1.0m Jacobus Kapteyn Telescope (JKT)

General Description of the Telescope

The JKT is an equatorially mounted (German type) telescope with a 1.0m diameter primary mirror. The asymmetric equatorial mounting allows the telescope to work either on the west or east side of the polar axis pier. The counterbalance box is mounted low on the polar axis in a torque tube to maximise the available working space in the dome. There is a choice of two secondary mirrors: n.b. In 1998, the f8 focus has been decommissioned due to the JKT being used now ONLY for direct CCD imaging at the f15 focus.

The tube is of open construction (Serrurier truss) with the optical components (primary and secondary mirrors) supported at the upper and lower ends of the truss. The top end ring supports the secondary mirror assembly and the focus drive.

The lower trusses carry the mirror cell containing the primary mirror. Below the mirror cell is a manually driven turntable (normally kept at a position angle of 0o) to which the JKT CCD A&G BOX (JAG) is mounted with a cryogenic CCD camera. The JKT is now a single instrument telescope and the JAG is seldom removed except for maintenance or when the mirror is removed for aluminising.

The declination axis rotates in two opposed pre-loaded taper roller bearings in hollow housings mounted at the top of the polar axis tube. This complete assembly is mounted on a base box which houses the polar axis bearing. This is seated on a wedge adjusted to the latitude angle. Fine adjustment in altitude and azimuth is available at the base of the wedge. The Polar axis rotates in two opposed pre-loaded tapered roller radial bearings.

An equatorially mounted telescope only needs to track at sidereal rate in R.A. to maintain the object at the focal plane. The polar axis points to true north and is inclined at the latitude angle equal to that of the observatory (approx 28.75o N). n.b. In this document, the terms HA (hour angle) and RA (right ascension) I have used freely. They simply mean the movement of the telescope in the east - west direction; the meridian being the north - south centre line of the axis. Hour angle being the more correct term as this value changes with time.

For a detailed description of the JKT refer to:  The JKT Operation and Maintenance Manual Vol. 1

The total weight supported on the tapered radial bearings of the polar axis is made up of the individual weights of each telescope component. These being:
Secondary mirror assembly 150kg
Top end ring 3500kg
Telescope Tube 1500kg
Centre section 11000kg
Counter weight 7000kg
Electronics cabinet 300kgs
Mirror cell 1000kgs
Primary mirror 215kgs
Rotator 250kgs
A&G box 250kgs
Polar axis 15000kgs
Total weight 40000kgs (40 metric tonnes)



Telescope tube

The telescope tube is a conventional Serrurier open truss design with a rectangular centre section to which the declination bearing trunnion is attached. The Serrurier truss provides equal and parallel deflections of the top end ring and the mirror cell at all altitude angles. The trusses are tubular steel sections which are welded to the centre section and to the top end ring.

The lower trusses are closed by the main mirror cell, the upper by a fixed end ring which can support one of two secondary mirror assemblies. The main mirror petals are suspended below the centre section piece on A-frames which support the base plate carrying the petals and drive assembly.

Cables run inside two of the upper trusses for the focus motor, encoder and the flat field lamps. The lower truss pairs carry the mirror cell which is secured with bolts at the four cardinal points.

Associated drawings: TB6 200

Secondary mirror cell

The secondary mirror assembly is mounted on locating pins on the top end ring. This ensures that optical alignment is reasonable well maintained if the secondary mirrors are interchanged. The secondary mirror is supported by a 6-point axial mechanical support system contained within the cell. A system of counterbalanced levers control the axial deflection at the outside edge. Focusing is achieved by moving the cell up and down in relation to the primary mirror; a range of about 20mm.

Associated drawings: TB6 210  TB6 290

Focus assembly

A stepper motor drives a lead screw through a 2:1 reduction gearbox. The screw pitch is 1mm, giving 0.5mm linear focus movement for each turn of the motor shaft. Stop pins are provided to prevent the focus mechanism from over running and jamming. The focus slide is supported by 8 pre-loaded linear ball bearings which run on rigidly mounted precision ground cylindrical rods mounted in the spider barrel.

