AP1200 Dec test.

I had noticed an increased error in Dec tracking which first looked like just bad seeing / windy conditions.
But the dec response became worse on the next 2 nights.
It would take 5 or more dec corrections after a reversal before any movement was seen and oscillated back and forth from zero. But I couldn't feel any dec wobble in the ota.
The lack of wobble meant the worm-wheel gear mesh was probably ok and something else was loose.
I ran Ray Gralaks PulseGuide program DEC test routines No.1 and No.2, images shown below with North down.

Quite a mess ! The left hand image should be a cross and the right 3 images a saw-tooth shape.

The left hand image shows the dec gearbox with the cover removed.
I removed the worm cover cap (on the other side of the gearbox) and turned the end of the worm looking at the drive gears at the same time.
There was 15 - 20 degrees movement of the worm before any of the drive gears moved.
The shoulder bolt and large upper gear was removed revealing the final drive gear attached to the end of the worm shaft (right image). The grub screw that attaches this gear to
the end of the worm was loose. It was give a gentle tighten.
As a note, when I had the large gear removed, it was much easier to 'feel' the mesh of the worm against the wheel without having to turn the whole drive train.

I repeated the Dec tests using PulseGuide with the result shown below.

If you are experiencing less than perfect tracking in dec and have checked the worm mesh as per the AstroPhysics procedure it could be worth taking the cover off the gearbox had having a look. Otherwise run the Pulseguide test and send the images to AP for evaluation and advice.

Dave Rose
28th Feb 2004
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