The Mark IV telescope uses a radius arm driven by a string, a lead screw, and a stepper motor to allow tracking of the sky with about 15 arc second precision for long exposure times.
The RA axis of the telescope is connected to a 10" long section of a wheel cut with a radius of 12". The radius section is wrapped with two SS cables so they can pull the wheel both E and W. The range of good motion is +/- 20 degrees, though the error for additional motion is small for the next 10 degrees or so. The two ends of the cable are attached to a carriage that is driven by a 1/4-20 rolled machine screw. The machine screw is driven by a Hurst PAS 3206-041 stepping motor. This motor requires 200 full steps to make one revolution of the output shaft.
To track the sky, the angular motion needs to be 15 arc seconds per second of time. This is 0.00007278 radians per second. At 12" this requires 0.00087346" per second of linear motion. With the 1/4-20 screw, this requires 0.01747 revolution of the lead screw per second. To achieve this running the motor half stepped, requires 6.987 half steps per second. This is measured best with a period meter and is a period of 0.1431 seconds.
This period can be measured between test points WB (Signal) and WC (Ground) on the motor board.
A VFC110 intgrated circuit provides a voltage controlled oscillator. The base frequency of the VCO is provided by a potentiometer. DAC 12 trims the potentiometer setting. The frequency is counted down by a 4020 scaler. A 151 data selector allows selecting a wide range of rates of 7 powers of two. See code in the subroutine RAMove for details. An eighth input into the 151 allows single stepping the RA drive from the computer. Several circuits translate the square wave pulse drive from the 151 data selector into the proper half step logic to drive the UC3717 stepper motor drivers.
For return after tracking, the RA drive can go at up to 16x the forward tracking speed. This is where it is currently set. The home position is determined by a limit switch. See code in subrouting RAMove for details.
The VCO potentiometer should be adjusted so that the period measured at WB is about 0.143 seconds. DAC 12 provides about a 1% trim on this period. A more positive voltage increases the stepping rate. The scale factor is roughly 0.05 step per second per volt. The minimum DAC step is 0.1 volt. The result is the ability to adjust the VCO to less than 0.1% Calibration
There is a thermometer mounted close to the VCO. This will allow future closed loop adjustment of the VCO as the temperature changes. The VCO temperature is the third (02) item on the read out list. Early data for TOM indicates a change in period of +200 ppm per C. This suggests a DAC adjustment of +0.03 volts per C. This is a very preliminary calibration. This calibration is expected to change from unit to unit, and may be positive or negative. For exposures under 500 seconds, it is unlikely that any VCO trim adjustment will be needed, even for winter to summer changes.
It is recommended that the best VCO trim is measured at the warmest and coldest opertaing temperature so that need for VCO trim can be evaluated.