replaced my previous discrete digital component based stepper controller with a picaxe microcontroller to perform same functions high speed, low speed and tracking.
have included acceleration for high speed slews to 24000Hz at 1/16 microstepping, thats around 450 rpm.
the picaxe is an extremely easy to use microcontroller, uses basic to program, does not need a programmer, just uses 3 pins from the serial port and very inexpensive. recommended for anyone looking at microcontrollers. the one used here is a picaxe 28X2 running at 16Mhz clock and stepper is driven with the a4983 stepper driver.
http://www.youtube.com/watch?v=oO68oe5R-Ws
extremely easy to adapt for a motorized focusser as well.