The goal: Create an instrument optimized for visual (deep sky) which delivers the best experience using the available
degrees of freedom to that end.
Feet on the ground.
Maximum (sensible) aperture; probably 32" or there abouts.
Let's up the ante and make it a binocular instrument.
In my experience (using a pair of 20" binoculars) I found that 95% of my time was spent with a 3mm exit pupil.
If one is prepared to release the constraint of achieving larger exit pupils, the system f/d can be relaxed to f10.
If we assume that we will be using a sub aperture corrector to tidy up field aberrations, why not release the
constraint of achieving a specific aspheric on the primary. The optician is then free to concentrate on producing
a smooth figure of revolution with the residual SA being corrected by the lens assembly after measuring the Z
of the primary upon its completion... in this case, who cares if we miss the target Z by a couple of percent?
Incidentally, one aspect of the DK design which few people are aware of is that the coma penalty is pretty much
all in the choice of the primary f/d, and not significantly influenced by the overall system f/d. Ergo, if we slow
the primary down to f/5, the aberrations become much better behaved. Whilst there is no such thing as an easy 32"
optic, an f5 Z=-0.6 would be a doddle compared to f3 Z=-1, especially if you didn't have to nail the exact Z directly
on the head.
Now, let's try and visualize the layout...
Separation between primary and secondary would be a little over 110" using an 8" (spherical) secondary, with the
focal plane 30" above the primary. If we chew up 24" of bfd folding the lightpath out the side of the tube assembly,
the eyepieces will be located something like 60" up the ota (away from the primary) So it would be a similar
observing position as you might expect to deal with as a 32" f2-ish Newtonian... give or take.
Choice of eyepiece(s):
Why not use something simple like a 5 element Masuyama, 32mm 84 degree?
Use the degrees of freedom in the field corrector to compensate for its field curvature and astigmatism.
Sure, it would be a bit of a one trick pony, but I think could live with that.
fwiw) Magnification would be 254x with a field of 20 arc minutes @ f/10 and 3.2mm exit pupil.
Anyway it would be one heck of a visual instrument.
Edited by clivemilne, 12 August 2017 - 10:33 PM.