Hi Neale, very good question - you’re right, what I need to detect is the angular position of an axis.

The challenge is the level of resolution – target is to get 10 ticks – or pulses- each arcsecond which means 36,000 ticks each degree.
As a consequence a full revolution of an axis would call for a 36,000 x 360 = ca.12 million ticks.

There are encoders that can go that high but they are top of the bunch and I suppose pretty rare and expensive (I guess 1,000/2,000£ easily).

I’d love to salvage an encoder like that but I think it’s not realistic.

On the other end I can fit a linear scale to a 1200mm diameter ground surface that is solid to the axis and read with a read-head.

If you want to have an idea of how the arrangement looks like I'm posting pics and vids here:

Instagram: #800mm_telescope
Facebook https://www.facebook.com/800mm.telescope
YOUTUBE https://www.youtube.com/channel/UCC4...IGmvUwCV9ASGUQ