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.
The tube is of open construction with the optical components (primary and secondary mirrors) supported at the extremities of the upper and lower Serrurier trusses. These attach to the top end ring and the massive centre box section. The top end ring supports the secondary mirror assembly with its focus drive or the prime focus turntable.
When observing at the f/3.29 Prime focus, the secondary mirror assembly is removed and the Prime Focus Cone Unit (PFCU) with Wide Field Camera (WFC) fitted.
Fitted to the centre box section are the trunnions which form the declination axis. These are supported on gimbal mounted bearings on top of the fork tynes. This complete assembly is mounted onto the polar axis disc. The base section housing the polar disc bearing is mounted on a spherical load point at the north end of the north/south centre line. The polar axis disc is supported by hydrostatic pads.
At the bottom of the lower Serrurier trusses, the mirror cell is attached to the tube by four high tensile bolts. The mirror cell contains the 2.5m primary mirror along with the mirror support system and associated electronics. Below the mirror cell is the Cassegrain instrument rotator and cable wrap.
The Cassegrain instrumentation with electronics racks is fitted below the rotator. The normal configuration consisting of the Acquisition and Guider box (A&G box), the Intermediate Dispersion Spectrograph (IDS) and the Faint Object Spectrograph (FOS) n.b. FOS now de-commissioned. These can be removed and a specialist instrument fitted if necessary. The A&G box is seldom removed.
The electronics for controlling the telescope is housed in racks (bays) in an air conditioned room on the second floor at the west side of the building known as the CLIP centre (Control, Logic, Interlock and Power). The telescope control computer, a DEC ALPHA is located here as is the ING Time Service, Data Acquisition System computers (UDAS) and some of the older instrument controllers e.g. The PFCU MMS and integrating TV system (GRINNELL).
For a detailed description of the INT refer to: The INT Operation and Maintenance Manual Vol. 1
The total weight supported by the hydrostatic bearings (the Polar axis
disc), is made up of the individual weights of each telescope component.
|Secondary mirror assembly||1000kg|
|Prime Focus Cone Unit||1000kg|
|Top end ring||9000kg|
|Centre box section||25000kg|
|Rotator and cable wrap||6000kg|
|Acquisition Guide Box||1250kg|
|Total weight||~ 90 metric tonnes|
Associated drawings: TB1 201 TB1 286
A 6A stepper motor driving a lead screw through a slipping clutch and gear train adjusts the focus position. Limit switches control the overall range of movement. The motor is controlled by a stepper motor drive board in the CLIP centre. If a limit is reached when the telescope is in Computer Mode, Engineering Mode will be returned and it will be necessary to drive the mechanism in the opposite direction to clear the limit before computer control can be returned. The position of the secondary mirror (or prime focus camera) is measured using absolute and incremental piston transducers
Originally the secondary mirror had a double petal mirror cover system, but this proved troublesome and was removed many years ago, however, the motors are still fitted to the tube.
Associated drawings: TB1 291
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 trusses are tubular steel sections with spade ends which are attached to the centre box section with brackets mounted on the top end ring. A cable duct runs along one of the truss tubes carrying cables, fibre optics and compressed air to the prime focus connector panel.
Associated drawings: TB1 201
Associated drawings: TB1 220
The mirror covers are controlled from two push buttons mounted on the services panel of the Engineering rack in the control room. Limit switches operating relays sense the OPEN and CLOSED condition. Indicator lamps within the push buttons show when the petals are fully open or closed. The power is then removed from the motors.
Mirror status is also sent to a CAMAC PR2402 input register (Address: B4 C3 N12 A1 - Bits 6 and 7) .
The contactors for driving the mirror cover motors and the mirror cover relay control circuit board are now located in the middle section of the Mains Distribution Cabinet in the CLIP centre.
n.b. This board is a new fabrication which ONLY includes the primary mirror cover relays. As the secondary mirror cover has been removed, the relays haven't been included. However, should there be a problem, the old relay board will work in this position.
