Re: ATM Jamming of concentric cylinders - Tapered edge lenses and tapered lens cells

["Bob May" <bobmay@nethere.com>]

Del, You have to remember that stressing the glass of a lens affects the
light direction modification properties just like stress does in a somewhat
different fashion to mirrors.  In other words, stress the glass and the
image suffers.  Actually, making lenses semirigid in their cells is a good
idea but when you have the two lenses the same diameter, they will both sit
on the cell edge in the same position irrespective of the position of the
cell and thus provide the rated viewing of the sky.  The secret here is
that, unlike the strong off-axis changes in image quality that mirrors have,
well designed lenses tend to provide relatively more gentle errors due to
the angle of the lens to the eyepiece.  Thus, the mirror moving about gets
out of it's sweet spot a lot easier than that of a lens.
The next problem is that you have to mount a lens at its edge which means
that it is going to flex in the cell.  Somebody recently calculated the
deformation of a lens in its cell and came up with what seemed to be a
horrific error from the weight of the lens distorting the glass.  In
reality, the distortion has just changed the radii of the two surfaces and
the biggest thing that does is to change the Petzval surface radius more
than anything else.
On the other hand, pressing a lens at three points will produce a nice
triangular image from the effects of the stress on the lens.  Better to
leave the lens only lightly supported as necessary and try to make that
little stress as even as possible to just hold the lens in place in the
cell.  Soft supports do help but you don't want to rigidly hold a lens in
its cell or it will not provide the image that it could.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net