Re: ATM Questions on The Perfect Machine

[Anthony Stillman <atmer@flash.net>]

>...what the issue was that
>was solved by a spoke design

Deformation and weight.


>2) p345 John Anderson was responsible for grinding and
>figuring the 200 inch disk

Anderson with Brown were responsible for testing.  Brown was responsible
for grinding, polishing and figuring.  This might seem like hair splitting,
but in my opinion, too often the ivoryed get the elevator while the
un-ivoryed get the shaft.  I know it annoys the hell out of some people
that I don't buy into this, and that suits me just fine.


>He and a Frank Ross saved a lot of time when they came up
>with "a scheme that would use a [relatively] small half-
>silver flat mirror and a special lens to precisely test the
>zones on the mirror"...
>...what they did?

They used the Ross null.  A simple lens which introduces enough spherical
abberation to cancel that of a paraboloid tested at its ROC.  The Ross null
differs from the Dall null in that the light cone passes through the lens
twice.    Null lenses can be quite ordinary for slow mirror.  However, as
the surfaces get fast the tolerance of the nulling apperatus, its
construction, and placement become excruciatingly small.


>"...opticians to grind deeper, faster mirrors,
>to finer tolerances, in a fraction of the time".

Florence doesn't have a clue.


>What's stress-lap polishing?

The local ROC for aspheric surfaces changes as a function of the radial
distance from the optical axis.  Further the sagittal and tangential ROCs
are likewise different.  The latter is often ignored.  However both can be
delt with by actively adjusting the ROC of the lap, i.e. a stressed lap.
Such laps are made on deformable backers.  The polishing substrate is
either also deformable or segmented.  I've seen the deformation implemented
with both pistons and a computer or wires, levers, and a cam.

There isn't much call for stressed laps in the ATM world.  Group 70 did
consider a stressed lap for a while, but ever smaller tools and localized
polishing has worked well enough so far.


Anthony