The 4.2m William Herschel Telescope

General Description of the Telescope

The WHT was one of the first of a new generation of telescopes to use the alt-azimuth mounting instead of the classical equatorial form. An altazimuth mounted telescope needs to be moved in both axes simultaneously to follow celestial objects as they rise and set. With modern computer technology and servo systems, this has now become common practice with large telescopes. The alt-azimuth design is also more compact and allows for a smaller dome.

The WHT has a 4.2 metre (167 inch) diameter f/2.5 parabolic primary mirror made of Cervit (a ceramic/glass product of zero thermal expansion). It has a Cassegrain, prime focus and two Nasmyth focal stations. The Nasmyth foci are known as the cable wrap side (CWS) and the drive side (DS). The CWS focus is used by the GHRIL (Ground based High Resolution Imaging Laboratory), the DS focus by GRACE (NAOMI). The Cass and Nasmyth foci are f/11 and either one can be selected by the movement of the Nasmyth flat mirror located within the lower light baffle tube above the mirror petals. The prime focus is f/2.8.

Historical Note

James Nasmyth 1808-1890 (Victorian engineer and inventor of the Steam Hammer) He was also an amateur astronomer. He came up with the idea of incorporating a third (tertiary) flat mirror to divert the secondary beam through a hollow altitude bearing. This brought the focus to a fixed observing position no matter where his telescope was pointing. He could sit at the eyepiece and control his 'home made' telescope with handwheels driving the altitude and azimuth axes. Very comfortable! His telescope is (was) on show in the Science Museum in London. (ejm)

The alt-azimuth design of the telescope causes the field of view to rotate as the telescope tracks. This is compensated at the Cassegrain focus (and prime focus when fitted) by rotating the instrument turntable at a speed equivalent to the tracking rate. This keeps the image stationary at the focal plane. At the Nasmyth foci, there are two compensating methods:

  1. Mounting the instrument directly on to the turntable (e.g. INTEGRAL)
  2. Fitting an optical image de-rotator on the turntable. Used for large 'fixed' instruments  (e.g. NAOMI)
The tube is of open construction with the primary and secondary mirrors supported at the extremities of the upper and lower Serrurier trusses. These attach to the top end ring and to the massive centre box section (cube). The cube houses the motorised balance weights used for fine balancing of the altitude axis. Fitted to the cube are trunnions which form the altitude axis. These are supported on hydrostatic bearings carried on massive tynes which attach to the base box. This complete assembly is mounted on a ring girder which is supported axially and radially by hydrostatic bearings, this forming the azimuth axis. Outboard of the tynes are the Nasmyth platforms. These are instrument platforms which are surrounded, but separated by access platforms. Below the cube is the mirror cell containing the primary mirror and the electronic/pneumatic controls for the mirror support system.

Mounted below the mirror cell is the CASS instrument rotator and cable wrap which carries the Cassegrain instruments. This consists of the A&G box and a spectrograph; usually ISIS. The A&G box is always present, but ISIS is sometimes removed and a different instrument fitted. SAURON for example.

For a detailed desciption of the WHT and a complete list of drawing numbers both ELECTRICAL and MECHANICAL refer to :

Grubb Parsons TECHNICAL MANUALS   43 & 44          The WHT Operation Handbook Vols 1 & 2

A full list and breakdown of the WHT ED drawings can also be found here.

Weights of components

The total weight supported by the azimuth bearing is made up of the individual weights of each telescope component. These being:
Secondary mirror assembly 613Kg
Top end ring 13,700Kg
Telescope tube 3,700Kg
Telescope tyne  CWS 17,500Kg
Telescope tyne  DS 18,800Kg
Nasmyth platform CWS 13,000Kg
Nasmyth platform DS 13,000Kg
Centre section + Alt assy 32,000Kg
Cntrwght Nas turret assy 1,500Kg
Azimuth cable twister 416Kgs
Mirror cell 21,000Kgs
Mirror 16,000Kgs
Rotator and cable wrap 8,000Kgs
Acquisition Guide Box 1,250Kgs
ISIS 2,250Kgs
Rising floor 5,250Kgs
Base box 29,000Kgs
Total weight of telescope Appox. 190  metric tonnes


Top End Ring

The top end ring closes the upper Serrurier trusses and provides a mounting for the FLIP RING, it's drive components and latches. Mounted on the top end ring are the prime focus and secondary mirror focus encoder line driver boxes and the flip ring control box. The Access Park 3 tie rods are located on the top ring and are used when the telescope requires anchoring at the AP3 park position.

The drive for the flip ring is a two speed three phase induction motor fitted with an electrically operated brake. The locking mechanism for the ring is provided by two motor driven bolts mounted on the fixed ring and driven between two pairs of rollers mounted on the rotating ring. Limit switches operated by cams provide the necessary sensing to control the operation of the bolts and control of the brake on the ring drive. Further switches are operated by strikers on the moving ring to select the speed and direction for the rotation of the flip ring and to check the balance of the assembly.

The operator's buttons are located by the side of the access park AP3 gate clearly marked for a prime focus or secondary mirror rotational flip. The flip over can only be made close to zenith since there are mercury operated switches mounted in the flip ring control box. When the Prime Focus is fitted the drive for the ring is inhibited.

