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To find the optimum setting for a non-TeleVue eyepiece, start at the outermost setting, focus the center and carefully evaluate the shape of the stars at the edge of the field. They won't be perfect.
Then, move the Paracorr tunable top in one setting, focus the center and compare the edge again.
Repeat.
There will be one setting that gives the best results (the tiniest stars at the edge). Write this down for each eyepiece.
Then, use a small label maker to put a number on the Paracorr at each of its 5 settings (e.g.1-2-3-4-5), and put a corresponding small sticker on each eyepiece that corresponds to the correct setting for that eyepiece. That way, in the field, you'll never have to remember which is the correct setting--you'll just match the numbers up.
When I change eyepieces, I install the eyepiece first, then loosen the tunable top and move it to the right setting and retighten the tunable top screw. Takes 5 seconds, and works like a charm.
I had the same remarks about the Paracorr, and especially noticed more stars in the field. I think this is due to a lot of stars at the limit now becoming visible because their star images aren't smeared out to invisibility.
I've used it for a couple years, and never noticed any deleterious effects, only positive. All the way to my 5mm eyepiece (365X).
I have an f/5 scope, but tried it recently in an f/5.5 scope and also noticed the improvement in star images, just not as great. The literature shows some positive effects in very wide fields all the way to f/8. I believe that now.
On the other side, a friend's f/3.66 dob is usable with one and is pretty bad without it.
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