May 20 2104
setting up sky tracking turntable at CTIO with Newport rotation stage.
Rotation stage is Newport RVS80PP. 1:180 worm gear so one rev of shaft is 2 degrees of turntable motion.
micro stepper drive determines how many pulses needed to turn shaft by one rotation. For N steps per shaft rotation, we get 2 degrees/N per step.
Sky rotates at 15 degrees per hour. So we need to rotate shaft at 15/2 = 7.5 times per hour= 0.002083333 Hz=f_shaft
So if controller is set to send out N micro steps per revolution, we need a drive frequency given by
where f_shaft is number given above. So f_drive=N*f_shaft. So if N=1000, f_drive=2.08333 Hz. That gives us steps that are 2/1000 degrees each=7.2 arc sec.
Pixels are 6.5 microns in size, focal length is around 70 mm so we have theta=6.5/70E3 radians per pixel so 19 arc sec per pixel.
Microstepping controller manual: manual, PDF file
Rotation stage manual: Newport_stage_manual.pdf
So to drive the rotator at sidereal rate would require about 106 Hz drive frequency, as a TTL square wave.
More precisely, at dec of -6 deg the sky moves at 15cos(dec) arcsec/sec = 14.92 arcsec/sec. So sidereal rate at geo would be 14.92/0.1406 = 106.10 Hz.
Twice sidereal is 212.2 Hz. FOV is 14.8 degrees. It takes 24 hours/360 deg * 14.8 deg~ 1 hour to move by one FOV.
DB-15 connector on turntable is male pins.
1: phase 1+
9: phase 1-
2: phase 2+
10: phase 2-
stepper wants rms of 0.71 A so full amplitude of drive current is 1A, into 1.7 Ohms.
Microstepper drive unit wants switches 1,2 and 3 all on to get this current limit.
24 Volt supply should be OK. Stripe is (+) on AC adapter.
Wired up DC adapter and function generator. Worked best with microstepper controller switch 1 set to increase motor current one notch above the minimum. Seems to do a fine job of driving the turntable.