Associated drawings: TC1 752  ED125

Secondary Mirror Cell

The f/11 mirror cell attaches to the focus mount by an adapter which also houses the collimation drive. The cell is located on the adapter at three points. One is a spherical bearing about which the cell is free to tilt under the action of the drive. The other two are pads on the mirror cell which bear upon eccentrics on the collimation drive. At each of these locating points, the cell is held to the adapter by a pair of loaded ties which provide a positive axial force at the points of contact. Pusher screws in the focus mount bear against hardened steel inserts in the adapter flange to provide centering adjustment. Two pads are provided on the outside of the adapter for lifting.

The collimation motors are single phase, reversing induction motors driving through a reduction gearbox and a worm. Mounted by each of the three points are LVDT's (Linear Voltage Displacement Transducers). These LVDT's are used to measure the exact position of the mirror within the cell and monitor any small movements due to flexure. See Transducer system.

The adjustment of the collimation push rods is achieved by operating a handset either at the telescope (Cassegrain connector box) or by plugging the handset into the connector on the engineering desk in the control room.

Associated drawings: TC1 632  ED160


The telescope tube is an open Serrurier truss design with a rectangular centre box section to which the altitude bearing trunnions are attached. The lower trusses are closed by the main mirror cell, the upper trusses by a fixed end ring which supports the secondary mirror and prime focus assembly. Two annular girders are supported from the centre section by spider vanes. The girders carry sky baffles on their outer flanges and between their inner flanges support the Nasmyth mirror turret. The mirror cover petals are suspended below the centerpiece on A-frames attached to a baseplate.

The Serrurier trusses have been designed to provide equal and parallel deflections of the top end ring and the mirror cell at all altitude angles. The upper trusses are tubular steel sections with spade ends bolted to the centre section and bracket mounted to the top end ring. Ducts carrying cables for the secondary mirror, focus and prime focus units run vertically between the drive and cable wrap side trusses. The lower trusses are of tubular steel welded to the mirror cell attachment block with spade ends bolted to the centre box section.

The mirror cell attachment block is connected to the mirror cell by hydraulically strained Pilgrim bolts. These bolts are released when the mirror cell needs to be removed, e.g. to gain access to the mirror for re-aluminising. When the cell is back in position, the bolts are inserted then pressurised using a hand pump to 28000 psi. This locks the bolt in the thread of the cell attachment block achieving a perfect fit. A knurled nut is added to the end of the bolts and tightened by hand only, the pressure is then released. The effective weight on each of these 2" diameter Pilgrim bolts is 9 tonnes each.

Centre Box Section

The centre section (or CUBE) is a square fabrication of four box girder sections. Trunnions with bearings are fitted to two opposite sides to form the altitude axis. Brackets are attached to provide the anchor points for the eight spider vanes which support the Nasmyth turret and sky baffles.

Motorised Balance Weights

Motorised counterweights are fitted in each corner of the centre box section providing a trimming facility for fine balancing of the telescope in altitude. The counterweight is driven along a leadscrew by a 3-phase geared motor. At the lower end of the leadscrew is a reduction gearbox which drives a 10 turn potentiometer to provide an indication of the counterweight position. This is displayed on a panel meter on the telescope control desk. The counterweights are driven in parallel.

Associated drawings: TC1 408  ED166  ED176

Nasmyth Turret

The Nasmyth flat mirror is mounted on a rotating turret which can set to one of four positions:
  1. Stowed                   (CASS)
  2. Nasmyth CWS        (GHRIL)
  3. Folded Cassegrain   (WYFFOS CALIBRATION LAMP BOX)
  4. Nasmyth DS            (GRACE)
The turret and drive are supported from the centre box section by spider vanes attached to two tie rings. The lower ring carries the bearing on which the turret moves, the upper ring holds the sky baffle. A geared synchronous motor either removes the mirror from the light beam (Cassegrain focus) or drives the mirror into the beam at 45o when the Nasmyth foci (or folded Cass) are used. A 3 phase motor rotates the mirror to the selected Nasmyth focal position.

Limit and opto-switches provide position and status of the mirror. These are displayed by lamps on the main control console. Latches activated by pneumatically operated solenoids lock the mirror once in position. The pneumatic circuits are fed from the telescope's dry nitrogen supply line and a pressure regulator is connected in series with each solenoid.

The control box for the Nasmyth turret is located inside the cube. Access is via a short ladder to the top of the mirror covers. It is quite safe to stand on these, but LOCK OFF the telescope first.

Associated drawings:  TC1 750  TC1 751  ED123  ED124  ED181  ED188

Nasmyth Rotators

These are used to remove image rotation (due the Alt/Az mounting of the telescope) at the Nasmyth foci. These being: A trunnion fitted to each Nasmyth platform carries the turntable which is fitted with a precision gear. A DC servo motor drives the rotator and the position is read using T+R incremental and absolute encoders. There are no rotator limits if the instrument is stationary,  but the limit switches can be actuated by fitting cams to the turntable if required to prevent cable (or fibre) wrap around.

A local control box mounted nearby allows the rotator to be moved +/- when engineering mode is selected. These switches are also duplicated in the control room on the Nasmyth engineering control panel in Bay 4.

See:  Turntable servos and encoders for more information.