Associated drawings: TB1 260 ED020
The mirror cell floor has a machined surface where 36 pneumatic pads; known as Belloframs, are fitted in three concentric rings for supporting the mirror. The belloframs are divided into 3 sectors of 120o with each sector being controlled by a JOUCOMATIC servo control valve. These valves control the air pressure (input supply = 100psi) to the twelve belloframs in each sector ensuring that the mirror alignment to the optical axis stays correct.
Three load cells (axial definers) measure the compressive/tensive 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:
These are used to maintain the correct alignment of the mirror to the optical axis of the telescope at any attitude and thus set the working height for the mirror.
Adjacent to each axial definer unit are dial gauges which are used to measure the height of the mirror within the cell. It is necessary to read these before and after mirror re-aluminisation to ensure that the mirror has returned to its correct operating position.
Associated drawings: TB1 222 TB1 227
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. These are mounted at the NE, NW and S positions.
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.
Associated drawings: TB1 221 TB1 223
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.
n.b. When the new mirror support system was installed, the two boards in Card Crate 6:
The rear face of the mirror cell provides the mounting for both the Cassegrain turntable and it's cable wrap. The turntable is in the form of a ring supported on a 1219mm diameter wire race slewing ring and is driven by a single motor/gearbox unit with an anti-backlash drive. The pinion engages with a spur gear cut into the circumference of the table.
The motor is a DC torque motor type: G19M4 fitted with a tacho-generator to provide velocity feedback and a solenoid operated brake. An idler shaft with a pair of pinions mesh with the turntable spur gear and drive a 36000 count 5 digit BCD absolute encoder giving a possible 360o position indication. A limit switch limits the travel of the table to +/- 200o . This constraint is due to the parameters of the cable wrap.
Associated drawings: TB1 272
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.
The electronic cubicles for the Cassegrain instruments are mounted on the underside of the cable wrap ring.
Associated drawings: TB1 274
Associated drawings: TB1 120 TB1 130
Two types of accumulators are used. One maintains a set oil pressure in the bellows pads and also serves as a buffer. This effectively takes out pulsations produced by the oil pump which could be transmitted through to the telescope. The larger accumulator is connected to the main feed to maintain oil pressure to the polar disc for several minutes in the event of oil pressure failure. This allows the telescope to come safely to rest.
The oil pump is switched on from the engineering control rack, but can be disabled by changeover switches in the oil plant room on the ground floor.
On each pad there are a series of 4 microswitches. When the hydraulic plant is running, these sense when the polar disc is raised off the pads. n.b. In practice, due to hysterisis in the microswitches, these are very difficult to adjust and its not usual to see several oil pad alarm lights illuminated on the control desk even when the working oil pressure has been reached. This is not a problem
The important alarm light is oil pressure. Failure of the oil pressure switch contacts not closing will cause a break in the alarm and interlock chain and will prevent the telescope being driven in HA both in computer and engineering mode. However, for engineering purposes, the telescope can be moved in DEC quick and slow motion without the oil pump running.
Associated drawings: TB1 180 to TB1 189
Fine balancing of the CASS turntable is achieved by loading weight plates
into slots below the mirror cell or with weights added or removed from
racks mounted on the cable wrap ring.
Cables that don't pass through the elephant trunks serve the HA (RA) drive of the telescope:
n.b. Cables that go to the control desk and engineering rack come back up from the CLIP centre. There is very little in the way of cabling coming from the telescope that terminates in the control room. Just some of the groups of fibre optic cables and a couple of alpha cables used for RS232 links. These can be found under the raised floor below the observers working area at the west end of the control room.
Associated drawings: TB1 120 TB1 141 TB1 146
TB1 153 TB1 155 TB1 156 TB1 157 TB1 220
When the telescope is tracking, the HA QM motor provides an anti-backlash torque to keep the worm and wheel in constant mesh by applying a force in the opposite direction to the rotation of the wormwheel.
On the DEC axis, the QM motor is powered on to maintain a constant load (anti-compliance torque) to the DEC SM tangent arm.
