See GA drawing IC/000. This holds the multiaperture masks precisely in the focal plane of the telescope (the telescope must be focussed onto the masks as part of the general setup). The latching mechanism also ensures that the displacement of the masks orthogonal to the optical axis is kept very small.
The wheel is moved by a stepper motor driving through a reduction gearbox and a tensioned toothed belt. The wheel hub is fitted with a rotary encoder which provides the local controller with a zeroset pulse. In order to maintain a fixed relationship between the wheel and the encoder, the wheel is dowelled to the hub and the encoder is dowelled to the wheel shaft.
The clamping mechanism consists of a plunger mounted on a linear slide which is driven by a pneumatic cylinder actuated by a solenoid valve. When the valve is not actuated, this drives under spring pressure into a V-block located on the wheel rim at each of the 8 mask positions. Shoes mounted on the other side of the wheel make contact with cam-followers acting on a line with the plunger. Thus the axial position of the wheel is precisely defined. When the solenoid is activated the clamp is lifted out of the V-block allowing the wheel to be moved by the stepper motor. The default position of the plunger, when no power is applied to the solenoid, is such that the wheel is clamped.
The position of the wheel in a tangential direction (i.e. orthogonal to the optical axis) is defined by the V-block. The job of the stepper motor is simply to get the correct V-block into approximately the right position.
The mounting arrangement for the aperture masks is given in Section 4.1.
Each mounting hole is clearly labelled on the outer face of the V-block from 0 to 7. '0' is normally kept clear.
See GA drawing IE/000. This is similar in principle to the aperture wheel but with the following difference.
The wheel contains 8 round apertures, each of which may be mounted with a filter+cell. The positions are clearly labelled near the V-block from 0 to 7. 0 is generally kept as the clear position and positions 1 and 2 are normally kept for Hartmann filters HL and HR. See section 4.2 for details of how to mount the filters.
See GA drawing IF/000. This is similar in principle to the aperture wheel but with the following difference.
The wheel contains 3 square holes for the grisms and 3 round holes for filters. The square holes alternate with the round ones. One of the round holes (labelled '0') is kept for the clear position. The others are labelled (close to the V~block) sequentially from this position. See section 4.3 for details of how to mount the grisms.
See GA drawing IH/000. The camera is mounted in an assembly which allow the position of the camera along the optical axis to be altered by +/-0.4mm. The assembly consists of a movable central section on which the camera is mounted supported by two large linear slides off a section which is fixed securely to the inside of the bottom unit of the structure. A stepper motor driving through a worm gear pushes down on one side of a lever. The other end is attached to the movable part of the focussing assembly and so lifts the camera upwards. A handle may be attached to the shaft running through the stepper motor to allow manual control of the camera position. Springs linking the movable and fixed sections keep the movable section locked against the lever head.
An SLVC (Super Linear Variable Capacitor) is attached to the fixed part of the assembly with the spring-loaded probe head resting against a bracket attached to the movable part. Thus it senses the axial position of the camera. The SLVC has signal conditioner unit which is fixed via an insulating plate to the inside of the bottom unit of the structure. The three leads link the probe to the signal conditioner unit are very delicate and must be handled with care.
HANDLING PRECAUTIONS
Handle the SLVC leads which join the probe to the signal conditioner unit with care. Do not overbend them or subject them to any excessive stress. Do not clamp them in a vice. Do not attempt to modify them.
A spare SLVC probe and signal conditioner unit is provided. The setting of the SLVC probe should not be altered since this will destroy the calibration between the physical position of the camera and the SLVC readout.
TAMPER WARNING
Do not remove the SLVC probe from its mounting bracket unless absolutely necessary. If it is removed, replace it so that the collar butts up against the mounting bracket. Do not remove the collar.
See top unit GA drawing II/001. There are two doors which can be locked and unlocked pneumatically via the control system. Each door has two handles which operate by pulling outwards (like ISIS etc.). Sensors attached to the latching mechanisms (a bar which rotates to insert or withdraw pins from the tongues of the latches) detect when the latching mechanism has operated and another set detect whether the door is actually open or closed. These sensors are Hettisch-type proximity switches. It is necessary to give the doors a good bang after they have been closed to ensure that the latching pins fall into in place and that the door sensors detect that the door has in fact been closed. The doors are lined with gasket material to make them light tight.
The doors are interlocked with the shutter and so will only open if the shutter is closed. The interlock may be overridden using the manual handset.
The doors may be opened manually by removing a small circular cover plate (held on by three tiny screws) at the top of each door. This reveals a hole through which a small bore rod may be pushed (e.g. a small screwdriver shaft). Moderate pressure applied in this way allows the bar connecting the latching mechanism to be rotated so that the latching pins are withdrawn from the tongue of the door latches. This is the only way to open the doors if the power is off.
See GA drawing VX/200. This is a Prontor shutter driven from the shutter control card in the local controller. It is mounted off the grism D-plate. It has two manual control handles. One opens and closes the shutter, the other locks it in the open position. A proximity switch in the shutter unit detects the open/closing of the shutter. Electrical signals go via a military connector mounted on the box which contains the manual controls. It is very difficult to plug this connector in when the grism wheel D-plate is mounted in the bottom structure.