Associated drawings:  ED6006 - 6013  ED6016  ED6052  ED6061 - 6078  ED6080 (Servo boards  ED6006 - 6082)

Primary Mirror Cell

The primary mirror cell is a steel fabrication consisting of a flat disc with a box section cylindrical surrounding wall. Stiffening the assembly is achieved by radial webs. Brackets for the lower Serrurier trusses are attached to the outer plate of the cell wall. The underside has a cell extension piece which carries the Cassegrain instrument turntable and the cable wrap. The cell floor is a machined surface for the seating of the pneumatic axial mirror support pads. Radial support devices (to balance the mirror and prevent it twisting) are fitted to the top face of the cell wall with their counterweights inside the box section.

Mirror Covers

The primary mirror cover is built up of 16 petals made from an aluminium honeycomb material for strength and lightness e.g. When any work on the Nasmyth turret is required, it is necessary to stand on the petals.

The petals are connected via leadscrews and sprockets to an endless chain driven from three geared induction motors. Limit switches operated from the petals stop the motors overrunning. Push buttons with status lamps on the telescope control desk, OPEN, CLOSE or can STOP the mirror petals at an intermediate position. This would be used to stop-down the mirror aperture if so desired.

A sine/cosine potentiometer driven from the mechanism generates a signal to indicate the percentage of obscuration to the MIRROR COVER POSITION meter on the control desk. The potentiometer's sine output is fed to board (BC1 105) along with a 10V regulator in Card crate 3. The output signal from the board drives the mirror cover position meter.

Associated drawings: ED159  ED176

Mirror support system

The primary mirror is supported axially by sixty pneumatic pads (Belloframs) set in three concentric rings. These pads are connected in groups of twenty to form three sectors of 120o.   These being :
  1. Mirror Top Sector 1
  2. Drive Side Sector 2
  3. Cable Wrap Side Sector 3.
Each sector is controlled by a JOUCOMATIC servo control valve. These valves control the air pressure (input supply = 100psi) to the twenty belloframs in each sector ensuring that the mirror alignment to the optical axis stays correct.

Three load cells (axial definers) measure the compressive forces acting on each sector and send signals back to the Mirror Support PID electronics crate for processing. This drives the JOUCOMATIC valves so forming a servo loop. A radial defining system consisting of counterweights and cranks is used to balance the mirror and to prevent it from twisting. The components of the mirror support system are:

Pneumatic Pads (Belloframs)

Each pad consists of a cast base with a gas (compressed air or N2) inlet connection. A diaphragm sleeve is clamped to the base with a V clamp ring. A rolling diaphragm is held between these two parts with its central horizontal surface bonded to the support pad piston. As the air pressure in the control system varies, the piston can rise or fall on the rolling diaphragm to apply the required supporting forces to the mirror.  There are also thirty rest pads which are spring loaded and support the mirror when the pneumatic system is not pressurised.

Axial Definer system

Three of these units; consisting of an adjustable rod and load cell  (one for each sector) are fitted between the cell floor and brackets cemented to the mirror's edge. These are used to maintain the correct alignment of the mirror to the optical axis of the telescope and thus set the working height for the mirror.

A load cell is screwed into one end of the definer rod and is so designed that only pure compressive forces are applied to the load cell. A microswitch fitted on a bracket from the load cell attachment point is operated by a cam on the lower housing. If the mirror support system becomes over pressurised, the microswitch is activated and sensed by the servo control electronics which signals an alarm.

Adjacent to each definer are dial gauges which are used to monitor the height of the mirror. These must be read before the mirror is removed for re-aluminisation. On mirror replacement, when the support system is pressurised, these are read to ensure the mirror height within the cell is correct.

The axial load cell values in pounds are indicated on panel meters below the mirror cell and on the telescope control desk.

Radial Definer system

This weight relief system consists of 12 pairs of counterweights with cranks fitted around the mirror cell. The cranks pivot in brackets cemented to the mirror edge. These are fitted so that each pair supports an equal weight and provide the required tangential supporting force for the mirror. They are set up to give minimum deflection of the radial definer load cells from zenith to low elevations.

The radial position of the mirror within the cell is fixed by three radial (transverse) adjustable defining rods each fitted with a load cell. Like the axial definers, these are designed to ensure that only pure compressive loads are measured.

The radial position of the mirror is monitored at four points using dial gauges. These can be withdrawn from the mirror's edge when the mirror is removed for re-aluminising. As with the axial definers, these must be read before and after mirror re-aluminisation to ensure correct alignment.

Radial load cell forces are also displayed on the panel meters below the mirror cell and on the telescope control desk.

Mirror motion

The definers are not absolutely rigid. There is compliance in the load cell and in the glue used to fix the brackets to the mirror. Load cell compliance is 1 micron per 3Kg. Glue compliance is non-linear starting at 1 micron per 1Kg up to 5 microns then tailing off rapidly.

A 1 micron tilt of the mirror ( i.e. 1 micron in 4.2 metres) gives a 0.05 arc-second tilt of the mirror and 0.1 arc-second tilt of the axis at prime focus. At Cassegrain, there is a de-magnification of approx. 3.7, thus a 1 micron tilt gives approx. 0.027 arc-seconds or a 10 microns tilt, approximating to a 0.25 arc-second error.

Mirror Support PID Servo Control

The electronics for the mirror support system are housed in a 3U crate mounted within a compartment below the mirror cell. This is a PID (Proportional - Integral - Derivative) system and consists of the modules: The full description and setting up procedure can be found in the manual: WHT/INT MIRROR SUPPORT SYSTEM

Cassegrain  Turntable

The turntable carries the Cassegrain Acquisition and Guidance unit (A&G Box) and the instrument; normally ISIS.