Associated drawings: TB1 143 TB1 153
There are limit switches and slotted opto switches (operated by a metal strip mounted on the anti-backlash nut) that determine the position of the tangent arm. These being:
If a final limit is reached in computer mode, engineering mode will be forced. It will then be necessary to turn the LIMIT keyswitch to the O/R (override) position and drive the telescope out of the limit manually. Once the limits are cleared, the DEC SM drive will automatically re-centre itself.
n.b. It should be noted that once the demanded position is reached, the DEC QM and SM drives to all intents and purposes are held stationary to keep the DEC axis rigid. This is done by keeping the motors powered up. The only time the DEC SM drive operates is when the telescope is tracking and autoguider corrections are needed.
A hollow shaft Baldwin incremental encoder is mounted on the lead-screw shaft. This sends positional data back to the TCS via CAMAC. However, since the DEC axis was fitted with a Heidenhain tape encoder, the Baldwin is no longer read. See: Heidenhain encoder
Associated drawings: TB1 141-02 TB1 154 TB1 155
Once the demanded position is obtained and the SM clamps are applied, the HA SM motor turns the wormwheel at sidereal tracking rate to keep the object under study on the focal plane. Autoguider corrections are sent to the HA servo by either speeding up or slowing down the SM motor.
HA limits are fitted each side approximately 100o from the mid position. The pre-limit switches operate 7o before the final limits are reached. The telescope should NEVER be driven pass the pre-limits.
Associated drawings: TB1 141 TB1 141-02
n.b. To get access to the DEC clamp motors or microswitches, the large cover needs to be removed. Mechanical support staff should be called as the cover is heavy and requires the use of the dome crane.
The HA quick and slow motion clamps are located in the polar axis disc and requires removing cover plates to get access to them.
Two groups of shoes fitted with friction pads, each driven by an endless chain and 3 phase motors make up the QM and SM clamp assemblies. The clamps are spring loaded and work off a cam driven by the chain. The chain is always driven in the same direction. Microswitches operated by a seperate cam determine whether the clamps are engaged or are free.
The QM clamps are for engineering use only and are always engaged. These are controlled by the CLAMP keyswitch on the engineering rack. This puts the telescope in AXIS FREE mode so that it can be moved by hand for maintenance. The axis free indicator will illuminate if the QM clamps are released. The telescope MUST BE IN BALANCE before the QM clamps are operated.
The SM clamps will be dis-engaged when the telescope is fast slewing and engaged when the telescope is at the demanded position and is tracking. Sending the telescope to zenith park will also engage the SM clamps when the telescope has stopped.
Before the SM clamps are engaged or dis-engaged, a series of conditions needs to be meet. Logic circuits with relays fitted in the CLAMP LOGIC tray in Bay 2 in the CLIP centre control the clamp action. See the Clamp Logic for more information.
The 3 phase contactors for both the DEC and HA QM and SM clamp motors can be found in the middle section of the Mains Distribution Cabinet.
Associated drawings: TB1 144 TB1 156 TB1 146-02
Other systems read or written to the TCS via CAMAC include the time
service, dome drive and position, focus and turntable control. Autoguider
corrections are processed using a VME system connected directly to the
DEC ALPHA computer. A engineering (manual) mode is also available
for driving the telescope, dome and other systems.
RA: EC3 DEC: EC5 (in Card Crate 4).
The error (or difference) voltage generated from the comparison is then taken to the QM power amplifiers. The tacho voltage backs off the input demand until the required velocity is achieved. The tacho signal is also used to disable the SM clamps from being engaged until the telescope has come to rest.
The demand signal after buffering is passed through an active notch filter which is tuned to the computer loop sampling frequency of 20Hz. This prevents mechanical resonance of the telescope. It is then amplified and compared to the tacho signal in the SM pre-amplifier boards. These being:
(RA: EC1 DEC: EC4 (in Card Crate 4)
The error signal generated from the comparison is taken to the SM power amplifiers. The tacho voltage backs off the input demand until the required velocity is achieved. As the operating conditions of speed and load on the worm/wheel encompasses an area in which slip-stick may be anticipated, an additional dither signal is introduced into the servo.
