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

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

David Whysong wrote:


> I'm looking at the Paul-Baker design - it's similar to the LSST. There
> is some light loss due to the combination of a large obstruction and 3
> large reflecting elements, and the secondary and teritary mirrors are
> very large! Tempting... if we were experienced at grinding optics, I'd
> probably do this. The one concern is field curvature, which will
> always be a problem (especially at fast f-ratios) while CCDs are flat!
>

Mirrors don't have to be large. It is pretty much optional, with the 
tertiary
needed somewhat larger than secondary for optimum field illumination.
It is only for baffling the tertiary that a large front obstruction is 
needed.

Field can be flattened by increasing the secondary-tertiary separation,
which results in r.o.c. of the tertiary becoming longer than that of the 
secondary.
The consequence is slower f-ratio, and need to aspherize the secondary
(also, somewhat more obstruction needed to baffle the tertiary). An example
would be the following 28" f/4.9 system (still with f/3.67 primary):

R1: -206.97"
K1: -1
primary-to-secondary: -75.6
R2: -51.77
K2: -0.575
secondary-to-tertiary: -69.1
R3: -69.1
K3: 0

The flat-field blur size 21.5mm off-axis is ~0.5um, slightly
larger than that in the field center. It occured to me that the two
PB mirrors could be inserted in a Newtonian arrangement
and used without removing its diagonal/focuser assembly.
Of course, it would require appropriate (heavier and more
complicated) structure.

Vlad




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