Hi,
The calculations are as follows:
- aVal = steps per axis revolution = "motor gear ratio" x "worm ratio" x (360/"step angle") x "microstep mode"
- sVal = steps per worm screw revolution = "aVal" / "worm ratio"
- IVal = interrupts per step = found by trial and error by minimising sidereal rate speed error
- bVal = interrupts per second = "IVal" / (86164.0905 / "aVal")
The "bVal" is essentially the base timing value for the system. A timer is configured to run at "bVal" interrupts per second, then the motor will be stepped after a certain number of interrupts. The number of interrupts sets the speed of the mount. To run at the sidereal rate the mount would make a step every "IVal" interrupts. To go at twice the sidereal rate it would step every
"IVal"/2 interrupts, and so on. This is why when "IVal" gets too low there are fewer possible high speed values because of integer rounding of the reciprocal.
The AstroEQ config utility needs to be able to calculate an "IVal" of <1200 (I forget the exact reason now as it was a long time ago that I worked it out, I think it was due to higher values confusing EQMOD but I could be wrong). Additionally the interrupt rate in the system is limited by both clock frequency, but also the number of clock cycles it takes the processor to perform each step of the motor (make the step, update encoder positions, update the IVal counter, accel/decel routines, bVal dithering for higher speed accuracy, etc.) which if both motors are running flat out is about 480 clock cycles. This gives a second upper limit on "IVal" of
(86164.0905 x 8000000.0)/("aVal" x 480).
Looking at your DEC axis values basically your gear ratios are not high enough and so the calculations violate the upper limit of "IVal". The only solution to this is to either increase the gear ratios, or get motors with a lower step angle. However if you find yourself pushing the upper limit of IVal it is usually going to result in a very low resolution of the mount (you'll be ending up with more than 1 arc second per microstep of the motor).