12/28/04 DPC

 

Notes regarding Perkins focus mechanism from discussion with Ted Dunham

 

End result = 9600 motor steps per inch of secondary motion, with negative stepping moving the secondary closer to the primary.

 

The chain is as follows:

 

1)     The stepper motor is 200 turns per revolution, and is operated in full stepping mode

2)     It drives a belt connecting two gears with a 4:1 drive reduction ratio

3)     The final gear drives a 12 turns per inch screw that moves the secondary

4)     Hence 200 * 4 * 12 = 9600 steps per inch of screw travel

 

Also, Rich Oliver notes that the voltage readout of the focus position on the Perkins console is tied to the focus encoder and is independent of the OMS motor controller. So, one should find the best focus using the OMS via LOIS, then write down the focus voltage-encoded value and save that information.