Q: Has anybody considered using a motor with a built in encoder as an alternative to a stepper motor for an absolute motorized focuser?

I can think of at least a couple advantages. Namely I expect they will run quieter and faster and provide accurate position feedback, while a stepper can skip steps and lose accuracy.

An example of a motor that got me thinking about them:

www.robotshop.com/ca/en/pololu-12v-301-g...-64-cpr-encoder.html

The Celestron focuser uses an encoder motor. It's essentially the same motor and contol firmware as they use in their mounts.

Interesting. I wonder what would be required to DIY a compatible motor controller.

IMO making firmware compatible would be more work than writing firmware with your own interface and writing a new iNDI driver.

Stepper motor firmware is far simpler than encoders. It can be no more than setting one output to specify the direction and sending a series of pulses on another. Or sending the coreect sequence of four signals to a ULN2803.

Encoder firmware needs an ISR to handle the gray code from the encoder and some sort of PWM current control for the motor that handles acceleration and speed, with suitable feedback from the encoder to get rate and position data. Maybe a PID controller.

Or get a commercial encoder motor controller. This won't be cheap nd will come with it's own control protocol. One way to do this would be to get the Celestron focuser of course.