The Paracorr's lens sits in the scope's light cone at a certain, distinctive, position, relative to the focal plane for it to correct coma correctly.
Don, I was thinking about this process.
Does this take into account the varying amount of in-focus required between dissimilar EPs?
I have the Type 2, not used it yet.
I figure I can calibrate it with either the Radian 14, or the Panoptic 27.
It just seems to me that an EP with a different point of focus would require prime focus set first, then coma adjustment with the tunable top.
Or... I'm not yet understanding how it works..
Likewise, the distance between the focal plane of the eyepiece and the Paracorr lens has to be a certain distance for the eyepiece to be able to focus on the coma-corrected focal plane of the scope + Paracorr.
That's why there can be a new focuser that incorporates a Paracorr lens. The lens is fixed relative to the telescope's light cone, and the eyepiece is adjusted in and out (using the focuser) until it comes to focus.
This is essentially what happens when you set the Paracorr for a known eyepiece and focus. You're putting the Paracorr at the correct distance from the focus of the scope and adjusting the tunable top to put that eyepiece at the right distance from the lens. When you adjust the tunable top (which has 7/8" of an inch of adjustment not counting the extra height of the 1.25" adapter), you are essentially doing what the person with the paracorr-in-focuser is doing, which is adjusting the distance between the focal plane of the eyepiece and the lens of the Paracorr.
Now, not every eyepiece will fall into the range of motion on the Paracorr's tunable top. Some 2" eyepieces might need an extension tube to get them far enough away from the lens (as would, for instance, an 8 Ethos used as a 2" eyepiece), and, conceivably, there could be an eyepiece that needed to get closer than the in-most setting.
But the range should be sufficient for most eyepieces.
Since focusing with an eyepiece of known setting has placed the Paracorr lens relative to the focus of the scope, you can dial in any other eyepiece by simply adjusting the tunable top to focus, leaving the Paracorr where it is by not using the focuser (which would move the Paracorr's lenses relative to the scope). So fixing the Paracorr lens using a known eyepiece and then focusing a new eyepiece by using the tunable top simply finds that new eyepiece's correct Paracorr setting, i.e. adjusts the correct distance from eyepiece focal plane to Paracorr lens.