I mounted my new 40d on the DOB for the first time using a 2" close coupler I got from scopestuff.com and I do not have enough back focus to get focus. The focuser is a feather touch. Any suggestions?
DSLR Focus problem
Started by frainiea, 05/12/2008 02:02PM
Posted 05/12/2008 02:02PM
Opening Post
Posted 05/12/2008 07:37PM
#2
Sorry, I get confused on the terminology here.
Does not enough "backfocus" mean you run out of travel on the way in or the way out?
If you run out on the way out--just loosen the setscrew, and pull the camera out a bit, or add an extender tube--both of which are pretty easy.
If you run out on the way in, it is somewhat more difficult. Assuming you have the shortest possible imaging train--the thinnest adapters, etc, the easiest thing on a dob is to screw or unscrew the collimation bolts so that the mirror moves as far forward as possible. (After years of re-collimating, it seems the primary mirror migrates towards the back of the tube.) If that does not give you enough, and this is a true Dob, then it is generally pretty easy to move the whole mirror cell up a bit. Just redrill the holes in the side of the tube that hold the screws to the primary mirror cell.
Hope that helps.
Alex
Does not enough "backfocus" mean you run out of travel on the way in or the way out?
If you run out on the way out--just loosen the setscrew, and pull the camera out a bit, or add an extender tube--both of which are pretty easy.
If you run out on the way in, it is somewhat more difficult. Assuming you have the shortest possible imaging train--the thinnest adapters, etc, the easiest thing on a dob is to screw or unscrew the collimation bolts so that the mirror moves as far forward as possible. (After years of re-collimating, it seems the primary mirror migrates towards the back of the tube.) If that does not give you enough, and this is a true Dob, then it is generally pretty easy to move the whole mirror cell up a bit. Just redrill the holes in the side of the tube that hold the screws to the primary mirror cell.
Hope that helps.
Alex
Funding Member
