The Prime Focus Cone Unit was the original imaging platform at prime focus and contained an offset X-Y autoguider probe and a filter wheel unit. Since the introduction of the Wide Field Camera (WFC) its purpose now is solely to act as a turntable to carry the WFC. The PFCU also contains the corrector lens which is essential for prime focus imaging and the focus drive motor and absolute and incremental piston transducers which measure the focus position. See WFC interconnections drawing.
The focus position is measured using an ASL absolute LVDT and recently, a SONY incremental piston transducer has been added to improve accuracy. The focus can be measured better to 0.01mm. More details can be found in the INT Focus section as the secondary mirror uses the same configuration.
The rotator is clamped using a series of shoes with friction pads driven by a chain. There is also a tapered plunger rod which locks the turntable at a cardinal point. When the plunger is inserted and the table clamped, the accuracy of positioning is to 2 arc minutes.
The sequence of events is important e.g. After rotating the turntable, the plunger must be inserted first before the clamps are applied. Stepper motors drive the plunger and clamps and a large 6A stepper motor rotates the turntable. The rotation position is encoded using a high precision 10 turn potentiometer (Helipot) driven by a steel bowden cable wrapped around the diameter of the rotator. A tensator spring keeps the cable taut thus keeping the shaft of the potentiometer rigid.
All mechanisms that use stepper motors are controlled by double height eurocard boards (one for each motor) which contains the motor coil phase switching and power (4 power transistors) circuits. A 6821 PIA chip handles the input signals from the mechanism's associated position sensors (Hettich slotted inductive switches) and also provides the direction and speed TTL toggles to drive the motor circuits. The ramp up/down of the stepper motors is done using a hardware circuit.
Serial communications to the 6850 ACIA board in the MMS is via an RS232 link (9600 7E1). There are two ports, one for engineering (where FORTH commands can be entered) the other for computer control. The computer port connects to a 3340 communications module: CAMAC Address: B4 C3 N17
The engineering port can be routed to two locations; either to the prime focus connector panel on the top end ring when the telescope is at Access Park where a VT220 terminal can be connected or via the network (portserver) to the ICS computer in the control room.
Refer to the INT Technical Manuals: 49
50 51 52 for full details and circuit drawings for the Prime Focus
Details of the MMS boards and circuit drawings can be found in INT Technical Manuals: 54 and 55
Testing the PFCU rotator in engineering control can be done in two ways:
All commands must be in UPPER CASE. At the ok prompt, type: HELP. You will be presented with the screen below:
'RETURN' key the execute 'Z' key is EMERGENCY STOP CLAMP_ON CLAMP_OFF PLUNGER-IN PLUNGER_OUT CLAMP, PLUNGER, ROTATION, XPROBE, YPROBE or FOCUS will select a mechanism. ?MECHANISM tells you which motor control switch tests transducer < direction > +LIMIT ?POSITION STOP HOLD -LIMIT STEP SLOW ?LIMIT MED FAST ?CARDINAL xxxx POSITION try REPEAT ?POSITION and ?ERROR okSome of the commands listed above don't work due to de-commissioned mechanisms. However, the ROTATION command is still valid. Type:
ROTATION to select the mechanism CLAMP_OFF to release to clamps PLUNGER_OUT to remove the plungerThe turntable can now be moved to the valid positions using the xxxx POSITION command. These being:
0o 200 90o 1100 180o 2000 270o 2900 360o 3800e.g. 1100 POSITION will move the rotator to 90o. Using ?POSITION will read the actual value from the potentiometer.
After a move is completed, the plunger must be inserted first to ensure that the turntable is 'pulled' into position and the cardinal point switch activated before applying the clamps.
PLUNGER_IN Lock the turntable to a cardinal point CLAMP_ON Now apply the clampsOther commands that work in all modes:
+LIMIT and -LIMIT To query a particular limit switch ?LIMIT Has any limit switch been activated? ?CARDINAL Has the plunger been inserted correctly?
N No error. Mechanism has moved to demanded position O Operation not completed Mechanism still in motion C Command error Invalid mechanism identifier or command qualifier sent T Timeout error Mechanism has not reached demanded position in time L Limit error Upper or lower limit switch has been activated S Switch error Both switches are true! Possibly a disconnected or broken cable D Demand error Demanded position is outside the normal range.