Ferrell's mirror: fine grinding update
Doug Ferrell (dferrell@adoc.xerox.com)
Wed, 12 Jul 1995 11:28:11 PDT
It's been a while since my last update (partially because I
haven't done as much as I should have). I spent about 3 hours grinding
with #220 with about 2/3 of the time MOT and 1/3 TOT. I used the
standard 1/3 diameter center over center stroke the whole time. I
spent the extra time after looking at my surface under a microscope
at about 100x. WOW! You can really tell that those pits are actually
chips out of the surface. Upon finding some larger pits which measured
from 3/1000 to 5/1000 inch across I decided to spend a little more time
on the #220, hence the extra hour. My focal length was just a bit under
53 inches when I stopped.
Currently I am debating whether I've done enough grinding with
#320. Having spent about 2.5 hours (same method as with the #220) I think
I should be moving on to #400, but under a 10x loupe I see a mostly
uniformly granular surface with some larger pits. These are about 2/1000
to 3/1000 inch across. Are these small enough to be removed by #400 grit?
It seems like they are to me, but I'd hate to waste a couple of hours
grinding to find out. Does anyone have a list of pit sizes vs. grit sizes
showing how big a pit can be removed with a given grit (in a 2 or 3 hour
period)? I guess the depth of the pits is the real issue here, but that's
a bit harder to measure (unless you have a SEM handy). Thanks for the
help.
Doug Ferrell Xerox Desktop Document Systems
dferrell@adoc.xerox.com Palo Alto, CA
p.s. I'm planning on building that 2" Crayford focuser based on the design
that someone put out on the web recently. Has anyone else tried this? Any
caveats?