Re: ATM Focal length

[Anthony Stillman <atmer@flash.net>]

>...the focal length of a parabolic mirror
>...From which zone...?   Edge zone?  Center zone?
>...Intuition tells me...the center zone....right?



Steve,

For the purposes of optical design the radius of curvature of a parbolodial
mirror is measured from the center zone.  The focal length is defined at
half the radius of curvature.

Generally ATMs use the 70 percent zone to define both the radius and focal
length.  This makes sence, as under star test or autocollimation it is this
focal length (half radius) that is measured.  Hence it defines such things
as the tube length and component positions.

It is usually easier to measure the radius to the edge of the mirror, so on
occasion it is this radius that is given.

Most data reduction techniques want the center radius.  However, most data
reduction techniques are somewhat insensitive to small changes in radius of
curvature and most ATM sagittas are proportionally small.

To muddy the water a bit more, and if you want to split hairs, you should
consider the effect of profile errors and obstructions.  The question then
becomes, which paraboloid do I choose?  Center zones and their effect on a
best fit should be ignored as the center is generally obstructed.  This is
the reasoning I've heard from some who use the 75 percent zone.  An ideal
best fit should be weighted for surface area.  Hence edge zones, where
there is a lot of glass, should drive the choice of radius.  It goes on and
on.  The bottom line----most data reduction techniques are somewhat
insensitive to small changes in radius of curvature.

Anthony