Re: [ATM] Better way to Foucault test- (never see false astigmatism)

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

Aw, gee, looked at your tile tool article.  That was a complex
and complicated method of making a tool!  Easier and faster this
way:

Materials -
Vertical blind material long enough to go around the mirror.
Metal flashing will also work here.
Plastic bag big enough to cover the mirror plus.  Or use the next
item.
Dishwashing soap or thick oil, either mineral or veg. as to your
choice.
Enough cement, hydrostone or dental stone for the tool.
Tiles and nippers for them.
Stuff to mix tape or string to hold the dam.
the cement or stone well.

Instructions.
Plop the mirror face up on the workbench.
Place the bag over the mirror or coat it with the soap or oil.
Wrap the blind material or flashing around the mirror and secure
with tape or string.
Cut tiles as necessary to fill the surface of the mirror without
extending to the absolute edge.  )I use tiles which are tied
together with blobs of RTV which go on top away from the mirror's
surface but tiles with a paper backing will go paper down).
Mi9x and pour the cement or stone to the depth you desire.  Note
here that cement takes a while to set although there are fast
setting cements available.  I use dental stone which setup in
about 15 minutes at the worst and 7 minutes typically from the
first mixing which means that it will be less than 10 minutes
after the pour that the tool is almost ready for use grinding.
When the stone is warm and no water is showing on the surface,
check that it is stiff and at that point you may remove the dam
and start working the back for flatness.
Seperate the tool from the mirror while it is still soft and
check the face for problems like bubbles and so forth.
Additional stone powder may be put on the face and some water and
that mixed and rubbed into the bubbles.
Take the tool and start grinding with it, being careful at first
as it is still a little soft.  After a half hour of grinding, the
stone will be fully hard and you can really work it to its max.

Tools for something over 18", I'd probablly use flashing instead
of the blind material as the tool will be getting too thick for
the width of the vertcal blind stuff.

I'll also note that the only way to avoid an astigmatic image
with a Foucault tester is to have the light sourcce at the same
level as the return beam.  This means that you need a
beamsplitter of some kind to do this  Otherwise, you will always
see the light shadow go up or down a surface with a really fast
mirror, depending upon whether the light source is above or below
the return beam.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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