On Aug 27, 8:07 am, "jamesgangnc" wrote:
wrote in message

...



On Aug 26, 10:52 pm, Wayne.B wrote:
On Tue, 26 Aug 2008 19:31:21 -0700 (PDT),
wrote:

Progress while waiting for Fay to go away. Finished rough grinding
yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror)
and finished the #120 grit and have started on the #180. My
magnifications will be a low of 15 X an a high of about 30X.

How do you know when you have the shape just right? What do use as a
blank?

At this stage, one simply uses a depth micrometer (or more simply a
feeler guage under a straight edge) to measure the depth of the
"bowl". The one grinds the "tool" (another glass blank) against the
mirror in such a way that they naturally form a concave and convex
spherical surface. Later, to change from spherical to paraboloidal,
one uses an amazingly simply shadow test called the Focault test to
measure the figure errors. It is so amazing that this actually works
and was figured out over a hundred years ago.

My father-in-law does this as well. Surprisingly (to me anyway) hand ground
mirrors are amazingly accurate. Because if the randomness of a human doing
it. So much so that duplicating the level of precision with manufacturing
machinery is quite difficult. As I recall it's also very messy and time
consuming. You sit with a piece of thick round glass in your lap and dish
it out by hand using various grit sizes of compound and another piece of
thick round glass as a tool. Once you finish, you send the glass off to
have the mirror coat applied. On the smaller sizes if you apply a value to
your time it's cheaper to buy a telescope. On bigger sizes like 8 or higher
you can save a fair amount of money making your own mirror. Of course there
is the satisfaction of knowing you made it your self. For a boat I might
try one of the cradle style. My daughter has a 4 inch one. You can take it
out of the socket and just sit the ball in your lap.

http://www.telescopes.com/telescopes...pes/tasco30x76...

I know, I can buy one for less than what my time is worth. I just
wanna do it for the love of optics and math.
However, I can relate this to boating even more. In one of the
Patrick Obrien books, Jack Aubrey grinds a telescope mirror being
tutored by Lady Herschel if I remember right. He uses the "Finest
Pomeranian Sludge" for polishing whatever that means. At that time,
they were still making such mirrors from metal because they either
couldnt polish the glass well enough or they couldnt make a front
surface mirror. I remember from somewhere that the best metal for
this was called "Bell metal". In a later Jack Aubrey book, he
attempts to rig his telescope from the ships rigging in such a way
that it stays oriented regardless of the ships rolling in order to
view the moons of Jupiter using them as a timepeice to avoid the need
for a chronomator. This didnt work in the book as it did not work in
real life. I think the reason was that they had to use very long
focal length telescopes because they did not have the Foucault test
for parabolizing a mirror and the diff tween a sphere and parabola for
long focal lengths is minimal. Long focal lengths mean high
magnification so any movement of the ship would be magnified.