Associated drawings:  TB6 210  TB6 290

Centre box section

The centre box section (or CUBE) is a square fabrication of four box sections with a bearing trunnion fitted to one side to form the declination axis. The tubular trusses are welded onto the centre section at each corner. All cables for the telescope pass through the declination bearing into the cube. The cables are secured at the setting circle side of the declination axis using a clamp. This allows the cables to twist within the declination bearing when the tube is lowered in altitude.

Primary mirror cell

The primary mirror cell is a steel fabrication consisting of a flat disc with a box section cylindrical surrounding wall. The mirror cell is bolted to the lower trusses by four equally spaced bolts, the shear being taken by angled wedges at each of the four cardinal positions. Stiffening of the assembly is achieved by radial webs. The underside of the cell has an extension piece which carries the Cassegrain Instrument rotator drive motor and encoder. The cell floor has a machined surface which carries the rest pads and the pneumatic mirror support system.

Associated drawings:  TB6 220

Mirror support system

The primary mirror is supported on twelve pneumatic pads (known as Belloframs) set in three concentric rings. The JKT mirror support system differs from the WHT/INT systems in that all the Belloframs work together as a single sector (not in 3 sectors of 120o as is the case with the WHT and INT). The sector is controlled by a single JOUCOMATIC servo control valve. Although there are 3 load cells to measure the forces acting on the mirror, the mirror support system only servos on one. The load cell used is selectable from a switch on the mirror servo electronics crate mounted on the cube. 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 rest pads which support the mirror when the pneumatic system is not pressurised.

Axial definer system

Three of these units are fitted between the cell floor and brackets cemented to the mirror's edge. These are used for aligning the mirror position within the cell to the optical axis of the telescope and thus setting the working height for the mirror. A load cell is screwed into one end of the definer rod. This has been designed to ensure that only pure compressive/tensive forces are applied to the load cell. Adjacent to each axial definer are dial gauges which are used to monitor the height of the mirror. These need to be read before and after mirror re-aluminisation to insure the mirror returns to the correct height once the support system is pressurised.

Associated drawings:  TB6 227

Radial definer system

Twelve counterweights provide the required tangential supporting force for the mirror and prevent it from twisting. These are connected by levers which can move freely in brackets cemented around the edge of the mirror. Fine balancing of the mirror is achieved by adjusting the position of each of the counterweights along its screwed rod. In practice, these are never touched and small lead weights are strategically placed around the mirror to achieve this.

The radial position of the mirror in the cell is fixed by three radial (transverse) defining units. These are pre-set to the required length and positioned close to the axial definers. These enable the mirror to return to its original position in the cell after re-aluminising.

Associated drawings:  TB6 223

Mirror Support PID Control

The electronics for the mirror support system are housed in a 3U crate mounted externally on the mirror cell. This is a PID (Proportional - Integral - Derivative) system and consists of the modules:
DSGA (Digital Strain Gauge Amplifier)
This module processes the signals coming from the load cells.

DMSC (Digital Mirror Support Controller)
There are 3 of these modules, each which contain a PID servo. Each controls a sector JOUCOMATIC valve. Switches on the modules select different operating modes and can change the sensitivity of the servo. 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.

MSCF (Mirror Support Control Facilities)
This module has the facility to drive the PID in manual mode for engineering tests. The +24V can be switched off, manual or auto mode selected and a Manual Control (multiturn pot) to change the demand to the PID servo.

CANBUS network
This module enables data to be sent via a co-axial cable to a PC.  A program running under Windows can be used for diagnostic purposes.

There are also PSU modules supplying:  +24V, +/- 12V and +5V.

Full details for the mirror servo are beyond the scope of this document. Refer to the manual WHT/INT Mirror Support System (INT workshop copy) which contains information for the JKT also.