The rear face of the mirror cell carries an extension piece. This provides the mounting for both the Cassegrain turntable and it's cable wrap. Two motor/gearbox units fitted with integral tachos and brakes drive the main turntable spur gear through a gear train. This is arranged to provide antibacklash control.

Two further sets of antibacklash pinions drive the T+R absolute and incremental encoders from the main spur gear. Another pinion is coupled to a geared rotary limit switch by a flexible drive shaft. The switch limits the rotation to +/- 270o. This constraint is due to the parameters of the cable wrap. An `A' frame mounts tangentially from the slewing ring and is attached to the operating arm of a Penny Giles linear potentiometer which is fitted to the cable wrap.

As the turntable rotates, it displaces the transducer which generates a signal to control the cable wrap servo. Limit switches operated by a cam on the apex of the `A' frame disconnect the signal if the wrap moves more than 3.5o out of phase with respect to the turntable.

Associated drawings: TC1 703  TC1 757  ED199  ED204  ED209

See Turntable servos and encoders for more information.

Cassegrain Cable Wrap

The Cassegrain cable wrap supports and guides the cables that need to move with the turntable. It also provides the mounting for the electronics racks associated with the Cassegrain instrumentation.

The cable wrap is mounted on the mirror cell extension piece between the cell baseplate and the turntable. It is driven from a disc armature motor-gearbox with integral tacho and brake via a torque limiting clutch which is set to slip at 100Nm.

The cable wrap consists of two loops of  KABELSCHLEPP plastic drag chain. Each loop passes around a roller mounted on brass bushes in the open ends of an almost annular aluminium channel section known colloquially as the banana. Cables are fed from the mirror cell connector plate, pass through ducting into the extension piece and then enter the cable wrap via slots. On exit from the wrap, the cables pass through two slots to the two Cassegrain connector boxes.

The cable wrap cable population is a important factor for trouble free operation. This as been achieved by having the same number of
cables (and of a similar diameter) in both chains. The cables are clamped at each end of the chains. This is important because if the
cables stretch or pull through the clamps, hernias can form and damage the cables.

n.b. Although this mechanism has been greatly improved, it is far from perfect. Cables can get damaged after a long period of operation. To change a damaged cable within the cable wrap requires at least two days of `downtime' on the telescope. The cable wrap is best checked for damage when the mirror is removed for re-aluminising.

Associated drawings: TC1 758  ED198

Azimuth Cable Twister

All cables for the telescope, instrumentation, service cabling, oil hoses etc. must pass from the pier to the telescope mounting. To achieve this a cable twister is used.

The twister is suspended by four steel ropes rigidly clamped to separator plates carrying the full weight of the assembly. This ensures that no stress is induced in the cables. A group of rollers are fitted to prevent the bottom end of the twister from turning as the telescope rotates in azimuth. At the top of the twister, the cables and pipes are fitted with connectors supported on four plates mounted around the central aperture in the floor of the base box. The cables then seperate into various routes within the telescope. The cables and services at the bottom of the twister are also fitted with connectors. This allows for the easy removal of a damaged cable from the twister. The cables then rise on cable trays within the pier surrounding wall and across the dome void and pass through fire bricks into the control room.

Within the control room, the instrumentation cables are terminated with free connectors and are laid into cabinets designated for that particular system of the telescope. The telescope cabling is directed to the cabinets known as the CLIP centre bays. These are either terminated on tagstrip blocks or with connectors mounted on panels.

Associated drawings: TC1 500  TC1 563  TC1 700

Cable runs and services

Cables and services to the telescope tube pass via a simple cable wrap and bridge arrangement over the altitude bearing. This is known as the ALTITUDE AXIS CABLE WRAP. These cables can rise or fall in a compartment below the GHRIL depending on the altitude of the telescope. These cables then divide into two defined groups:

Telescope Tube and Prime Focus

These cables connect to equipment on the telescope tube and within the centre box section. The motorised counterweights, Nasmyth mirror control, temperature sensors, mirror covers, etc.

The prime focus cabling goes to the focus drive, the collimation motors and the prime focus instruments: PFIP, AF2 (when in use).

Associated drawings: TC1 700  TC1 701

Mirror Cell and Cassegrain Focus

These cables are terminated at the mirror cell connector panel solely for the need to separate the mirror cell from the cube when the mirror is removed for re-aluminising. These cables serve the needs for two areas.

Firstly for the telescope eg. mirror support, Cassegrain rotator, cable wrap control, encoding etc. and secondly, which form the majority of the cables, for the Cassegrain instruments and detectors.

Associated drawings: TC1 702  TC1 703

Hydraulic System

The oil for the bearings is contained in a single reservoir on which the distribution and monitoring systems are also mounted. Three pumps are mounted, each on a separate bed. There is one pump for each axis. The third circulates the oil from the reservoir through a cooler located in an outside building. A heat exchanger triggers off the cooling circulation plant to effectively reduce the temperature of the oil.

Accumulators on the downstream side of the flow dividers remove pressure pulses to reduce vibration induced into the telescope structure. The monitoring equipment includes alarms for oil level, oil pressure high and low, oil temperature high and low, filter condition alarms and rotation detection for the flow dividers. Auxilliary contacts on the main pump contactor provide a pump tripped alarm.