There is only one difference between the HA and DEC slow motion pre-amplifiers. This is the facility to centre the DEC slow motion drive.. This is achieved by micro switches which are connected to the clamp logic electronics. According to whether the lead screw is left or right of centre, a + or - voltage is applied to the DEC SM pre-amplifier which will drive the lead screw until the DEC CENTRED switched is activated.
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 the HA and DEC encoder values and computed the velocity demand value for the DAC. Other tasks such as moving the dome to compensate for telescope position 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 SM clamps on both axes are released and 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.
This is achieved by the TCS reading (at 20Hz) the Ferranti encoder (HA) and Heidenhain encoder (DEC) during fast slewing until the error between actual and demanded has reached a low level (minutes of arc). When this is achieved, the TCS will switch from QM to SM drive. The SM clamps are engaged and the Baldwin encoder (now HA axis only) brought into the calculation to increase the positional accuracy to within seconds of arc.
n.b. It should be noted that the HA and DEC demand rates are chosen so that when the telescope axes are driven in QM, they arrive close to the demanded position at the same time. The change over from QM to SM drive in HA and DEC always happens simultaneously.
Computer generated levels are sent to a CAMAC output driver module (Address: B4 C3 N15 A0 ) to engage or dis-engage the SM clamps. These signals are sent to the CLAMP LOGIC circuits which in conjunction with the state of relays in the ALARM and INTERLOCK crate decide whether the SM clamps can be operated safety.
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.
The tacho signal is used to maintain the velocity and to switch in the
QM motors which act as anti-backlash devices when the demanded position
has been reached. These motors work by applying a light force in
the opposite direction to which the telescope is tracking. This occurs
when the SM clamps have been re-engaged and the demanded position reached.
The TCS Info Display will then indicate that the telescope is tracking.
Moire Fringes are formed when a section of the grating known as the index is superimposed on a scale grating of identical structure, but with two sets of lines at a slight angle. A collimated beam produced from a IR emitting GaAs chip at the focus of the lens projects the index line structure onto the scale. Fine adjustments of the angle between the index and the scale gratings enables the phase of the signals to be set accurately to 90o . Any distortion in the grating or curvature of the tape will cause the phase to deviate from the quadrature condition.
There are three reading heads mounted 120o around the tape to compensate for disc (tape) ovality. Each reading head produces two sinusoidal signals with a 90o phase shift between them. Depending on which one is leading the other by 90o, this will be the lead pulse, the other the lag pulse.
The signals from each head are sent to the Ferranti processing modules in the CLIP centre. Here they are converted into two TTL pulse trains, but still retaining their phase shift. The lead pulse will cause the modules to count up, the lag pulse to count down.
The pulses from the three processing modules are then sent to individual CAMAC RGO32Bit up/down counters (Address: B4 C2 N 2-5-8) where the serial data is converted to a parallel binary up/down value which the TCS can read.
Note: Fitting this system was a success, so good in fact that the DEC Baldwin encoder is no longer read by the TCS. The reason being that encoding directly from the DEC axis removes the error that was always present due to the Baldwin being fitted to the slow motion mechanism. It was planned to replace the HA Ferranti tape also, but this was never done due to cost!
The Heidenhain system uses two heads to reduce error due to ovality in the tape. The output from the heads are TTL quadrature signals that go directly to two CAMAC RGO32BIT up/down counters (Address: B4 C2 N 11-14)
IMPORTANT: The HA axis will NEVER get into tracking mode as the position error generated will be too high without a working Baldwin encoder, but this is NOT the case for the DEC axis which uses the Heidenhain tape encoder and does not require the Baldwin.
The Baldwin optical incremental encoder has two quadrature outputs of 12000 counts per revolution. There is also a 1 pulse per rev signal generated. The output signals go first to line driver boxes mounted close to the encoder and then on to CAMAC RGO32Bit up/down counters (Address: B4 C3 N 3-6).
These modules multiply the number of pulses by a factor of four making the total number per revolution to 48000 counts. One Baldwin bit is equivalent to 0.01875 arc seconds in RA and 0.01 arc seconds in DEC.
Note: The Baldwin encoders have long been replaced with
versions that use an LED for a light source. So the ominous task of replacing
encoder lamps is no longer neccessary!
