An update on my RPi4 project. I’ve installed a Waveshare HAT which allows me to run two stepper motors (or DC servos), and also power the Pi board underneath (Rev B).

I made a mounting plate from an old piece of aluminium tube that was lying around, attached to the Crayford  focuser on my WO Zenithstar 80IIED doublet (Rev C). Attached to this is an L bracket with Neema 17 stepper motor, attached to the 1:10 fine focus shaft using a flexible coupler (Rev D).

I chose this setup as an Indi driver has been developed by K Ross which he has generously made available via GitHub. This allows me to use the inbuilt autofocus module in EKOS. 

The Star Adventurer mount only tracks in RA. The eagle eyed among you might have noticed a second shaft coupler on the manual DEC bracket. I will attach a second stepper motor to this, and then try and find a way of sending DEC+/- signals to this from PHD2 or the inbuilt Stellarmate guiding.

The autofocus routine worked making minor adjustments to identify the lowest HFR, although some tweaks are needed to fine tune this, and also turnoff power to the motor when not needed. Rev A shows Arcturus in fine focus, far better than I can achieve manually with my homemade Bahtinov mask. 

Rev E shoes the whole setup teetering on top of the tiny SA mount. Finally a single frame of M13 using a 30s unguided exposure (Rev F).

[postscript - problems with tracking which I put down to excessive weight - total 6.3kg on my 5kg max load SA2i mount, I managed to shed 0.5kg by using a smaller stepper motor, and removing an extra counterweight - no imrpovement?

I 'tracked' down the problem to loss of power. I had been supplying my RPi4 and stepper motor HAT with 12v, which powers the RPi4 board with an inbuilt 5v inverter. Stepper motors require power to hold their position, and it was clear this was drawing too much current, resulting in loss of power to the mount. I use a 240v to 12v 5A inverter, the RPi4 requires 3A, and the stepper motor 1A. The solution, power the Waveshare HAT and RPi4 separately - solved! Hope this helps someone else].