Re: [ATM] confusing results

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

If you don't measure the sag. accurately enough then your
calculations will lie to you.  I measure the sag. with my
spherometers to 0.0001" to get a decent accuracy on the focal
length and still can be off by an inch or more.  You also need to
have the radius of the spherometer to fair accuracy or your
measurement really isn't worth much.  Then you have to do the
formula correctly to get the right answer.
If you're measuring across the whole mirror, 1/2 of the diameter
of the mirror is the number to be used for the radius for the
spherometer (generally given as "r").  I'll also remind you that
you can't mix measuring systems or you will definitely end up
with problems in your answer.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Addresses have been munged up to keep spammers from bothering me.
replace the obvious words with the proper character.
Thank you for your understanding!

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