It seems the paracorrs like the TeleVue just slide in to the focuser and then the eyepieces go into that, sounds easy enough, but I saw a comment about eyepieces being used with them that implied there is some complication? Maybe it has to do with losing parafocal characteristic for a set of eyepieces? I see one TelVue model has 5 settings (A ... E) what is that for?
Is there more to using these than just putting them in?
In addition to all the points made by others, one of the advantages to the Paracorr's adjustable top is that once you have just one eyepiece whose setting you know,
all other eyepieces can have their settings determined by dropping the eyepiece into the Paracorr and focusing using the Tunable top of the Paracorr.
Whatever setting is derived is the correct setting for that eyepiece.
You can do a whole set of eyepieces that way.
In use, in the field, you pre-dial the letter that corresponds to the setting for that eyepiece, and when you insert the eyepiece it will be nearly in focus.
Your total focuser adjustment after finding the settings for every eyepiece may be as little as +/-1mm.
If you have one TeleVue eyepiece, you can look at their chart, set the Paracorr top for that eyepiece, and focus the scope.
That puts the Paracorr in the right position in the light cone from the primary mirror. Any/all other eyepieces would have their positions identified by focusing using the Paracorr's top.
If you don't have a TeleVue eyepiece, there is another way to find the right position for the Paracorr in the light cone:
Set the top to "E", and place translucent scotch tape across the 2" opening of the Paracorr.
Point the scope at the Moon and focus the Moon on the tape (no eyepiece). When the Moon is in focus, you have found the optimum position for the Paracorr.
After that, without moving the focuser at all, focus each eyepiece with the Paracorr's Tunable top and write down the setting you identify for each eyepiece.
Those will be the letters you dial in for every eyepiece before you insert them.
IF the tunable top of your Paracorr moves smoothly enough, you can actually focus using the tunable top, but pre-dialing the letters and using the fine focus on the scope
is probably easier.
If you share the view with someone who is nearsighted and is not wearing glasses, have them dial the tunable top of the Paracorr to focus the eyepiece for their eye.
Then return the eyepiece to the letter setting you've determined is best and focus with the fine focus knob on the scope to return the Paracorr's top to the right setting for your eyes.
One point: having no spherical error in your eye is important when setting up the Paracorr, so if you use the tape trick, wear your glasses to find focus.
Don't remove your glasses if you need correction. You can determine the settings of the Paracorr's top later without glasses if you observe alone, but that initial
setting should be done with glasses on if you wear glasses to correct a spherical error in the eye.