Tripping of any pump will shut down the telescope drives. Switching the pumps on can be done locally (in the oil plant room) or switched to remote control and made available at the telescope control desk.

Associated drawings: ED161


Altitude Drive

The Altitude axis is formed by two trunnions each supported on a pair of inclined hydrostatic bearings. Lateral location is provided by a hydrostatic bearing at each side loaded against the rim of the trunnion. Pressure switches monitor the oil flow to each bearing, any fault will be indicated by an alarm on the telescope control desk. In the event of a fault developing, the telescope drive will be shut down. The bearing trunnions are cylindrical steel forgings which bolt to the outer face of the centre box section and whose bores form the light paths to the Nasmyth foci. Machined finishes provide accurate mating surfaces to achieve perfect alignment.

The drive and control system consists of two DC servo motors combined with tachogenerators to provide velocity feedback. These are controlled through the Marconi servo control equipment. Absolute and incremental encoders provide positional data.

Drive limits are set by cam operated switches. These switches are:  + /- Pre-limits and Final limits. These limit switches are activated only if a problem or malfunction exists or if the telescope is taken to the access park position for maintainance or instrument changes. There are also software limits set to work just inside the pre-limits for normal operation. Any limit condition is indicated by an alarm on the engineering control desk.

Moving the telescope to ACCESS PARK, (positions AP1, AP2) can be either done via the engineering control desk push buttons or from the GALLERY control box located at the AP3 access gate on the dome walkway. Moving the telescope to AP3 can ONLY be done from the gallery as the guard-rail needs opening and a floor section raised to achieve this. If the EMERGENCY STOP button is operated when the telescope is travelling below AP2, limit bypasses are removed and can only be re-introduced by hand winding back to AP2.

If a final limit switch is reached, control relays drop-out breaking power to the dome and the servo electronics. From this position, the telescope can only be moved out from the limit by rotating the handwheel which connects to the motor shaft via a clutch and spring arrangement. The handwheel, along with the altitude drive gear is located in a compartment within the upper DS fork tyne. Access is via `swing out'  ladder and through a hatch below the GRACE Nasmyth platform.

Azimuth Drive

The telescope is supported in azimuth by externally pressurised hydrostatic bearings. The ring girder is carried on six axial units. Three master units define the plane in which the ring girder lies in order to maintain the axis vertical. Three slave units carry equal shares of the load. Beside each axial unit, a radial unit is fitted to define the transverse position of the axis. Pressure switches monitor the oil flow to each bearing. Any fault will be indicated by an alarm at the telescope control desk. In the event of a fault developing, the telescope drive will be shut down.

The drive and control of the azimuth axis is identical to the altitude axis. The only difference being that the motors are mounted 180o apart from each other.

The handwheel and azimuth drive equipment are located within the telescope pier, access is via a wooden staircase and a door which leads to the mid level of the cable twister. One needs to crawl under the ring girder to work in this area.

Servo control system

The WHT servo control system was designed and manufactured by Marconi Radar Systems Ltd together with the telescope alarm and interlock logic. Except for their mechanical specifications, the servo systems associated with the azimuth and altitude motions are identical. There are two modes of  operation: For a full description of the WHT control system, see:

Technical Manuals 43/44       Grubb Parsons Operation Handbooks Vols 1 & 2
Technical Manuals 45/46          Marconi Servo Control Equipment

Detailed information on setting up the COUNTER and RATE GENERATOR boards can be found in Martin Fisher's documents

Manual or Engineering Mode

Manual controls for the telescope are situated both on the telescope control desk and on a panel mounted on the CWS tyne. The required operating station is selected by means of a keyswitch alongside the manual controls on the telescope. As the drive rate is supplied from potentiometers mounted on the telescope control desk, manual control is normally done from there. To have full control of the telescope from the ACCESS PARK position on the dome balcony, a keyswitch is provided to switch between local and remote stations. Remote being either the telescope control desk or the telescope tyne controls. Local control from the GALLERY control box is set for only pre-defined movements of the telescope.

These being:

Analogue Control

In the Manual Mode of operation, a DC voltage fed via the drive rate potentiometer is summed with the tacho signal in the Analogue Process Board (APB). The error signal generated from the summation causes the brake to be disengaged and the servo to run. The signal from the tachogenerator backs off the input demand voltage until the required velocity is achieved. Loop stability is controlled by circuits on the Process board.  With no DC voltage i.e. The drive rate potentiometer fully anti-clockwise, the telescope will decelerate under servo control until a time delay relay has energised causing the brake to be applied.

Computer Mode

In computer mode, the WHT is controlled by a DEC ALPHA computer running VAX VMS. The software controlling this is called the Telescope Control System (TCS). CAMAC is used to interface the control systems such as the positional data from various encoders and to drive the telescope using the Marconi servo equipment. Other systems read or written to the TCS include the time service, dome drive and position, focus and turntable control.

Providing the alarm and interlock logic is in the correct state, turning the the Engineering/Computer mode keyswitch and the pushing the button marked COMPUTER RESET located on the services panel on the telescope control desk will select computer mode. The velocity demand will now come from the computer via CAMAC.

n.b. The switch over from engineering mode to computer mode will NOT take place without the TCS software running

Digital Control

In the Computer Mode of operation, a digital word generated in the computer is sent to the Rate Generator board (RGB) via a CAMAC dual 24 bit parallel output register type OR48.

