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.
|Secondary mirror assembly||150kg|
|Top end ring||3500kg|
|Total weight||40000kgs (40 metric tonnes)|
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
Associated drawings: TB6 210 TB6 290
Associated drawings: TB6 210 TB6 290
Associated drawings: TB6 220
Associated drawings: TB6 227
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
DSGA (Digital Strain Gauge Amplifier)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.
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.
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.
Associated drawings: TB6 260 TB6 700 02
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
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:
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)
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)
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
Associated drawings: TB6 603
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.
Polar Axis and RA drive: TB6 130 TB6 141
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
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
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:
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
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.
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:
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
In practice, there are 5 opto-switches fitted on the polar and declination axes. These being located at:
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
More details can be found in the JKT Operation and Maintenance Manual Vol 1
Associated drawings: TB6 710 BB6 101 BB6 112
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:
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
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
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
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
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