]I communicated with Mel to get his advice on this, and this is what he had to say:
"Yes, design for the primary's focal plane: do not adjust it. Use the
normal focal plane distance. Do make the 15% smaller field adjustment.
Imagine that you place the eyepiece in the focuser sans coma corrector.
You'll see stars that are focused at the center but that literally are
stretched into invisibility at the edge. Putting in the P2, stars
sharpen up amazingly to the edge, and the focal plane changes and the
field shrinks by 15%. Think of a barlow lens that shifts the focal plane
to increase the focal length. In fact, as you no doubt know, the Ethos
line of eyepieces first lens set functions as a barlow to spread out the
light so that the rest of the lens sets can more effectively do their
job. The field stop is inside the eyepiece as a consequence of the
Since there is an optimal position for the P2, it does not need the
focus range of an eyepiece. While the P2 shoulder strictly calls for
3/4" more inward travel, the inward travel otherwise required to
accommodate some eyepieces can be dispensed with, keeping an allowance
for near sighted people. Also important but little discussed is the need
to vary the P2 position slightly for certain eyepieces. For example, the
8mm Ethos is strongly overcorrected and can use a little P2 position
change to fix spherical aberration. I cannot see a visual degradation of
coma at the edge.
Overall, I design my focusers to position the P2 in its recommended
position and I add 5/16" more for near sighted eyes and for making
adjustments to eyepieces that change the spherical aberration strongly.
Positioning the focuser is confusing. That's why I standardize on the
dimensions that I show in my graphic. Actually the best design is for a
small adjustment focuser for the P2, then a longer range focuser for the
eyepiece (get rid of the tunable top)."
So just use the Diagonal Calculator and make the 15% adjustment, and you are (almost) done.
According to a diagram shown earlier in the thread, the distance from prime focus to the base of the P2 is 89mm.
So, with the zero intrusion distance of 10.55" (268mm) + 89mm = 357mm (14.05"), and with the 41mm FS EP, reduced to 35.7mm (1.40") we have the diagram below. This looks perfectly acceptable to me (and I could intrude into the light path a little if I chose to widen the fully illuminated zone, but doubt I could see the difference). So a 4" secondary it is!
[attachment=1244154:Screen Shot 2019-04-10 at 7.40.32 PM.png
Edited by careysub, 10 April 2019 - 09:46 PM.