MARCONI CAMAC crate:  Address:  B6 C1 N9   registers :  A0 (ALT)  A1  (AZ)

The Rate Generator provides a pulse stream which is fed via anti-coincidence and steering logic to one of the inputs of an up/down counter located on the Counter board (CTR). This can be frequency divided via on board switches. The other input to the counter is the pulse train from the gear driven incremental encodern.b. (In Azimuth, this could also come from the Inductorsyn tape encoder if enabled).

As counting proceeds, a parallel word is presented to a D to A converter which generates an error signal which drives the servo. The encoder signal subtracts from the input and the demanded velocity is achieved. Loop stability is controlled by circuits on the Process board  (APB)

Marconi Power Amplifiers

Associated with the altitude and azimuth servos are 4 chopper FET power amplifiers type PA1944 which drive the DC servo motors with integral tachogeneraters to provide velocity feedback.

When the telescope is fast slewing, both motors rotate in the same sense. When the demanded position has been reached, one of the motors on each axis drives in the opposite direction to provide an anti-backlash torque when the telescope is tracking. Motor currents are shown on meters both on the PA trays and on the control desk.

See Technical Manual 46   for more information..

Altitude and Azimuth Encoding

ALT and AZ axis encoding consists of a pair of coarse and fine 8 bit and 16 bit INDUcoder optical absolute encoders which are geared driven from the main spur gear therefore providing a known position for the telescope at startup. These encoders were installed recently to replace the obsolete BALWIN types, with their constantly blowing light bulbs. Another encoder, 17 bit geared ITEK incremental encoders provide the high accuracy needed for pointing and tracking.

Associated drawings: ED206  ED207  ED208  ED210

A description of the new absolute encoders and their installation can be found here.

Absolute encoders

Two optical disc absolute encoders coupled together are used on each axis. A 16 bit INDUcoder encoder is used as the fine encoder and an 8 bit version as the coarse encoder. This, in theory, gives an angular resolution to 24 bit accuracy, however the coarse encoder increments on fractions of complete turns of the fine encoder so the resolution in practical is less then 22 bits. For a deeper explanation on this subject check the description here.

What used to be the ENCODER LINE DRIVER BOX is now only used as a junction and PSU box. The box contains 10V psu to supply both encoders. Twisted pair cables carry the signals back to the appropriate connection bays in the control room.

The absolute encoders are read by progammable display units which also convert the encoders SSI output signals to parallel data. This data in turn is read by the TCS via a CAMAC PR2403 input module located in the NASMYTH crate (Addresses:  B6 C2 N14 A0  (ALT)   B6 C2 N14 A1 (AZ).  The absolute encoders are accurate to 0.483398 arcseconds.

All absolute encoder values are read on startup by the TCS. These values are used to reset the incremental encoder counters to the same values. During slewing and tracking, the absolute and incremental encoder values should be in step, but there may be a small discrepancy due to the continual updating of the telescope info display window.

Incremental encoders

The 17 bit incremental ITEK encoders are connected to one of the inputs of the up/down counter contained on the Counter board  via the Rate Generator board as well as being interfaced to the telescope control computer via CAMAC RGO32BIT counter modules.
These are located in the MARCONI crate:  Addresses:  B6 C1 N3  (ALT)     B6 C1 N5 (AZ)

The light source for the optical disc reader within the encoder is an LED and is thus very reliable. The incremental encoders can resolve an angular position to 0.0298570 arcseconds.

The output signals from the incremental encoders (Lead, Lag and 1 Pulse per rev) are fed to an ENCODER LINE DRIVER BOX. These boxes are mounted close to the encoder to minimise noise pickup. The box contains differential line driver chips and a 5v psu to supply both the encoder and the logic. Twisted pair cables carry the signals back to the appropriate connection bays in the control room.

Azimuth tape encoder

An INDUCTORSYN tape incremental encoder has been installed on the azimuth axis of the WHT. This consists of a precision track etched onto a tape mounted on a ring girder and is fitted around the azimuth base box. Access is via the wooden staircase to mid-level of the pier.

Four inductive reading heads with sine and cosine outputs are mounted around the ring and send positional data back to a processing electronics crate located in Bay 6. The digital outputs from the processing crate go to four RGO32BIT up/down counter CAMAC modules fitted in the MARCONI Crate.  (Address: B6 C1 slots N12 - N13 - N14 - N15)

n.b. At the time of writing, the INDUCTORSYN tape encoder is not currently read by the TCS.

n.b.  The ALT and AZ roller encoders are not used, However the RGO32BIT counter in the AZ roller position (Address: B6 C1 N6) is used to read the average of the 4 Inductorsyn tape encoder heads.

Checking the encoders

Checking encoders operations can be done on the on the ENCODER PAGE (page 2 of the TCS Info display window. Type: page enc to read the values. The values of ALL the encoders will be displayed and the absolute and incremental values in each group should be in close agreement.

Synchro Indicators

Course and Fine synchro dial indicators are mounted on the control desk above the ALT and AZ rate controls. These are used to give an indication of where the telescope is pointing when using engineering (manual) mode.