Mirror cover petals

The primary mirror cover is built up of 16 petals made from aluminium for strength and lightness. Each petal is moved by a nut and screw mechanism. The screws are driven by an endless chain from two geared single phase AC motors. Limit switches operated by levers mounted on the petals sense the open and closed positions. The circuitry has been designed so that one push of either the OPEN or CLOSE button on the control desk will initiate the operation. The petals cannot be put into a semi-open state (as can the INT or WHT) for stopping down the beam. Lamps built into the push buttons indicate when an open or close operation has been completed. Mirror status is sent to a CAMAC PR2402 dual input register, Address : B4 C2 N7 A0 (Bits 12 and 13) and can be read on the ALARMS PAGE of the TCS.

Associated drawings:  TB6 260  TB6 700 02

Instrument turntable

A turntable is provided for mounting instruments to the telescope. This now only being the CCD Acquisition and Guidance box (JAG). Generally, this is maintained at a position angle of 0o. There is also another TV acquisition unit available known as the Ellis A&G box, but this is seldom used these days since the JKT became a one instrument telescope.

The turntable moves in an opposed pre-loaded pair of radial angular contact ball bearings and is driven by a single phase 240V motor/gearbox unit by a pinion which engages with the spur gear on the rotator ring. Two push buttons +/- on the mirror cell move the turntable. n.b. The turntable CANNOT be moved under computer control.

Before the turntable can be moved, it is necessary to unscrew the clamping knob on the mirror cell. As a safety feature, when the turntable is clamped, power is removed from the motor.

Associated drawings:  TB6 272

Telescope cabling

All cables from both the control room and the CLIP centre on the lower floor are carried in cable trays below the rising floor. The cables then pass through a slot in the concrete pier and emerge in the base box. Cables and pipes that peel off at this point are: Some of the cables go to a tagstrip accessible by removing a side cover on the base box near the gate of the rising floor.

Cables and air pipes that go to the telescope tube are then routed on an external cable tray from the base box to the bottom of the polar axis tube (through the HA setting circle). The cables pass through this tube and are terminated with connectors on a circular connector plate at the top of the polar axis. This allows broken cables to be replaced relatively easily without having to change a complete cable run through the building and telescope superstructure.

The cables continue on from this connector plate and are fitted to another external cable tray which takes them to the outer hollow tube of the declination axis (through the DEC setting circle). The cables then pass through declination axis and into the cube. From here, most go to the instrument connector panel mounted between the mirror cell and the finder telescope. Many of these cables are now obsolete due to many of the instruments that were once used on the JKT having been decommissioned. Those still in use are:

The rest split off and go to the mirror cell or up two of the truss tubes to the top end ring. These being: Clamps are mounted at the top and bottom ends of the polar axis and the outer end of the DEC axis. This has been arranged so that when the telescope is pointing to the zenith, the cable bundles lay untwisted in the polar and declination axes.

n.b. It should be noted that this arrangement is NOT as described in the Grubb Parsons Handbook. The JKT was completed re-cabled in Oct 1987 due to cable stretching in the polar axis that was causing bad tracking.

Associated drawings:  TB6 700 02 (4 sheets)

Lubrication system

The hydraulic plant comprises of an oil tank, pump and a pressure switch. This is located under the rising floor adjacent to the pier. Oil is pumped to the worms and the worm-shaft roller bearings at a constant flow to lubricate them. The oil drains back from the polar axis worm box under gravity.

The declination drive casing is drained at all times by one of four suction pipes. The suction pipe is selected according to the orientation of the telescope by two mercury tilt-switches operating solenoid control valves. This is a very simple switching arrangement and has replaced the original more complex Grubb Parsons circuit which used logic gates.

Suction is provided by a scavenging pump which also has a small continuous feed of oil from the supply pump to maintain its own lubrication. The oil pump on/off switch and the oil pressure alarm indicator are mounted on the control console.

The oil pressure switch forms part of the alarm and interlock chain and power will be removed from the telescope servo amplifiers if the oil pressure fails.

