Re: [ATM] convex secondary

["vladimir sacek" <vlad@copper.net>]

I was wondering why is that focal plane sitting right in front of the 
primary, at a quite awkward location. Then realized that it is due to 
primary being not perforated.
If so, you may want to take into consideration hyperbolic (aplantic) 
Loweday. It is a 2-mirror 3-reflection design, in which concave secondary 
reflects light back to the primary, which sends it through an opening on the 
secondary to a final focus. The design requires less aspherizing than either 
classical or aplantic Cassegrain, and has better field quality than 
Ritchey-Chretien.

An example of a system that could be made with your mirrors is this f/7.75 
aplantic Loweday:

R1=-1600mm
K1=-1.0372
distance to secondary: -600mm
R2=-413.333mm
K2=-1.2949

On best surface (340mm curvature) diffraction-limited field radius is nearly 
20mm (0.36 degrees), which compares to nearly 12mm (0.18 degrees) of an f/9 
Ritchey-Chretien (-290mm field curvature) made with same mirrors. Better 
curved field performance in the Loweday is mostly due to nearly twice lower 
astigmatism.

On flat surface, at 12mm off-axis the Loweday has nearly round ~0.025mm 
blur, vs, ~0.0042x0.0022mm blur in the Ritchey-Chretien.

The design requires carefully planned and executed baffling, but nothing 
really extraordinary compared to other 2-mirror systems. And, one thing's 
for sure - it
is a sort of unique.

Vlad


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