Hi,
I recently upgraded my mouse encoders and the way they mechanically interface
on the GEM to radically improve the pointing accuracy.
Using mice internals on scopes is not new.
There are several great sites showing how to home brew your own
'poor man's version of an Argo Navis system'.
Mine are part of the
Bartels Stepper Drive system I built years ago.
I have used mouse encoders on my homemade GEM for years but
it was only ever accurate down to 1/5 of a degree. This is quite accurate
for general viewing and wide field pointing accuracy but for deep sky, narrow
field it's not.
This improvement gives me approximately 8500 counts per 360 degrees on DEC axis and
a whopping 25,000 counts per 360 degrees on the RA!
That's 0.042° or about 2.5minutes DEC (was previously about 12 minutes)
and 0.0144° in RA or about 0.85 minutes.
Here's what I did:
I selected a 320dpi mouse with fine slotted wheels instead of the more common coarse 180-250 dpi
types. These types have better IR transmitter / detector pairs suited for fitting higher
resolution encoder wheels.
Using an old PS/2 mouse (with mini Din6 connector), I cut the two transmitter / detector pairs
off from the mouse circuit board with a Dremel.
I carefully chose the placement of the two cuts so that minimal re-soldering of broken tracks
was needed. The paths for the two cuts are shown as yellow dotted lines.
The only wire link I had to re-instate is also shown in yellow.
For the DEC axis: I used a 144 slot optical window wheel I found on the back of
a DC servo motor years ago. I kept this optical wheel knowing I might use it for
just this purpose
The rubber capstan wheel and shaft assembly are from an old photocopier and
fitted to a piece of aluminium extrusion with a pair of nylon bushes.
The encoder pair is mounted on an adjustment plate to zero out false counts.
Once wires were soldered to the pair, I made the fine wiring safe with some
5 min Araldite.
The DEC encoder assembly fits exactly where the lower res version sat.
That is, resting on the outer circumference of the old original setting circle
disks.
For the RA axis I used a small reduction gearbox/ optical wheel assembly I
pulled out of an old hand scanner I pulled to bits years ago
This gear train is perfectly suited for this purpose and that's also exactly
why I held on to it!
A rubber capstan and shaft were also fitted to another little box section extrusion
and the detector pair fitted to an adjustment plate.
In the Bartels software, encoder increments are just entered as an integer for
ALT and AZ (DEC and RA on a GEM) and you're away!
I have tested this all for about a week and the pointing accuracy is a huge
improvement.
After initializing on a known star and entering the co-ordinates, I am able
to drive to the galaxy or object of interest nearby and it's now always
in the field of the DSI Pro II on the live view.
Steve