Re: ATM 8" refractor project

["Bob May" <bobmay@nethere.com>]

You will want to build several spherometers as Dave indicates.  Difference
in them is the radius you measure to.  Also several dial indicators are
helpful.  A good place to get dial indicators is at a used machinery shop.
I've got a place called Grandpa's shop near me that is a little treasure
trove of odds and ends of machine tools, etc. and he's got a cabinet which
had several different dial indicators.  A working .0001" indicator usually
goes for $30 and one that needed a little oil in it went for $10.  Since I
can take a watch apart and actually get it back together and working right,
opening a dial indicator is no problem and the thing just needed to be
cleaned of crud and lubricated.  I did get one that had a bent shaft but
that wasn't any problem either as I just bent it back until it started
moving easily and lubed it and put it back together.
As to the spherometers, you generally know the radii that you are going to
need to use so sit down and calculate (use a spreadsheet to make things easy
on you and you might as well print the results out so you don't have to keep
on calculating for that spherometer!) and see what the step size is in the
areas of interest.
FWIW, knowing how thick a piece of material is (preferably metal) exactly
can help getting the right offset if the dial indicator is of a low travel
type to get the right zone for doing a measurement (you have one that goes
.050" (they usually go a little past that) max) get a shim that is .040"
thick and that will give you the ability to measure almost .090" for a sag.
when you set things up right.  You will probably prefer to have 3 of the
spacers so that you can go either way with the measurement.  I will note
that .050" on a 2" radius spherometer is really a pretty short radius tho.
I will note that the longer the travel on the spherometer and thus, closer
to the limits of the dial indicator, the more accurate the instrument will
be.
I'll also note that if you have a lathe, making a 3 ball spherometer is
quite simple but the simpler 2 ball one is quite as accurate and can be made
with just simple hand tools in short order.  I have one at the class that is
just a dial indicator pushed into a 1/4" (standard size for dial indicators)
hole in a stick that is fairly straight.  It works fine and tastes great for
doing quick and dirty measurements across a concave surface but it won't
work on a convex surface for the obvious reason that the ends won't be in
contact with the diameter of the circle of measurement!
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net