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.
||f = Focal length of Mirror
||N = Revolutions per minute
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
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
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. =
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."
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