Associated drawings:  TB6 122  (A new drawing in the folder replaces: BB6 003 01)


Counterweights moved by leadscrews are provided for both the polar axis and declination balancing. HA axis balancing is achieved by moving weights in the counterpoise box away or towards the polar axis tube. The upper and lower weights are moved using a removable handle. The positions of the weights may be seen through scaled slots. The coarse declination weights are located on the telescope truss and moved by hand wheels. External weights are also added to the cube or to the rotator for fine balancing.

n.b. As the JKT just uses the CCD A&G box now, it is seldom necessary to re-balance the telescope.

Associated drawings:  TB6 133  TB6 203

Tie bars

When an instrument is removed, tie bars are fitted between the DEC and HA axes and the pier to support the telescope in an unbalanced position at the zenith. Fitting these bars operates microswitches in the pier brackets to prevent power being applied when the telescope is clamped. These switches also illuminate alarm indicators on the control desk. There is also another set tie bars for use when the telescope tube is horizontal. These are used when changing the top end ring optics.

Associated drawings:  TB6 603

The JKT servo drive system


As in the case of the WHT and INT, the JKT is controlled by a DEC ALPHA computer running VAX VMS. The software controlling this is called the Telescope Control System (TCS). CAMAC  (Computer Automated Measurement And Control) is used to interface the various systems such as the DEC and HA position encoders and the DAC to drive the servo amplifiers. Other systems read or written to the TCS via CAMAC include the time service, dome drive and position, turntable position, focus control and autoguider corrections.

A manual (engineering) mode is also available for driving the telescope, dome and other systems.

The Declination and Hour Angle (polar) axes rotate in two opposed sets of pre-loaded taper-roller bearings. A worm-wheel and spur gear are rigidly fixed to the axes. An INLAND DC torque motor with integral tacho-generator drives the worm through a flexible coupling connected directly to the worm shaft. The gear reduction is 540:1 on both axes. Position encoding is done using a BALDWIN incremental optical encoder mounted on the worm shaft just in front of the flexible coupling.

A second torque motor with gearbox and pinion drives the spur gear. This provides the anti-backlash torque required to keep the worm in constant mesh with the worm-wheel when the telescope is tracking.

Declination and HA +/- limit switches prevent the telescope from moving into forbidden zones and mercury tilt switches mounted in a box on the cube act as an horizon limit which operates about 5o above the horizon.

The worm drive assemblies are mounted on a moving plate restrained by a pre-loaded spring unit. This prevents excessive drive loads being applied to the worm-wheel teeth. Possible causes that could move this plate are the telescope hitting an obstruction, if it is grossly out of balance or in rare cases, earthquake tremors. If this plate moves by more than 1 or 2mm, a microswitch is operated and the worm overload display alarm on the control desk will come on.

The worm shafts are extended through the casing and are fitted with flywheels. These help to maintain inertia on the worm shaft. With the servo drives powered off, it is possible to spin these by hand and move the telescope.

Note: The fly wheels are grooved to allow a cord attached to a spring balance to be fitted. By wrapping the cord around the wheel and pulling the spring balance out at a steady rate, it is possible to check the balance of the telescope. This procedure is done in both directions to ensure the forces needed to move the telescope are equal.

Associated drawings:


Polar Axis and RA drive:  TB6 130  TB6 141  TB6 143
Dec Axis and drive :        TB6 134  TB6 151  TB6 153

Electrical:  TB6 700 02  TB6 702  TB6 703

A full description of the JKT control system can be found in the manual: RA and Declination Servo System

Engineering Mode operation

Manual controls for the telescope are located on the control desk. There are four push buttons for HA and DEC +/- movement. The drive rate is obtained from potentiometers adjacent to the push buttons. The inputs to the servo pre-amplifiers ramp up according to the voltage set by the potentiometers thus ensuring a slow build-up of motor velocity.

On selection of telescope direction, a DC voltage is derived from the RATE potentiometer (which forms part of a voltage divider chain) on the Manual Control card: EC3. This voltage level is then compared with the voltage coming from the tacho-generator (mounted on the motor). Both of these voltages go to a comparator on the pre-amplifier cards (RA: EC1 and DEC: EC2). The error (or difference) voltage generated is then taken to the INLAND servo power amplifiers which drive the motors.

