[ATM] re Anybody home etc.,

[David Weinshenker]

Richard F.L.R. Snashall wrote:
> Bill Kelley wrote:
>> At 92 I just made some 4.25  F4 flexed "Omniscopes "(local atm''er 
>> name for the contraptions).  Unflexed spheres at 5" from object being viewed they
>> function as microscopes at powers from 10X to around 200 X. Flexed through the conics
>> to a paraboloid they work at any distance to infinity as a fine small telescope.
>> As a microscope they have a very shallow depth of field at full aperture.
>> With careful fine focusing they are great. I have some decent pics of a 1 mm.
>> spotted spider on a rosebud. I use a Ronchi screen on a star for quick tweaking the paraboloid. 

> Why isn't the natural object/image point for the unflexed sphere
> at the sphere center?

Interesting: a point at the sphere center would focus back to itself
(e.g. "null" condition of Foucault test at center of perfect "uncorrected"
sphere); a point at the parabolic focus (F=R/2) would focus (neglecting
aberrations) a point source at infinity (e.g, astronomical case); a point
closer to the surface than that would focus to a diverging beam that would
have a virtual image behind the surface: could this be used as a sort
of large-aperture microscope?

Would the surface for least aberration then become a flattened ellipse?
If the instrument were being adjusted visually by the observer, perhaps
the condition of best focus is actually flexed slightly "the other way"
(relative to the setting for use as a paraboloidal telescope) from the
neutral sphere!

-dave w