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[ATM] Re: Aplantic Gregorian (was: 25" mirror possibilities)
Michael Lindner wrote:
"Reading Schroeder's "Astronomical Optics" (which is indeed a $100 book!)
I got
interested in the aplanatic Gregorian. After running through the
equations,
however, I discovered it was difficult to come up with a practical
design.
Secondary sizes tended to be 50% or larger and/or the mirrors became
quite
aspheric. My conclusion was that this is not an easy scope to make nor a
very
useful design to build - better performance for less work out of almost
any
other 2 mirror design."
It is not as easy to make as it may look like, but it shouldn't be
neither harder
to make nor inferior performance-wise to most 2-miror systems. It does
require
larger c.obstruction for a given F#, but considering that
Cassegrain-systems in general can't have
efficient baffling with obstructions much smaller than ~30%, it doesn't
make so much difference.
Its Petzval curvature is convex towards mirrors, unlike other 2-mirror
systems, which
offers an attractive possibility of having nearly flat astigmatism-free
field in combination with
matching positive lens system (such as an eyepiece, of focal reducer).
Two examples of an f/3/15 system with 0.15f1 back focal length
(f1=primary f.l.)
Aplantic Gregorian: - primary conic K=-0.982
- min. secondary size 0.29D
- secondary K=-0.48, ~f/2
- secondary-to-primary 1.29f1
- angular astigmatism 0.13Theta^2 (field
angle in radians)
- median field curvature k=R1/7.2
Ritchey-Chretien: - primary conic K1=-1.019
- min secondary size 0.19D
- secondary K=-2.44, ~f/3
- secondary-to-primary 0.81f1
- angular astigmatism 0.16Theta^2
- median field curvature k=R1/10.2
That said, the friendliest design to build and use I know of is
Dall-Kirkham.
Vlad
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