Does Paracorr affect focus?

I'm building a new telescope and don't yet have a Paracorr. But since it's a 12/5"f/5, I want to be sure that I'm buidling it to be "Paracorr-ready" in case I feel the need for one.

So, my question is whether I need to compensate for additional in-travel or out-travel when a Paracorr is used. In other words, if you are using a particular eyepiece by itself, then take it out, insert the Paracorr and put the EP back in, do you need to adjust your focus? If so, how much and in what direction?

Thanks.

I'm not curently using a paracorr but my recollection is that it requires additional in-focus travel.
Perhaps others who are currently using it can chime in...

David Martin said:

I'm building a new telescope and don't yet have a Paracorr. But since it's a 12/5"f/5, I want to be sure that I'm buidling it to be "Paracorr-ready" in case I feel the need for one.

The shoulder of the Paracorr's 2" barrel wants to be roughly 18-20mm below the focal plane (in other words, if you design the focal plane to be 1" above the focuser draw tube's shoulder when it's fully racked in, you have roughly 5-7mm of focuser in-travel left once you've focused).

But you usually need very little travel if you are using a Paracorr, because differences in eyepiece focal plane are handled via the tunable top (when changing eyepieces, you use the tunable top for rough focusing and then only your focuser for fine focusing, if you want the tunable top to remain at its optimal position for all eyepieces).

If you've designed the focal plane to hover only 0.25" or 0.5" over the focuser draw tube when it's fully racked in, tough luck, unless you can compensate that by moving the mirror on its collimation bolts (this reduces the fully illuminated field size, but the Paracorr makes every eyepiece use a field with a diameter that's 15% smaller because of its barlowing effect, so that's usually a wash).

And yes, the Paracorr is long; a reason to make the focal plane lie a bit higher than 1" over the fully racked in position if the focuser is a very low profile focuser, but even if it does clip off-axis light bundles, vignetting by the Paracorr is usually very mild for scopes with some aperture.