Paracorr and CCD

I'm building a 12.5" f/5.3 dob, which will be used for CCD photography. Since I'm looking to have a clean field over 26mm, I plan on using a Paracorr.

I have read about the 3/8" in-travel that the Paracorr requires for focus, but on the Al's web site he also states that "You must adjust distance from last lens surface to CCD surface to 55mm+4mm for best performance."

Does this mean that in order to use the Paracorr and CCD camera, I need to have total backfocus of 68mm (9+55+4)!? That seems huge. It would push my focal plane to at least 13" from the center of the scope and push me towards a 3.1" secondary.

This seems like a bit much, and I'm hoping that I'm misunderstanding something. Someone, please enlighten me. Thanks.

-David

Not exactly. You need to have 55mm + 4mm (actually, are you sure that isn't +/-4mm?) from the paracorr rear lens to the ccd camera. You may or may not need to use some type of tube extender to achieve this, depending on your CCD camera's layout. The paracorr's rear lens will be well down in the focuser tube. To figure out your back-focus, add 9mm to the distance from the top of the focuser to your CCD chip for your camera. That should let you get the CCD chip to the focal plane with the paracorr in place. Actually, I would add 5mm or so more for slop. And don't forget that you mirror collimation bolts can also move the mirror up and down in the OTA a bit.

Jarad