The tacho signal gradually backs off the input demand until the required velocity is achieved and loop stability maintained. This action also assists the in the prevention of lock-up of the worm/wheel which may occur under certain conditions such as if the telescope is out-of-balance or rapid deceleration occurs.

Computer Mode operation

Providing the alarm and interlock logic is in the correct state, turning the keyswitch from engineering to computer mode (and pushing the green reset button), telescope control will be passed to the TCS if its running.

The CAMAC system is clocked at 20Hz (50mS). During this period, the TCS will have read the time service (UTC), have converted UTC to Local Sidereal Time (LST), will have read in the instantaneous values stored in the RGO32Bit up/down counters and computed the demand value to the DAC to drive the telescope. Other tasks such as moving the dome, checking the focus/temperature tracking etc) are also performed during this period.

In computer mode, two servo loops are generated:

Position loop

The JKT uses BALDWIN encoders; one on each axis, which the TCS reads (at 20Hz) to determine the required velocity demand signal.

When co-ordinates have been entered for RA and DEC, the TCS calculates how far the telescope needs to move and at what velocity. The telescope will drive to reduce the error between the DEMANDED and the ACTUAL  position co-ordinates. The greater the difference is between them, the faster the telescope will move. As the errors get smaller, the velocity decreases until the error between the actual and demanded position is the same.

Velocity loop

A digital word (the DEMAND value) generated by the TCS after reading the encoders is sent to the servo pre-amplifiers via a CAMAC 9085 Digital to Analogue Converter module (DAC)  (Address: B4 C1 N12)  This samples at 12bits.  The channels being:

   1. A0  HA (FAST DAC)
   2. A1  HA (SLOW DAC)
   3. A2  DEC (FAST DAC)
   4. A3  DEC (SLOW DAC)

The DAC is set for Bi-polar mode operation; the output levels being: +/- 5V. A value of 2048 being equal to 0V  i.e. No demand being sent to the servo amplifiers. When fasting slewing (a large position error generated), the TCS outputs a demand to the DEC and HA fast DAC channels. At a pre-determined point, control will then pass to the slow DAC, (the velocity being reduced by a factor of 10) until the position error is reduced to zero. Once tracking is achieved, the HA axis will drive at sidereal rate; a velocity demand of  ~ 1990  being sent to the DAC A1 channel.

The tacho signal is used to maintain a constant velocity and to switch in the anti-backlash torque motors when the demanded position has been reached. These motors work by applying a light force in the opposite direction when the telescope is tracking through a pinion driving a spur gear rigidly attached to the wormwheel.

In computer mode, a preset voltage is applied to the anti-backlash motors to provide a constant torque via the Torque Control cards (RA: EC4 and DEC: EC5). These circuits are switched off when in manual mode or when the tacho voltage exceeds a pre-determined value such as when the the telescope is slewing.

The TCS will drive the telescope to constantly reduce the error between the demanded and the actual position co-ordinates. When tracking, the position error on the TCS Info Display will be very low and usually show: 00:00:00.

Click here for a detailed block diagram of the JKT Servo system.

Position Encoding

This is done using BALDWIN optical incremental encoders mounted on each worm-shaft. Note: These encoders use an LED as a light source, so the need for lamp replacement has been eliminated.

The encoders have two tracks (LEAD and LAG) producing a quadrature output of 12000 counts per revolution. There is also a 1 pulse per revolution signal generated. To achieve a point of positional reference before observing, the Baldwin encoders have been set up radially so that the 1 pulse per rev signal occurs midway during a pulse from the datum (meridian and zenith) switches.

The encoder TTL output signals are fed to line driver boxes where they are logically ANDED with the zeroset pulse. These signals are then passed to CAMAC RGO32 Bit counters.  Addresses:

allowing the TCS to calibrate to a known position on startup. Once this has been determined, the TCS can then keep track of where the telescope is pointing and to where it needs to move for acquiring targets.

The RGO 32Bit counters have the facility to multiply the number of pulses by a factor of four thus making the total number of pulses per revolution to 48000 counts. Taking into account the 540:1 reduction bewteen the worm and wheel, this gives an angular resolution of 0.05 arc secs per baldwin bit.

