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1st Advanced
Mirror Making Seminar
18th January 2003
I was fortunate enough to attend this meeting, its purpose was to bring
together professional opticians and other interested parties to discus the
grinding and polishing of the 42 inch diameter mirror to be installed in
an observatory at The
Astronomy Centre.
The mirror is 42 inches in diameter, 3 inches thick and has a
pre-generated curve to F4. The blank weights 400lbs and is BVC, a
laminated material produced by fusing together thin sheets of glass, and
supplied by ASM products
in Canada.
The meeting was hosted by Steve Taylor, the technical director of the
Astronomy Centre. The day consisted of a number of sessions dedicated to
the various aspects of mirror making from design through to aluminising.
There were no formal speakers as such, people would just chip in to share
their experiences. Those
present included Peter Wise, John Wall, Norman
Oldham, John Owen,
Dave Owen and Peter Drew the founding father of the Astronomy Centre.
Amongst the subjects discussed was the possible optical configuration of
the telescope. Peter Wise proposed an all spherical system, he displayed
an eight inch telescope with the same optical system.
In addition to the primary there were two lens elements. For the 42
inch mirror one of these would need to be
somewhere in the region of 10 inches in diameter and was a doublet.
On the plus side the system required a spherical primary, relatively easy
to figure, on the negative side was the making of the large
additional lens elements.
Dave Thompson of the Liverpool sidewalk astronomers contributed much useful
information about the two 30 inch mirrors he had been involved in making.
The first attempt was with a 1 inch thick blank, which was a partial
success, the second with a 1.5 inch blank was by all accounts a good
mirror. Both mirrors are of plate glass.
The business of testing the mirror was discussed. Steve Taylor said he
could borrow a good quality 1-meter diameter flat if required. This
tempted me to suggest the primary be tested by autocollimation. Even
though the test flat was not
of the required diameter it could be used off set. The mirror could then
be rotated to enable viewing of the remainder of the surface. An
additional flat would be required to turn the light on to the side for
viewing and positioning the light source.
John Owen then
described the Offner null test and suggested this would be a suitable
solution. The test is very similar to the Dall null test in which a plano-convex
lens is used to introduced spherical aberration equal but opposite in sign
to that produced by the mirror under test. The result is that when a knife
edge is introduced as the radius of curvature the mirror surface is seem
to darken evenly (null) across its surface. The two tests just mentioned
assume that a paraboloid is required for the final figure. The difference
between the Offner and Dall tests being that the former utilises two lens,
a more desirable configuration when a high quality surface is required.
This was the test used to figure the Hubble space telescope mirror, I
hesitate to mention this for fear of putting the mockers on the project!
John Owen when on to talk about various aspects of mirror making which I
found absorbing.
Paul Rees who works for TTL
gave an interesting insight in to the design process used by his company
when asked by a customer to produce a telescope. He went on to give an
excellent description of the surrier truss type tube assembly.
Peter Drew
concluded the meeting with an appeal for ideas on how to attract more
people to the astronomy centre. This is a unique group in the country and
I believe the amateur astronomical community needs to support and make use
of the centre.
An excellent day
was had by all present and it was agreed that another such event should be
organised. Full marks to Steve for putting the event together and to those
who helped out with domestic arrangements. I look forward to the next
event!
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