Re: [ATM] Parabolizing 6" f/4.5 Issues

[Mike Lockwood <melockwo@uiuc.edu>]

Doug,

Douglas Arion wrote:
> I'm trying to finish figuring on a fast 6". Here are the
> data:  ROC: 1410 mm
> Everest pin positions: 25, 40.7, 57, 71 mm (Radii)
> Data: (Average of four sets, very consistent among them,
> standard deviation of 0.03 mm, normalized to zero at first
> zone):  0, .60, 1.24, 1.54 mm
> Suggestions??? Edge appears raised; zone around 40% raised,
> at least via FigureXP.

So you're using a moving source tester, I assume?

If so, yes, from one viewpoint the edge is raised.  It might seem that 
the easiest way to correct the mirror is to work the edge down, using 
mainly long center-over-center strokes (the central part of the 
parabolizing stroke).  I don't recommend this.

 From another viewpoint, the outer zone is just fine, but the three 
zones inside of it are raised by ~150nm.  Drag the offset slider in 
FigureXP to the left until it says "Offset from best fit ROC = -0.154 
Millimeters" and look at the plateau.

For this interpretation, you need to remove glass inside of the edge. 
  With a soft (or warmed) lap, do the normal parabolizing W-stroke, 
but using the most action when the mirror is offset by 1/2" to 1". 
That is, when doing the W-stroke, do more forward and back movements 
(strokes) while the mirror is overhanging by 1/2" to 1.5".  Shorten 
the long strokes that you would normally do when the mirror is 
centered over the lap (or you will work the outer zone, too), and 
don't do too much when the mirror is offset by 2" - 2.5" (or you will 
dig a hole in the center).

This will mainly work zones 2 and 3.  Zone 4 is fine, in this view. 
Zone 1 can easily be corrected more (it's easy to dig a hole in the 
center), so don't worry about doing it now.

For this all to work, the lap and mirror MUST remain in good contact. 
  After warming both and then doing a 5 or 10-minute press (they 
should still be warm after the press), you should feel the significant 
drag during polishing that is characteristic of good contact.  Keep 
the strokes slow, and the pressure light.  If the lap starts to 
slip/grab, you must slow the strokes or re-press.  I'm guessing your 
good contact will last about 10 minutes.  Then stop, clean up, and test.

Doing slow strokes with good contact will also smooth the surface. 
Keep the lap/mirror quite wet (do NOT let it dry out!) with water - 
don't overdo it on the Cerium.

	Mike Lockwood

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