Hi Rowland
Could you post some pics of your mount and hand controller? are you able to make manual corrections by depressing the hand controller buttons during sidereal tracking?
Does it have an RS232 port?
My suggestion for autoguiding was based on these facts
- that you already have a very sturdy mount that can track extremely accurately and already has low PE.
I've seen 5 min unguided shots and they're impressive.
So I don't see the need to alter or regulate the tracking rate directly using encoders. minor corrections from a guide camera would more than suffice.
- you're likely to stay within the 5 to 10 min exposure length for most objects unless you're going narrow band.
So all you're missing is the goto and the autoguiding.
to add autoguiding, your hand controller should be capable of manual guiding which I believe it is.
my understanding is that when the mount is tracking at sidereal rate, the e/w buttons would cause a 0.5x change in speed for the duration of the guide pulse if using st4 guiding or the duration of the button depress if using manual guiding.
so if your hand controller buttons do this, its quite easy to wire the optocoupler outputs from a guide camera to your hand controller.
The outputs from the ST4 port are normally high (+5v) and they're for the following
N = DEC+ = Up
S = DEC- = Down=
W = RA- = Right = Decrease track speed
E = RA+ = Left = Increase track speed
So all you do is work out the polarity of the switches, and wire the st4 output in parallel to the switches so they simulate a button depress.
the guiding software like phd will determine the length of the pulse based on the initial calibration routine.
As for goto, as mentioned, two 10k encoders, one for each axis, and a home built DSC (picaxe or arduino) or David Ek's DSC is ample.
Have you had a look at Rajiva's stepper controllers? they have heaps of features
http://astronomie.rajiva.de/
Cheers
Alistair