The synchros themselves are mounted below the cover which protects the coarse and fine ALT and AZ absolute encoders. These are driven through a gear train from the shaft of the coarse encoder. These may require racking (rotating the body of the synchro) at times to maintain accuracy.

Alarms and Interlocks

The Alarm and Interlock logic contains relays mounted on Eurocards in a double height crate in Bay 5 of the CLIP centre. Each card is identified by a letter. Upper case letters refer to cards with all relay terminations led out to pins on their back plane connectors; lower case letters refer to alarm relay cards. Each card has 4 edge mounted LED'S fitted to indicate when a particular relay is energised and thus facilitates fault finding.

The Alarm and Interlock logic is designed to disable most of the servo drive if a fault condition is detected either in Engineering or Computer Mode. A fault is flagged by a red indicator lamp on the alarm status panel. The panel is split horizontally between the telescope alarms including operational and oil bearing alarm lamps and the altitude and azimuth axes main bearing oil alarm status lamps.

Any alarm will stop the telescope drive and switch to engineering mode if computer mode was enabled and will not permit computer operation until the alarm(s) have been cleared. Every alarm is interfaced to the telescope control computer via CAMAC  (Address: B6 C3 N6) purely for monitoring purposes. The Telescope Operator can then call up the alarm INFO page on the telescope control terminal to see what alarm has been set if the telescope stops tracking.

Most of the alarms are self explanatory with written legends in the lamp cover. If the azimuth axis, altitude axis, Cassegrain turntable or cable wrap has moved into a pre-limit, driving in the opposite direction in Engineering Mode will clear the alarm. If a final limit for either the altitude or azimuth axis has been activated, it can only be cleared by using the handwheel on the appropriate drive.

For other alarm conditions, the cause will need investigation, e.g. GHRIL door left open, one of the Nasmyth gates not closed etc.

n.b.  The cable wrap alarm sometimes will not clear by driving in the opposite direction. If this occurs, operating the micro switch on the `A' frame by hand whilst pressing the direction button on the handset will clear the fault.

Associated drawings:  TC1 550-01  TC1 620  TC1 625  ED167  ED168  ED170   ED178  ED193  ED194

Azimuth Zone Sensing

Total movement of the azimuth axis is +/- 270 degrees. Zone sensing is required to give indication of the actual position of the telescope for enabling limit switch operation if the telescope moves into a forbidden area. A cam operated set of microswitches under a cover near the ring girder provide this facility.

n.b. The position of the cam is critical and if moved by hand into an incorrect position will prematurely move the telescope into a limit switch.

A two colour (RED/GREEN) LED above the azimuth coarse synchro indicator on the telescope control desk shows which zone the telescope is working in.

IMPORTANT  If the telescope is zeroset in azimiuth using the target, ( zeroset az target )  it MUST be done in the GREEN zone.

Associated drawings: TC1 550-01  ED194

Zeroset datums

If an ALT or AZ absolute encoder fails, a second method can be used to move the telescope in both axes to a known datum.

The zeroset equipment consists of an inductive sensor mounted on a rigid plate; it's output going to the Two Zone Module and a metal target fitted to the main drive gears and thus moves with the telescope. The distance between the target and sensor is less than 1mm and needs to be accurately aligned. There is one for both the altitude and azimuth axes.

n.b. There have been new zeroset units fitted. The AZ sensor and target is mounted on the OUTSIDE of the base box near the tape encoder.

Zeroset Two Zone Module  (old system)

The two zone modules are fitted into encoder style  line driver boxes and are mounted close to the sensors. There are preset pots on the module to adjust the zeroset marker pulse to appear within a very narrow window thus determining a very accurate datum. These adjustments both set the width of the window and also can move the window a small amount +/- in respect to the centre of the target.

The narrow output pulse from the two zone module is anded with the 1 pulse/rev from the incremental encoder (this is done in the incremental encoder line driver box) and is also fed to the RGO32BIT counters where it sets a datum flag that can be read by the TCS.

See Technical Manual 34     For details on the Zeroset Two zone Module.

Transducer System

To improve pointing accuracy, nine SANGAMO LVDT's (Linear Voltage Displacement Transducers) are fitted. Positional data from these transducers is fed into the TCS via a CAMAC ADC module. The transducers measure positional errors due to flexure in the mirror support systems caused by forces introduced when the telescope is moved away from the zenith and also any small changes in concentricity in the azimuth bearing. n.b. The primary mirror transducers (* Ch1 and Ch2) are not currently read.

These are located at the following positions:
Channel  1 Primary mirror Mirror 1  (0o) *
Channel  2 Primary mirror Mirror 2  (180o) *
Channel  3 Azimuth bearing Az-3
Channel  4 Azimuth bearing Az-4
Channel  5 Azimuth bearing Az-1
Channel  6 Azimuth bearing Az-2
Channel  7 Secondary mirror suspension point 1
Channel  8 Secondary mirror suspension point 2
Channel  9 Secondary mirror suspension point 3
Channel  10 Spare

The transducer sensitivites and scaling factors are:
Channel Number Sensitivity Scaling factor
1, 2 150mV 10mV per micron
3,  4,  5,  6 65mV 6.6mV per micron
7, 8, 9 65mV 10mV per micron

The outputs from the LVDT's go to the Sangamo/Schlumberger CR24 transducer processor crate. The output channels from the CR24 then go to a ADC 1232 module in the CLIP CENTRE CAMAC crate (Address: B6 C3 N11 )

