[ATM] Loveday alternative to Cassegrain

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

A few days ago, while "simplifying" Laux' three-mirror telescope, 
I stumbled onto a two-mirror 3-reflection design.
It took me a little while to realize that it is, basically, 
a Loveday arrangement. Originally, it consists from two confocal paraboloids, 
with convex secondary reflecting collimated beams back onto the primary, 
which then focuses through an opening on the secondary (not long ago 
I thought this is a Mersenne telescope; well, one learns as long as he lives, 
and dies dumb).

This arangement becomes aplantic at low focal ratios of ~f/20 and slower, 
with negligible astigmatism and field curvature. What I found out is that it is 
considerably easier to make it aplantic than a Cassegrain. Here are the basic 
parameters of a 16" f/3/8.1 system, with those of a comparable Ritchey-Chretien 
in the brackets:

primary conic: -1.096 (-1.129)
secondary conic: -1.927 (-6.353)
Seidel astigmatism: -0.0007 (-0.00164)
median field curvature: 785mm (400mm)
diffraction limited curved field radius: 17mm (11.3mm)
flat field radius within 0.025mm: 17mm (12mm)

Obviously, the Loveday is not only easier to build, but also 
offers considerably better image quality.
The Loveday would be easier to build than even classical Cassegrain, 
which would require secondary conic of -4.737 here. Its final focus is 
made accessible by placing a diagonal at some distance in front or 
behind the secondary. It would result in similar field illumination consequences 
as a front of the baffle in a Cassegrain, but Loveday's baffling would definitely be 
better due to the diagonal turned away from the incoming light. 
A minor disadvantage is a necessity of having separate supports for both, 
secondary and diagonal. A minor advantage is that a camera could be 
permanently mounted behind the secondary mirror.  

Miscollimation sensitivities are very similar for all. And yet,
both Cassegrains are widely known and fairly used, while
the Loveday seems to be in nearly complete obscurity. 
Is there something I am missing?

Vlad
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