If a Baldwin encoder or RGO32Bit counter module develops a fault, the telescope pointing will be completely wrong and its position unknown. In a severe case (high order bits missing) this could cause the telescope to shoot off until a limit or horizon switch is activated. A missing low order bit will have the effect that the telescope will never obtain TRACKING status due to a constantly high position error. As the Baldwin encoders now use an LED light source, it is more likely to be a faulty RGO32Bit CAMAC module than the encoder itself that causes these problems.

Associated drawings:  TB6 702  TB6 703  BB6 002 01

Zeroset datum switches

To move the telescope to a known datum  (i.e. to set the encoders at startup), slotted opto-switches mounted on the fixed sections of the polar and declination axes are used. Two metal flags 180o apart are fitted to the rotating components of the axes which creates a 'zeroset window' when the light beam is interrupted. This two flag arrangement allows zero-setting the telescope when the tube is positioned on either the EAST or WEST side of the pier. n.b. It should be noted that the telescope conventionally works on the EAST side of the pier.

In practice, there are 5 opto-switches fitted on the polar and declination axes. These being located at:

The telescope is always zeroset at ZENITH PARK. For this reason, the +30 degree zenith switch on the DEC axis (the latitude of La Palma is approximately 28.75o) and the 0o meridian switch in the polar axis are used.

n.b. Because there are effectively 5 positions where the telescope could 'zeroset', it is important to see visually that the telescope is vertical.

Associated drawings:  TB6 700  02

Alarms and Interlocks

Interlocks are built into the system which will disable the telescope if certain conditions are not met or exceeded. Each alarm is indicated by a lamp accompanied by an audible tone on the control desk. These being: If any of these are activated, power to the telescope motors will be removed. If the telescope is working in computer mode and an alarm is activated, engineering mode will be forced. Some of the alarms are 'fleeting' whilst others are not. To return control back with a non-fleeting alarm, an override keyswitch is provided to enable the telescope to be moved in the opposite direction until it is safe and the alarm clears. Needless to say, great care must be taken using the override switch. When an alarm is cleared, it is necessary to push the green RESET button to reinstate power to the servo amplifiers.

More details can be found in the JKT Operation and Maintenance Manual Vol 1

Associated drawings:  TB6 710  BB6 101  BB6 112

Servo pre-amplifiers

The servo pre-amplifiers consist of wire wrapped Eurocards and a modular PSU mounted in a 3U rack fitted between the power amplifier trays in a cabinet located in the TCS room (CLIP centre) on the first floor.  The board placement from left to right is: The position demand voltage coming from either the control desk (via manual control board EC3) or CAMAC is routed via relays to the pre-amplifiers. Their state being on the whether engineering or computer mode is selected. In depth information is available in the JKT SERVO manual.

Power Amplifiers

There are four INLAND power amplifiers mounted along with the pre-amplifier crate in the control cabinet. From the top down, these are: The input power to the amplifiers is derived from a full wave rectified three phase transformer with anti-surge circuit mounted in the bottom of the cabinet. This PSU is connected to the amplifier through a 20A fuse and smoothing circuit. Each amplifier is capable of delivering 30A at 40V. However, the current is limited to a maximum of 16 amps.

The INLAND power amplifiers are mounted on a removable tray with the connections being made via a crimp tag terminal strip. n.b. All amplifiers (including the ones used for the INT) are compatible with each other and can be interchanged between the INT and JKT telescopes and the positions they occupy.

The INLAND amplifier is modular in that it consists of a 'plug-in' encapsulated hybrid servo amplifier mounted on a frame consisting of two large heatsinks. A mains driven fan draws air through the frame. The hybrid amplifier drives 12 power transistors (2N3773) connected in an H-bridge configuration; 3 per leg, 6 mounted on each heatsink. n.b. It should be noted that the 3 transistors in each leg of the bridge are matched for gain so that the current passing through them is evenly distributed.

