To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


> From: "Nathan D. Spaulding" 
>
> Hey there,
>         Here is what yo do.  1) SCREW THE MATH!!!!!!!

Be careful, or the math may screw you back.  ;-)

Or as my father once put it.  "Make sure the screwing you get is worth
the screwing you get."

  2)Take your desired
> focal length, double it and make a beam compass that length.   3) Obtain
a
> piece of sheet metal, or alminum  4) Scribe an arc on it with the
compass 5)
> cut and file to the line.

As an example to highlight my concern, the sagitta of a 12 inch f/6 mirror
differs from the sagitta of a 12 inch f/8 mirror only by (please check my
math ;-)...about 0.030 inch...which is about 1/32 of an inch.

Can our mirror maker scribe a line that accurately?  Can he cut/file that
accurately?  Actually he probably needs to be accurate to about 1/64th of
an inch...or more.  Does our ATM have steady hands?  Good eyes?  Patience?

We need to ask the telescope maker in question the $64 question:  What
f/ratio is your target?...and more importantly...what is your tolerance
plus and minus?

Depending on his answer...the scribed template method may not meet his
needs.  Especially for a slower mirror that he is comtemplating.

>...This will result in the most usefull piece of
> mirror making equipment ever created.  Let me know if you do this, and
how
> it works.

Hmmmmm.  Our angst ridden ATM may not know how well it works until after
many hours of grinding and polishing to the point he can measure focal
length more accurately by other means.  (Assuming he does not make a
spherometer or use the sun/wet reflection test during fine grinding.)
Perhaps your advice will give him lots of frustruation instead?  ;-)

Leon Trotsky supposedly said "you may not be interested in revolution,
but revolution is interested in you."

Let's put that in math terms.  "You may not be interested in math, but
math is interested in you."  ;-)

OK, you can use a straigtedge and go/no-go test to measure sagitta...but
then you are not sure that you have a spherical shape during rough
grinding.  Other tests are needed.  If you make a spherometer with a 6 or 8
inch spacing of supports (for this 12 inch mirror example)...you can center
it on the mirror and take readings...then off-center...and if your mirror
is spherical you should get the same readings.

Tom Krajci
Tashkent, Uzbekistan

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