Oil and grime on the tapes can also cause problems. Before suspecting a faulty Ferranti or Heidenhain encoder reading head, the tape should be cleaned.
The Ferranti grating tape comprises a steel tape wrapped around the
edge of the polar disc. It is recommended that this tape is cleaned
every 2 years, due to dust and grease accumulating and therfore the
read out heads loosing pulses. There are 3 read out heads seperated a
120 degrees. To get access to the tape , the read out head has to
be removed, undoing the nut on the spring loaded bolt. Take th bolt
away. The head can now be removed, by pushing the unit agains the 2
spring loaded bolts on the front and tilting the unit of the track. The
positioning won't be lost, as this is defined by the metal bar.
You now have access to the tape for cleaning and inspection. You
need a second person in the control room to move the telescope while
cleaning the tape with propanol. The telescope has enough movement both
ways to clean the tape, first removing the head mounted at 300 degrees,
and then repeat the procedure after removing the head at 60 degrees.
This will avoid removing the head mounted at 180 degrees, to which the
access is much more difficult.
In the case of HA, (or if tracking cannot be achieved) a faulty
Baldwin encoder or its RGO32Bit CAMAC module could be the problem. n.b.
As the Baldwin encoders now use an LED light source, it is more likely
to be a faulty RGO32Bit counter than the encoder itself.
To zeroset (clear) all the encoder RGO32bit counters at startup; in the case of the Baldwins, this pulse is logically ANDED in the line driver boxes with the signals: lead, lag and 1pulse per rev. The datum pulse is also used to clear the registers of the Ferranti and Heidenhain RGO32bit counters this being applied to the zero ref input of the module.
Refer to (Drawing: ED079) which shows the general arrangement. The zeroset module setup is critical to set up. Refer to the technical folder on the zeroset module if it needs to be re-calibrated.
Associated drawings: BB1 006
Alarm status is also read by a CAMAC input register (address: B4 C3 N13 A0) and is shown on page 3 of the TCS Info Display window.
In the case of the telescope moving into a limit switch, the spring biased LIMIT keyswitch needs to be turned to O/R (override) and held whilst the telescope is driven out from the limit. Once the alarm is cleared, the keyswitch can be released and normal operation resumed.
The HORIZON LIMITS are a special case and stop the telescope (at any attitude) if the tube is getting close to the horizon (about 5o). When moving the telescope to ACCESS PARK, the horizon alarm will operate. As the telescope needs to move downwards further to gain access to the WFC when working at prime focus, operating the LIMIT keyswitch and re-selecting QM DEC will allow the telescope to move down to its final limit. When doing top end instrument changes, a clamp on the dome balcony operated by a hand wheel locks the tube ring when the telescope is in an out-of-balance condition.
The alarms below are fleeting alarms which simply sound an audible warning and illuminate an indicator. These do not trip the servo power amplifiers. Pushing the ACCEPT button mutes the alarm.
Note: Although there is a tie bar alarm relay, the contacts in the interlock chain have been shorted across for reasons unknown (at least to me!). In engineering mode, it is possible to move the telescope in DEC without the oil pump running and the tie prop inserted.
Do NOT attempt to move the telescope when tied. Serious mechanical damage may occur and the power amplifiers can burn out due to excessive currents being drawn.
Associated drawings: TB1 541 BB1 124 to BB1 130
Signals from the engineering rack push buttons and relays, HA and DEC SM clamp micro-switches, DEC centralising micro-switches and CAMAC are opto-isolated on Board 1 (Drawing: ED039) before being presented to the clamp logic circuit.
The clamp logic circuit on Board 2 (Drawing: ED040) consists of a series of TTL gates which read the state of the switches, push buttons and relays. Decisions are made according to truth tables whether to release or engage the SM clamps. Other signals are examined to check if the switch-over between the QM and SM pre-amplifiers can take place. Once SM drive is selected, a torque loop is engaged with a signal being sent to the QM pre-amplifiers. The DEC centralising logic is also contained on this board as is the logic to be able to select computer mode.
