Part 1, the upgrade of the mount:
Parts used:
2x
NEMA 17 stepper motors (400 steps/rev)
2x
DRV8825 Stepper motor drivers
2x 60 teeth GT2 pulleys from ebay
2x 15 teeth GT2 pulleys from ebay
2x GT2 belt from
smallparts.com.au
GT2 belts are timing belts designed for minimal backlash and are used in 3D printers. I believe that they are the best choice for reducing backlash.
My choice was to use 60 and 15 teeth pulleys to get a gearing ratio of 4. I would have liked a higher gearing but the size of the pulleys was the limiting factor. 4x gearing with 400 stepper-motor steps per revolution and 32x microstepping on EQ5 gives a final resolution of 0.17 arc-sec per step. I tested the performance of the motors with 32x microstepping and did not detect any missed steps.
After my build was finished SilentStepStick TMC2100 stepper motor controllers came out which allow up to 256x microstepping. I was considering upgrading to them but did not see any need to do so with my ED80. There is no guarantee that my stepper motors would work with such a high microstepping anyway.
The motors were mounted on custom designed mounts which were 3D printed in ABS. Red because I only had red filament at the time
edit: Added a CAD image of the DEC mounting that was 3D printed.