, r 4 / 8R3, where "r" is the zone radius of the mirror. and "R" is the radius of curvature of the centre zone.

For a 6"dia. f/8 mirror this is .0000114 at the edge. Equal to half a wavelength of light. (Apparent magnification of knife edge test. = 100,000

Optical Quality of Telescopes. "Sky and Telescope" March 1992

"Frankly, I am surprised by the results. Conventional wisdom among amateur astronomers suggests that a 1/ 10 wavefront mirror should easily reveal its superiority over a 1/4. wavefront mirror of the same design. This is not what I observed, and it suggests to that under typical bachyard astronomy conditions a 1/4. wavefront optical system is perfectly adequate. On those rare nights of excellent seeing, maybe the 1/10 wavefront mirror would show its stuff, but even then I doubt the difference would be noticed by most observers, except in rigerous side-by-side comparisons on selected test objects. Clearly this exercise suggests that there are huge gains in visual performance as optical quality improves to 1/4. wavefront, and less conspicuous benefits with further improvement.
The 1/4. wave mirrors intrinsic performance in this test was outstanding compared to that of many other telescopes I have used. To me this says two things. First, a good quality 6" f/8 Newtonian reflector- so seldom seen anymore -is a superb instrument for the casual or intermediate level observer. Second, if manufacturers advertise diffraction limited systems and deliver on their promise, everybody but the connoisseur of costly precision optics should be happy."

Terrance Dickinson