Re: [ATM] A polishing we will go!

["jcmulherin" <jcmulherin@opticalmechanics.com>]

Hi Ed:

Nice to hear that you're getting started. Since you are polishing with a
sub-diameter lap I would recommend taking a look at the figure right away.
It's easy to lose control of the mirror's figure with a sub diameter lap.
You'll want to monitor the figure as you polish to maintain a good spherical
surface. Have a look at it with the Ronchi test and make sure the edge isn't
turned or that you're not making zones. If things look good, just keep going
the way you are.

Good luck!

James

James Mulherin
Optical Mechanics, Inc.
jcmulherin@opticalmechanics.com
www.opticalmechanics.com
Tel: (319) 351-3960
Fax: (319) 351-3943

> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of edwski@comcast.net
> Sent: Monday, October 02, 2006 4:42 PM
> To: atm@atmlist.net
> Subject: [ATM] A polishing we will go!
> 
> Greeting ATMers,
> Having made an 8 incher and 10 incher, I wanted to go for the gusto.
> Originally I was going to get a pregenerated 16 from Newport. Those plans
> got delayed(luckily). A few weeks ago I purchased a ready to polish 18
> inch from James Mulherin at OMI. The mirror arrived very well packed and
> fine ground,  excellent! Looked too nice to polish. Our local mirror
> making group meets at the NCA(National Capital Astronomers) mirror class
> run by Guy Brandenburg(a big thank goes to Guy!).
> We had a old 12 inch pitch lap. After softening it with hot water, we
> pressed it with 100 lbs of weight. Then the journey began. I began
> polishing with cerox 1670. On the first night, I only manged to put in 45
> minutes of polishing. The center was beginning to polish out. :)
> A week later, I cold pressed the lap for 30 minutes and began polishing.
> Holy Pitch Lap! It is quite a work out, especially when you start to
> squeak. 45 minutes and I needed a break. 15 minutes of cold pressing and
> back to polishing.  All was great until I heard the most dreaded sound you
> can hear while grinding/polishing. @$%&* I had scratched the blank!!.
> There are a few, one or two that I don't think will get polished out. The
> pitch lap was made on a base of some-sort of poured material, this is ok,
> but there were still tiles from grinding under the pitch. GRRRRR. I was
> back again this past friday, we switched to another pitch lap, this one is
> better. Total actual polishing so far, 3.75 hours. The polish is all the
> way to the edge, no hazy areas, just a nice smooth polish. Hard work but
> fun. I plan on a total of at least 8 hours of polishing before taking a
> look on the test stand.
> 
> Until Then, Grind more and Enjoy the Sky
> 
> Ed Witkowski
> Woodbridge, VA
> 
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