My understanding is that for a paracorr to provide its best correction of coma, it needs to be at an optimum position with respect to the mirror. When using a TeleVue Paracorr, proper positioning of the device is achieved by setting the tunable top to a position specific to an eyepiece, and focusing the telescope. If a different eyepiece is placed in the Paracorr that is not parfocal with the first eyepiece, focus should be regained by turning the tunable top rather than adjusting the focuser.
Enter the myopic observer who wears glasses, but removes them to look through the eyepiece. Normally, this is not a big deal. The observer can simply adjust the focuser inward to compensate for their nearsightedness. But with a Paracorr in place, I believe this will result in the Paracorr not being in it's optimum position.
What made me think of this?
I was out this morning testing out a new-to-me Paracorr I in an f/5 newtonian using the Double Cluster as a test subject. My observing eye requires a spherical correction of about -6.5 diopters, and a cylinder of .25 diopters. This is a pretty strong prescription, so when I'm not wearing corrective lenses, I need to rack the focuser in pretty far to compensate.
Anyway, I used a 22mm Nagler Type 4 to set the position of the Paracorr while wearing glasses, and began swapping between the 22mm and a 31mm Nagler Type 5. At some point, I decided to remove my glasses to make looking through the 31mm more comfortable. It was then I noticed the the coma correction seemed to suddenly be less than great. At first I thought maybe it was my astigmatism. However, when I tried resetting the Paracorr using the 22mm without wearing glasses, and then put my glasses back on, I realized something else was happening. It was then it occured to me that by racking in the focuser, I had upset the position of the Paracorr with respect to the mirror.
Can anyone confirm my discovery? Certainly, from now on I will plan on using corrective lenses while observing with my dobsonian.