Both the CAMAC crate and the CR24 unit are located in Bay 5 in the control room. For more information and a full description of the transducers and details of the CR24 unit.  See:  Technical Manual 42  (Telescope Transducing System)

Turntable Servos and Encoders

There are 4 turntables, these being:
  1. Cassegrain                             (2 motors)
  2. Nasmyth Drive Side              (1 motor)
  3. Nasmyth Cable Wrap Side    (1 motor)
  4. Prime Focus  (when fitted)       (2 motors)
The Marconi turntable servos are identical to those used on the ALT and AZ drives with the exception of the power amplifiers. They use a linear type, model EM1811-01-B manufactured by INLAND. There are two motors fitted to the Cassegrain and prime focus turntables. One drives the turntable, the other (when tracking) applies torque bias to remove backlash. The Nasmyth turntables use a single motor.

In engineering mode, the Nasmyth turntables can be moved from +/- direction buttons in the Nasmyth rack (below the CAMAC crate) or from a local control box on the Nasmyth platforms. The Cass and Prime turntable can be moved from the engineering desk or if at the telescope; using the pendant box (at Cass) or at prime focus using the turntable +/- buttons on  the AP3 control box. The turntable speed in engineering mode is fixed.

The demand to the servo (Nasmyth Marconi crate) in computer mode comes from the CAMAC modules:

Two encoders are used on each turntable. These being:
  1. A T+R absolute encoder  which is read on startup to determine the position angle.
  2. A T+R incremental encoder which then takes over for slewing and tracking.
As with the ALT and AZ axes, the incremental encoder pulses are fed to the Rate Generator Board (RGB) in the Marconi  servo Nasmyth crate and to RGO32BIT counters. These are located in: The absolute encoders for ALL turntables are read directly by the TCS via HYTEC 450-4 Differential 24 bit input/output  modules. These are located in: Associated drawings: TC1 550-01  TC1 757  ED158  ED194  ED199  ED204  ED205  ED209  ED215

Cassegrain Cable wrap

The Cassegrain cable wrap comprises of a velocity servo, the DC error signal being derived from a linear potentiometer which is mechanically linked to the Cassegrain turntable. No error signal exists when the turntable and cable wrap are in alignment. When misalignment occurs, an error signal  is generated which causes the servo to run. The error signal is backed off by the cable wrap error signal and a steady state condition exists.

A tacho input from the Cassegrain turntable produces a feed forward term which together with processing provided by the Process board (PRB), provides system stability.

Associated drawings: TC1 550-01  TC1 758  ED194  ED198

Focus Drive

The focus drive mechanism is mounted centrally within the flip ring. Either the secondary mirror cell or the prime focus turntable can be attached to this assembly. It is carried on four pairs of pre-tensioned vanes terminating in two ring girders on which the structure is assembled.

The focusing system is powered by two printed armature motors, mounted opposite each other. A drive belt loops around the motors plus four gearboxes with leadscrews. These drive the assembly towards or away from the primary mirror to achieve the desired focus. Position encoding is achieved by an 20 bit absolute encoder with a 100 turn range of 1000 counts per turn.

Associated drawings: TC1 753  ED177  ED185  ED186  ED197  ED203

Focus servo control

The focus drive is fitted with a velocity loop similar to that fitted to the turntable and main drives. The digital loop however is different in that there is NO Rate Generator Board and no incremental encoder fitted. The demand and error summing point is created in the computer as the feedback term. The servo  power amplifier uses an INLAND unit.  Type EM1811-01-B

Focus control to the servo from the TCS is via a OR48 module in the MARCONI crate. Address: B6 C1 N11 A0

A 20 bit BCD absolute encoder transmits positional data directly to the computer via a Hytec 450-4 Differential 24 bit input/output module in the NASMYTH crate. Address: B6 C2 N21

Associated drawings: TC1 550-01  ED185  ED186  ED197  ED203

Focus tracking and temperature sensors

To allow the focus to track for small variations due to the expansion and contraction of the Serrurier Truss, seven platinum resistance thermometers (Type PT100) are fitted to the telescope tube and close to the mirror.

These being:

  1. Top End Ring  (ADC Channel A0)
  2. Upper Serrurier Truss CWS  (ADC Channel A1)
  3. Upper Serrurier Truss DS  (ADC Channel A2)
  4. Mirror side  (ADC Channel A3) *
  5. Lower Surrurier Truss  (ADC Channel A4) *
  6. Mirror air  (ADC Channel A5) *
  7. Mirror cover  (ADC Channel A6) *
* NOT read by TCS

The mirror sensor is fitted into the fog baffle close to the mirror surface. The sensors are attached by Araldite to nylon mounts for strength and to protect the leads. This assembly is then fitted into small diecast boxes mounted onto the telescope.

The top end ring sensor box also contains the temperature transmitter. This provides an output current drive which is proportional the resistance of the sensor (100 ohms at 0o C). The other sensors are connected to transmitters mounted in a box on the inner wall of the centre section. The outputs from the transmitters go to board (BC1 105) fitted with precision resistors to convert the current drive into a voltage level. This provides an input to an ADC 1232 module in CLIP CENTRE crate. Address: B6 C3 N12

Associated drawings:  ED157 ED176

Page last updated:  25th January 2011  rjp