Help with Low Altitude Mechanical Problems with My 8" LX200 Classic Focuser and Robofocus

American Association of Variable Star Observers (AAVSO)
Fri, 12/27/2019 - 15:18

Hello! I've been using a Robofocus on my 8" LX200 classic for photometry for a number of years with my SBIG ST-402 and focal reducer on its fork mount. Recently, I've developed a focusing problem I thought folks in this forum might be able to help me with.

    At low altitudes, the focus does not seem to change even though Robofocus rotates the focus knob.

    I've taken the focuser apart (I had installed Peterson's EZ Focus upgrade several years ago) and moved the mirror back and forth to redistribute any grease on the baffle on which the mirror rides. I installed a washer on the focus knob to take up residual slop. Still the problem persists.

   During a run, I can correct the problem by moving the mirror up and down several hunded moves through Robofocus (with my settings, 4 is a small move and 13 is a large move). However, the focus would then settle back into the previous pattern after a short while.

    This does not seem to happen when the telescope is pointing over about 45 degrees altitude.

    I'm wondering if the mirror has loosened in the actual mirror cell or if there just is not enough grease on the baffle that the mirror rides on so that at low altitudes it gets stuck. In either case, I think that  would mean sending the scope off since I'm not keen to take get that deep into the scope mechanics.

    I would appreciate any insights if there might be other causes of this problem that I should consider. Thank you and best regards.