Re: [ATM] need confirmation on sagitta depth

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

The formula that I use all of the time is the S = R - (R^2 -
r^2)^1/2.  Please note that the parens and the power to the 1/2
is another way to say square root and is used here as well as
elsewhere because the square root symbol isn't available to the
type used in these emails.  So what you need to do is enter the
Radius of curvature into your typical scientific calculator,
store that number in the storage register, hit the subtract key,
then open a parens and recall the number from the storage
register, square it then hit the subtract key again and put in
the radius of the mirror and square that.  Then close the parens
and do the square root (usually the 2nd key function of the
square key) and then hit the equals key.
As  a result, I did the calculation on my trusty Casio FX-81 by
entering 78 and multiplying it by two to get the ROC, storing
that then hitting the - key then the open paren key, restoring
the stored value, doing the 2nd function to get a square of that
number then entering 6.5 and squaring that also followed by
closing the paren and then doing a square root on that number and
finally hitting the equals key.  Answer was 0.135"+ a bit.  Same
number that you got and that should have put the best focus point
six and a half feet from the mirror if you used the sun as the
light source.  If you used a flashlight, you need to have the
light and the observation screen thirteen feet away as you are
looking at the Radius of Curvature rather than the focal length.
One thing to note is that the surface needs to be an approximate
sphere in order to do the test correctly.  Having a deep hole in
the middle of the mirror with a shallow curve everywhere else
won't work as most of the mirror will be at that longer ROC.
This problem is easy to do when you are using a small tool to do
the inital glass removal.  The flashlight test will easily show
when the ROC isn't a regular spherical shape as one part of the
surface will do one thing while the rest of the surface will do
somethng else and when you look at it all happening, it will look
like a fancy plate for your food.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Replace the obvious words with the proper character.

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