Board 3 (Drawing: ED038) contains the power transistor relay drivers and relays which operate the 3 phase SM clamp motor contactors. n.b. It should be noted that the contactors on the tray are obsolete. The clamp contactors are now located in the middle section of the Mains Distribution Cabinet and marked as HA and DEC SM clamps. The EMERGENCY STOP, power amplifier RESET and associated relays are also found on this board or the plug in relays within the tray. A break in the alarm chain or the Emergency stop pushed will cause the 50V supply to be removed from the power amplifiers.
When in computer mode, clamp control is passed to a CAMAC OD2407 output driver (Address: B4 C3 N15 A0). Bits used are:
A recent addition was a small board with a relay to enable the TCS to switch from computer to engineering mode. The relay is operated from a CAMAC output driver (Address : B4 C3 N15 A1). A new drawing: ED062A shows the switching arrangement and link through connections.
Refer to: INT Technical Manual 1 for more information on the Clamp logic tray and truth tables.
Associated drawings: TB1 550 BB1 302 BB1 303
On the SM pre-amplifiers, the demand signal after buffering is passed through an active notch filter which is tuned to the computer loop sampling frequency of 20Hz. This prevents mechanical resonance of the telescope. It is then amplified and compared to the tacho signal. The error signal generated from the comparison is taken to the SM power amplifiers. The tacho voltage will back off the input demand until the required velocity is achieved.
As the operating conditions of speed and load on the worm/wheel encompasses an area in which slip-stick may be anticipated, an additional dither signal is introduced into the servo. This signal is generated used an oscillator constructed from CMOS gates (IC5)
The HA and DEC QM pre-amplifiers contain a speed interlock circuit (IC4) driven from the tacho signal. The output drives a relay with contacts in series with the contactor coil and its switching transistor. This prevents the SM clamps being applied if the QM speed is too great.
Associated drawings: BB1 120 BB1 121 BB1 123
The input power to the amplifiers is derived from full wave rectified; three phase transformers with anti-surge circuits mounted in the bottom of two of the bays. These PSU's connect to the amplifier trays 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 Lucar crimp tag terminal strip. n.b. All amplifiers (including the ones used in the JKT) are compatible with each other and can be interchanged between the INT and JKT telescopes and the positions they occupy.
See: INT (JKT) INLAND power amplifiers for more details and how to change a faulty unit.
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 need to be matched for gain so that the current passing through them is evenly distributed.
A bi-metal strip 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 engineering rack. Each power amplifier has 3 indicator lamps on the front panel. These are:
The Cass turntable pre-amplifier board EC6 (Drawing: ED048) is located in Card Crate 4 along with the RA and DEC servo pre-amplifiers. It enables the turntable to be driven in a +/- direction and at SLOW or FAST speeds, either manually with push buttons in engineering mode or via a CAMAC output driver (Address: B4 C3 N11 A0) when under computer control.
Turntable position is measured using a 36,000 count BCD encoder (Drawing: ED050). An 18 bit line driver box mounted near the encoder carries the encoder data to the CLIP centre (Drawing: ED049). A line receiver/driver board (Drawing: ED018) with the aid of a decoder converts the BCD data to degrees and minutes of arc before passing the signals to a CAMAC input register (Address: B4 C3 N10 A1) that the * TCS can read. The outputs from this board are also taken to an LED display on the engineering rack (Drawing: ED016).
The conversion is done using a PROM DECODER (Drawing: ED015 sheets 1/2) which contains a look up table in firmware. Both the Cass TT line receiver and prom decoder boards are located in Card Crate 6 (Bay 3).
Historical note: Originally a Beckmann display for the Cass TT was available on the control desk, but when the TCS moved over to DEC ALPHA computer, this display no longer works. So ignore drawing: ED017. At one time, a Cass TT handset with LED position readout could be plugged into a mirror cell connector, but the cable got ripped out of its connector many years and we never managed to work out how it was connected? i.e. The Grubb Parsons drawings for this were wrong!
* For reasons unknown, the TCS after converting the BCD values to binary has to invert the top 3 bytes (whole degree part) to make sense out of the data.
