Prime Focus Cone Unit (PFCU)

How to use Engineering control

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.

Focus Drive

The focusing unit consists of an outer cylinder which fixes rigidly to the telescope prime focus mounting ring. An inner cylinder carrying the 3 element corrector lens runs in the outer cylinder on linear bearings and provides +/- 25mm of focus adjustment travel. A 6A stepper motor drives a leadscrew fitted with an anti-backlash nut connected to the assembly.

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 (turntable) is the only mechanism that is required by the WFC. As the WFC contains an integral autoguider CCD (offset to one edge of the 4 chip CCD science mosaic) there may be rare occasions when a guide star cannot be found. The turntable can then be rotated to one of four positions to find a guide star. The turntable normally rests at 180o, but can be moved to any of the four cardinal points (90o, 180o , 270o or 0/360o)

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.

PFCU MMS Controller

The PFCU controller is an MMS (Micro-Modular microprocessor System) based around the 8 bit Motorola 6800 processor and was designed at RGO (circa 1982). The software was written in FORTH and the object code is held in EPROMS on the ROM boards within the crate. The crate along with its rack mounted power supply is located in bay 6 of the CLIP centre.

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 Cone Unit.
Details of the MMS boards and circuit drawings can be found in INT Technical Manuals: 54 and 55


n.b. When the rotator is demanded to move from the ICS terminal, for example using the command: rot mount 90 the operations of releasing the clamps and removing the plunger is automatic as is the move to the demanded position and re-application of the plunger and clamps. Sometimes there may be a problem that the plunger does NOT line up with the detent and the mechanism times out. This could be due to the WFC being out of balance. By releasing the plunger and clamps using engineering control and moving the rotator by hand, any out of balance problems can be seen.

Testing the PFCU rotator in engineering control can be done in two ways:

  1. Plugging in the VT200 terminal on the balcony (gantry)  to the PFCU connector on the telescope connector panel
  2. Running a 'tip' session from the ICS computer in the control room. A window has been set up for this
A switch box in the CLIP centre at the top of bay 6 determines whether engineering control is passed to the telescope connector: gantry terminal or to the Portserver: ICS control in bay 9. If there are problems with the rotator, it is better to use the terminal on the gantry as one can see the mechanism in operation.

Gantry terminal

Plug the comms cable into the 12pin connector marked PF ENG CONTROL and switch on the terminal. Hitting return should bring up the ok prompt. Terminal should be set to 9600 7E1

ICS computer

Right click on the background and bring up the PROGRAMS window then select the option: Prime Rotator Info. This will bring up a green xterm window called Prime Rotator TIP Session. Pushing return should display the ok prompt.

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

will select a mechanism. ?MECHANISM tells you which

motor control   switch tests   transducer
< direction >    +LIMIT        ?POSITION


Some 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 plunger
The 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  3800
e.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 clamps
Other 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?

Status Messages and error codes

When a mechanism is activated, it will return a status letter when finished. These are:
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.

Last updated: Sept 21st 2002   ejm