A bi-metal thermostat which opens at 70oC is mounted on each heatsink. If it operates (usually a sign that the fan has failed), an alarm output will shutdown the telescope drive. This alarm will also illuminate the P.A. Overload indicator on the control desk.

The output to the motor is protected with a 25A fuse. In series with the fuse is a shunt with cables connected across it going to the motor current meters on the control desk. These currents should be equal when the telescope is slewing (around 3 to 4 amps) in both in the + and - directions. If the motor currents are unbalanced, the problem is almost certainly that the telescope is out of balance.

Each power amplifier has 3 indicator lamps on the front panel. These are:

Instrument rotator

The Cassegrain rotator is driven via a reduction gearbox by a single phase AC motor. It is operated by +/- direction push buttons mounted on the mirror cell. As an added safety precaution, mains power can only be directed to the push buttons when the turntable clamp is released. Once the required rotator position is obtained, the clamping knob must be re-locked to prevent the turntable from slipping.

The turntable position is generated using a 13bit absolute encoder driven by the turntable spur gear. The output signals are converted to TTL levels in the adjacent line drive box and are fed to a CAMAC PR2403 dual input register, Address: B4 C2 N14 A1. The rotator position is available from the TCS INFO DISPLAY window.

Associated drawings: TB6 704

Focus Control

The F8 and F15 secondary mirror top end ring assemblies use identical stepper motors for the focus drive. The connectors from the focus motors on the f8 and f16 top end rings are wired linked differently so that changing a ring will indicate which focus is being used. This is shown on a control desk indicator lamp. CAMAC can also read this information thus the TCS knows what focus the telescope is working at. (Address: B4 C2 N7 A0 - (Bit 10 =  f8 and 11 = f15).

n.b. As the JKT always works at f15 with the JAG and CCD, top end focus ring changes are never done now.

The focus motor is driven from a stepper motor driver card mounted in the bottom card crate in the control desk. This card is controlled from either the manual focus +/- buttons or when in computer mode via a CAMAC OD2407 output driver module: Address: B4 C2 N6 A1 (Bits 1 to 4 = FAST, SLOW, direction +, direction - )

Associated drawings: TB6 700 02  BB6 117

Focus position

A 4 decade BCD absolute encoder determines the focus position. The encoder outputs pass to a line driver box mounted on the ring before being sent to CAMAC. A PR2403 dual input register, Address: B4 C2 N14 A0 enables the TCS to read the focus position.

A second set of outputs from the line driver box are sent to a LOCAL focus LED display mounted in a box on the finder telescope. When the telescope is put into engineering mode and the remote handset plugged into the ENGINEERING connector on this box, the focus can be moved manually from the observing floor. The same applies to the FOCUS +/- push buttons on the control desk.

n.b. When the TCS changed over to a DEC ALPHA computer, the OBSERVER connector (which allowed HA, DEC and FOCUS corrections to be made when in computer mode) was never re-implemented.

Associated drawings: TB6 700 02  BB6 001

Focus correction for temperature changes

To allow for the focus to track with temperature changes, platinium temperature sensors (Type: P100 - 100 ohms at 0oC) are fitted to the top and bottom of one of the tube trusses and a lower truss.

The sensors are connected to temperature transmitter modules which are mounted in a box on top of the cube. These modules are current drivers which terminate on a precision 1K resistor network card which is located in the crate at bottom of the control desk (along with the focus stepper motor board).

The voltages measured across the resistors are proportional to temperature and are sent to a CAMAC ADC1232 module: Address: B4 C2 N13 - A0 top, A1 mid, A2 lower sensors) allowing the TCS to measure these values. A change in temperature of +/- 0.5oC will change the focus by 0.01mm.

Associated drawings:  TB6 700 02  BB6 118

Telescope power supplies

To provide power for the encoders and line driver boxes and associated electronics on the telescope, a crate containing modular PSU's is mounted at the bottom of the control desk. These supplies deliver  +5 and +/-12V

If a problem exists, e.g. with the encoders, it is best to check these first as they have failed several times in the past.

Last updated: Sept 2002  ejm