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Re: [ATM] Commercially available spherometer withMitutoyoDigimaticIndicator
Hello Bob,
Sorry I couldn't make last Saturday. I planned on making the 160 mile
drive, but my brakes decided they needed replacing instead.
I'm planning on making this Saturday, if you're going to be there.
Sounds like you know the spherometer in question. It actually has been
invaluable, even with it's challenges. I've already been fine grinding both
mirrors, so I only have one flat surface left. I would have two flat
surfaces for comparison, but I managed to grind the wrong side of the mirror
during one session, so I have one mirror with a .004 sagitta on the back
side. I'm going to grind both mirror backs flat by grinding them against
each other. Ultimately, I plan on having a good flat in inventory
specifically to act as a good reference. As far as the grits go, I'm just
about done with fine grinding, so pits are not an issue. We'll probably be
making the pitch lap this Saturday. I've been picking up the spherometer
and setting it back down. Found it to be the only way to work with the
sticking needle.
Looking forward to this Saturday! :-)
Regards,
Terin Lyr D'Amico
Camarillo, CA
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Bob May
Sent: Wednesday, February 22, 2006 6:22 PM
To: atm@atmlist.net
Subject: Re: [ATM] Commercially available spherometer with
MitutoyoDigimaticIndicator
I think that Peter lent you his plastic bodied spherometer. That
one does have the possibility of meauring oddly at times. The
mention of the bezel indicates that was the one lent ot you.
I'll also mention that some of the glass used for the reference
flat isn't that flat. Check both sides of the glass for the flat
surface just to make sure. Use another piece of glass (even a
window will work if you check to make sure that you've got the
flat surface by measuring the window in several locations as well
as other windows) to find out what the 0 reference is.
Next is that the coarse grits won't give a very accurate
meaurement as the peaks and valleys of the ground glass will end
up with the spherometer sitting differently than with the finer
grit surfaces where the peaks are less different than the the
valleys and their distance between them is a lot less. Also, the
finer grits allow the surface to be a lot more spherical than the
coarse grits where the surface is quite hyperbolic and you will
measure a lot longer ROC toward the edges of the mirror surface.
Lastly, to prevent the changing of the zero, don't slide the
spherometer across the glass as this erodes the feet and thus
change the zero point. Rather put the spherometer down and tap
it lightly to settle the feet down. This will tend to knock the
foot off of any high peak and get you a better meaurement. If
the rod for the indicator sticks out the top of the indicator, I
also tap that one to make the meaurement more accurate. It is
very easy to see a change of a thou or more when doing this.
Hope to see you when you come down again to Peter's place@
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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