From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 


The pitch that comes with Newport kits is way too soft, IMO.  I have a
friend who made a lap with that stuff, and it literally flowed off the face
off the lap after about 3 days of face-up storage at 75 degrees!  At 65
degrees I would use Gugolz 55, with maybe 5 or 10% Gugolz 64 mixed to firm
it up a little.  Just straight Gugolz 55 would be OK.  You might be able to
boil the Burgundy down to something harder.

You don't need to have separate laps for polishing and figuring.  But for
polishing, things go a lot faster with a full size lap, and it's easier to
wind up with a good edge.

For figuring, an 8" lap would be nice, since it lets you concentrate
work on the center of the mirror.  One of the best ways to figure a large
mirror is working tool on top with a sub-diameter lap.  Just lower the
center of the mirror to obtain a paraboloidal shape.

Since you're working on a 12" mirror, a smaller figuring lap also
helps keep the tool forces down to a tolerable level during figuring. 
Polishing against a sphere with a 12" lap is not too hard.  But once
the mirror surface starts to deform during figuring, the force needed to
push that same 12" lap can be amazingly high.  An 8" requires
much less force.

Resist the temptation to cut the 12" tool down to some smaller size
for figuring.  It's great to have a full size polisher handy for things
like fixing a rolled edge, reducing over correction, and generally
smoothing things out.  A full size lap is the closest you'll ever get to an
"Undo" button.

Bill T.

>I am nearing the time to go to polishing on my 12+"and wanted to get a
>sense of how hard the pitch should be. I have two pounds of burgundy
>pitch from Newport Optics (creamy yellow color). I am working on a BVC
>blank which is quite soft. My working space is pretty cool most of the
>time (in the garage so it tends to be in the mid-60's F). I have read
>Texereau where he advocates chewing the pitch to test it (maybe not). I
>have seen varying opinions here and on the Web. Any really good advice
>from those of you who have worked BVC or other softer glass before?
>Looking for tips on how to test he hardness, how to adjust it
>(turpentine?), etc...
>
>Also, should I plan on using two different laps for polishing and
>figuring? If so, should the pitch be softer/harder for one than the
>other?
>
>
>


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