John, et al.,
My wife just pointed out how much of a bonehead I was. It took her to show me the error in my logic. Now I eat crow....
The focal plane of the 8mm Ethos is 0.7" below the 2" shoulder.
When I put the eyepiece in the adapter, it automatically raises the eyepiece by the length of the 2" skirt (1.04"). Hence, the focal plane of the eyepiece is now 1.04-0.7 ABOVE the adapter (or 0.34"). So long as I also take into account the thickness of the adapter, the calculator should work fine.
According to the TeleVue website, that means the focal plane is above the reference surface (the bottom of the 2" skirt the eyepiece is resting on).
That means I should be able to plug the 0.34" figure into the calculator and get the proper setting for the Paracorr.
OK. I get setting 3.
It gets setting 4 for the Type 6 Naglers (agreeing with TeleVue), and setting 5 for the 22T4 (again agreeing with TeleVue), and setting 3 for the 17 T4 (same as TeleVue).
But I get setting 3 for the 8 Ethos (no agreement there--TV says 4) and setting 3 for the 16T5 (where TV says 4), but the difference is now 1 setting (0.125" or 3mm).
That says the 8mm Ethos is best used as a 1.25" eyepiece in the Paracorr.
So, John, is my math now better? Or have I overlooked something else?
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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