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:
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.
| 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 |
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
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 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.
Associated drawings: TC1 408 ED166 ED176
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
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)
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
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:
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.
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.
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.
LED's arranged in a bargraph on these modules show the servo in action. When the telescope is slewing, the red LEDs will be flickering + or - of centre depending on whether compressive or tensive forces are being measured from the load cells. The LED's are arranged so that the outer ones on the bar show large forces whilst those closer to the centre show the servo reaching equilibrium. The green LED in the middle being illuminated when the mirror servo has stabilised. By the time the telescope has got into tracking, only the 3 green middle LED's should be on.
n.b. If this is not so and red LED's are seen toggling, this
is a sign of mirror servo oscillation and will need to be investigated.
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.
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
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
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
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
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.
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.
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
These being:
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
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 encoder. n.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)
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..
Associated drawings: ED206 ED207 ED208 ED210
A description of the new absolute encoders and their installation can be found 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.
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.
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.
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.
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
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
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.
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.
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)
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:
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
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 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
These being:
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