A block diagram showing the Cass TT layout can be found in (Drawing:
Associated drawings: TB1 457 TB1 490 BB1 004
BB1 005 BB1 131 BB1 301
The circuit diagram is kept with the MMS circuits in Document Folder 15 in the INT electronics workshop.
The control board drives the motor at two speeds: FAST or SLOW. There are three input lines for direction, speed and power. The latter is used to de-energise the motor coils to save power when motion ceases. Focus control can be either via the push buttons; SLOW and FAST, + or - DIRECTION on the engineering rack or via a CAMAC output driver module, (Address: B4 C3 N11 A1) when computer mode is selected.
UPPER and LOWER limit switches protect the mechanism and if active will force engineering mode if computer mode was enabled. The mechanism needs driving out from a limit switch manually before computer control can be re-enabled. Limit switch status and focal station selected: PRIME or CASS (this is determined by internal wire links which are linked differently within the CASS and PRIME focus motor cable connectors) can also be read by CAMAC, (Address: B4 C3 N12 A1).
Associated drawings: BB1 312 BB1 313 BB1 315 ED076
The E725 transducer signal processor and a junction box are mounted in a large box near the top of the prime focus connector panel. A cable with a 9-way D-conn. carry the signals between the junction box and the transducer.
Manufacturers documentation on the LVDT and the E725 display unit can by clicking on the links.
The output from the E725 unit is RS485 2400bds 8bit NoPar 1-stopBit
protocol. RS485 is used, because the run to the CLIP centre is
excessive. Data is carried down over a 2-wire link.
In the CLIP centre, the data is sent to a CAMAC 3340 comms module (Address: B4 C3 N18). An RS485 line receiver re-converts the data back to RS232 and this unit is plugged directly into the D25 connector on the 3340 module. A small mains plug type PSU powers the line driver.
The calibration procedure for the LVDT can be found here
A suitable Absolute Focus Display has still to be identified to mount into the Engineering rack in the control room.
For this reason, a VT220 display monitor standing on top of the control desk is used to show the raw data from the absolute focus encoder. This VT220 display is connected in parallel with input of the above mentioned CAMAC 3340 comms module.
A single cable connects the transducer to the SONY processing module with LCD display. This is mounted in the side of a larger box (located at the bottom of the prime focus connector panel) to gain access if needed to the programming buttons. The processing module has an RS232 serial I/O configured for: 2400BDS 8N1 protocol. Because the cable run to the CLIP centre is excessive, an RS422 line driver carries the data down a 4 wire link.
Within the large box, a small PSU is fitted which powers both the SONY processing module and the RS422 line driver. As the display gives off a green glow in the dome, a black taped mask is put over this, but can easily be removed if needed to gain access to the programming buttons.
In the CLIP centre, the data is sent to a CAMAC 3340 comms module (Address: B4 C3 N19). An RS422 line receiver plugs directly into the D25 connector on the 3340 module. A small mains plug type PSU powers the line driver.
More information on this unit can be found at: SONY
focus transducer in the Ops-team web pages.
To allow the focus to track due to the expansion and contraction of the Serrurier truss with temperature, five platinium resistance thermometers (Type: PT100) are fitted to the telescope tube, top end ring, cube and against and above the mirror.
The mirror air sensor is fitted below one of the petals. The mirror sensor is mounted on a sprung arm and rests against the top edge of the mirror. A side cover needs to be removed from the mirror cell to gain access to this sensor. All the sensors (with the exception of the mirror) are clamped under a nylon mount to the surface to be monitored. Heat sink compound is spread onto the sensor and surface to improve thermal contact. The temperature sensors are mounted in small diecast boxes.
Located under a side panel within the cube is a box containing the temperature transmitters. These provide an output current drive which is proportional to the resistance of the sensor (100 ohms at 0oC). The outputs from these transmitters go to Board EC8 (Drawing: ED072) fitted with precision 1K resistors in Card Crate 4 in the CLIP centre.
The voltages developed across these resistors are taken to a CAMAC ADC 1232 module (Address: B4 C2 N20) enabling the TCS to process the temperature data and adjust the focus if so required.
The ADC channels are:
Associated drawings: TB1 714 BB1 310
Last edited: Nov 2012 rjp