Reflections Upon a Knife Edge/Bill's Compound Scope

[btk@ix.netcom.com (Bill Marriott )]

A Long Winded Post:


1/20th Wave?

For the past couple of weeks, I been trying to finish up my 8" f5 
(4.94), with the figure flopping between 1/2 and 1/4 wave. The goal has 
been to reach 1/8+ wave, as much for workmanship purposes as viewing 
ones. With urging from Bratislav, I have been using the program Tex to 
both anaylize what I've have done as well as planning out the next 
'attack'. The new slitless tester has allowed readings that regularly 
repeat with in a .002" range, giving me the confidence to keep on 
trying to get it as close as possible.

A lot of the success I attribute to the method of plotting out the 
readings graphically on one sheet, the theoretical readings on another, 
and overlay the two, adjusting their 'Y-axis' to see what parabola 
would be the easiest to achieve in terms of correction. With Tex, you 
can punch in numbers to see how far you need to go and where, to 
achieve the figure your looking for.

Anyway, after having problems getting better than 1/4 wave with normal 
procedures (also, as this is a learning experience on both what, and 
what not, to do, knowing that a good general W stroke seems to bring 
things back to the 1/2 wave state), I decided to try something 
different, feeling adventuresome. As the graph of the figure showed 
that the centers of zones 2,3,&4 were very close, and the center of 
only zone 1 needed to be knocked down, I proceded to lay out on the 
mirror the boundries of zone 1 with a fine tipped marker. I then made a 
small (1") pitch tool, using a thick (.5") 1" dia piece of glass, with 
a 3/16" thick layer of Gugloz 64 pitch . Heat in hot water and pressed 
out on zone 1 of the mirror, chilled, scored with a razor blade, warmed 
and pressed again, chilled, pressed, and then, mounted the mirror on 
one of the DC drive spindles at work, set for 4 rpm, and in a small 
circular motion, rotating the tool regulary, polished zone 1, blending 
slightly into the zone 2 region.

After 10 rev. of the primary, checked, and found slight improvement 
(10%-aprox 1/3.8 wave), but not to bad looking, no surface downgrade 
under the 40x scope, very slight zoning.

After 20 rev, with a little more uumph, and I find myself pondering the 
following:

Tex, using both centering methods, shows better than 1/20th 
wave, two zones being closer to 1/30 (Whatever that means really). The 
two parabolas overlap almost perfectly. I have tested the mirror left, 
right, upside down, etc, with same results)

However, as was predictable, the zone 1 of the 8" mirror, with a .860" 
radius center hole, and a 2.15" outer radius of zone one shows some 
slight zoneing, and very light dog biscuit. 

I have no doubt that I can clean this up, feathering in to zone 2, or 
using the full size lap to smooth the center, probably at some exspense 
of wave rating, but that maybe OK, 
which brings up my question (finally!):

In terms of viewing (the bottom line), what is more important here, the 
overall picture,  the smoothness of this least important center 
zone,(it's not that bad really, the way it is) or its specific radius 
as pertaining to the current 1/20th wave rating, and, CAN these 
differences be appreciated visually, versus the posibility that I could 
proceed to make things worse?  Also, with more zones in the mask, would 
the zoning effect the wave rating more? 

As usuall, comments are greatly appreciated,

Next step is the secondary, and I may leave this where it is till the 
secondary finishes up in case of design modifications that may leave 
the current figure irrelevant anyway!

 
Later.....

Bill Marriott
Forest Knolls, Ca. USA
btk@ix.netcom.com