Re: [ATM] Corrector/reducer for a fast Newtonian

["vladimir" <svla@isp.com>]

>> I'm looking at the Paul-Baker design...

Thought it would be useful to have PB parameters posted
within this thread, both for the design offering exceptional
image quality, and for the parameters being quite simple.

The original PB has parabolic primary, spherical convex secondary
with the radius R2=kR1, R1 being the primary radius, and
"k" the hight of marginal ray at the secondary in units of
the aperture (it can be expressed as 1-s/f, where
"s" is the primary-to-secondary separation, and "f" the
primary f.l.). Being confocal with the primary, secondary
creates a zero-power (parallel) output beam. Concave tertiary is
spherical, with the radius R3=R2, which also
equals secondary-to-tertiary separation.

For a flat-field PB, secondary-to-tertiary separation increases
to R2/(1-k), which is also the tertiary radius. Secondary mirror
becomes an ellipsoid with the conic given by

K2 = -1 + (R2/R3)^3

(Schroeder, Astronomical Optics 1st ed. p117).

It can also be written as

 K2 = -1 + (1-k)^3

This gives a flat-field system corrected for 3rd order spherical, coma and
astigmatism. For fast systems, some tweaking may be necessary to correct
for higher-order spherical. For the f/4.9 system posted earlier, such 
optimization
(change in the secondary conic from -0.578 to -0.574) would minimize the 
blur size,
but with very little effective improvement: the Strehl at 21.5mm off-axis 
would
increase from 0.995 to 0.998.

Note that secondary/tertiary can be as large as desired, but the minimum 
size
is determined by the size of tertiary in the converging cone from the 
primary.

Vlad 

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