Re: [ATM] FW: Figuring flats

[Mike Lockwood <melockwo@uiuc.edu>]

Philip,

Philip Lardner wrote:
> So far I have avoided having to work mirror-on-top as I was aware of the
> danger of hand-warmth. I am experimenting with using a very short stroke
> with little sideways movement to make the work more concave (from convex)
> and it seems to be working. I'm also trying out a paper ring when pressing
> to put the outer zone of the lap out of action and not work the turned
> zone - that also seems to be working... touch wood. I guess I'm just
> satisfying a curiosity doing this before I bite the bullet.

Yes, that should work very well.  It is one of the most dependable 
ways to remove zones - if the mirrors are figures of revolution.

> The print-through from the saddle-shaped back is beginning to make sense
> now. I have noticed a *very* slight increase in resistance when working
> perpendicular diameters (easy in the 12 to 6 o'clock direction, say, and
> more resistance in the 3 to 9 o'clock direction) that can't be accounted for
> by the CeO drying or thinning out. The difference is only just noticeable as
> a slight 'catching' of the lap when centre over centre.

Keep in mind that pitch laps are often directional, and they seem to 
grab more along certain diameters.  Make sure that the extra friction 
correlates with the mirror orientation, not the pitch lap orientation.

If this slight resistance only happens when you are going across 
particular diameters of the MIRROR, then welcome to the "I have felt 
astigmatism" club.  That generally means it's significant, maybe a 
wave or so, depending on the pitch/cerox/stroke combination.

Feeling that either means that you are feeling the bending of the 
mirror under polishing pressure, or you are actually helping to fix 
the problem.  I hope it's the latter.

> I haven't started
> grinding the backs yet... still hoping for a miracle - but I have been
> experimenting with ways of making a fine-ground surface translucent without
> having to polish it out. Turtle wax is not good - Vaseline is so-so but
> slippery - a drop of oil works nicely (but is oily... duh!), floor wax is
> reasonable, and clear cellulose lacquer scratches off too easily.

It may be possible to fix these flats with MOT work.  I figured a very 
tricky 8"x1/2" Pyrex flat MOT, with a wooden holder to help me get a 
grip on the mirror (without brushing my fingers on the pitch lap) and 
to insulate it from the heat of my hands.

	Mike L.

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