Check the following:
  1. Is there Telescope Power available?
  2. Have the ALT and AZ power amp breakers been switched on (or tripped out)? (Marconi amplifier rack... Bay 6)
  3. Are both the Bearing and Gearbox oil pumps running? (engineering console)
  4. Do all the alarm lamps associated with the oil system extinguish? (engineering console) n.b. Push the LAMP TEST button to check for burnt out panel lamps. Alarm indicators may appear to be extinguished simply due to a non working lamp!
  5. When pushing the ALT or AZ +/- buttons and slowly turning the variable speed controls clockwise, are currents being shown on the panel meters? Normally a relay can be heard when the buttons are pushed. If no current is seen on the panel meters, suspect a faulty Servo Power amplifier. If this is not the case, there may be a problem with one of the Marconi servo modules which will need checking out (or changing).
  6. Check also that the +5v and +24v power supplies are available to the Marconi servo rack (Bay 5). There are two circuit breakers/switches which may have tripped? The same applies to the Nasmyth rotator servos which have their own PSU in Bay 4.


If the telescope checks out ok in ENGINEERING MODE

  1. Is the TCS running?
  2. Check the CAMAC crates for activity by looking at the LED's on the bus monitor modules (e.g. DTM4 or similar). There is one of these installed in each crate.
  3. Are all alarms clear? A Nasmyth gate not closing properly or the main door to the GHRIL enclosure open can drop the telescope out of computer mode.
  4. Have any of the turntable limit alarms been activated? The telescope will not go into Computer Mode if for example the Cass turntable has moved into a limit.

  5. n.b. If the telescope is working at the Cass or Nasmyth focus, it is normal for the prime focus turntable +/- limit alarms to be on.


This can be due to many problems. Some examples are listed below:
  1. Is there anything PHYSICALLY preventing the telescope from moving freely? This may sound ridiculous, but some years ago, a metal framed CHAIR was jammed between the UES platform and the dome balcony and was being dragged around by the telescope!
  2. Check the ENCODER page on the Info display window and ensure that both the incremental and absolute encoders for each axis are in close agreement. Missing bits caused by a faulty CAMAC module reading an encoder may be the problem. If an absolute encoder is jumping by large steps, suspect a blown encoder lamp.
  3. Check the currents for the ALT and AZ motors and the Cass (or PFTT) rotator when the telescope is tracking. These should be relatively stable. Expect:
  4. Altitude:  Motor 1  -2.5A Motor 2 +2.5A  (+/- 0.5A)
    Azimuth :  Motor 1  -4.5A Motor 2 +4.5A  (+/- 0.5A)
  5. A slow build-up followed by a rapid fall-off could be due to an out of balance condition either with the altitude axis or the instrument rotator or that something is causing the system to drag. An instrument cable picked up by the Cass turntable driving mechanism for example.
  6. Another related problem could be the Cass Cable Wrap. This is driven by it's own servo and is separate from the main Cass turntable. Check to see that it is moving freely and that the PENNY GILES linear potentiometers are not impaired by a `floating' instrument cable etc.

    Cass turntable  : +/- 3A (+/- 0.5A)
    Cass cable wrap : +/- 1A
    If the Cass cable wrap is drawing heavy currents, it may be that a cable or hose within the wrap has become unclamped and is being chewed up by the chain. Since this was modified with rollers and separators some years the problem has almost disappeared, but cables can still get damaged or form a `hernia' by getting forced out of the cable wrap.

  7. Check the mirror support LOAD CELL meters. Is there any oscillation when the telescope is tracking? The Mirror servo although totally separate from the main telescope drive system can introduce problems that appear to be tracking errors.


Occasionally the lamps within the altitude or azimuth absolute encoders fail. Although most of the encoders in use at the ING that use filament lamps have been replaced, we are still stuck with this problem with the coarse and fine ALT/AZ absolute encoders in the WHT.

Before the zeroset option was available, this was a serious problem as the telescope would loose it's position on start-up or during the night and would necessitate a call out for the duty engineer!

If a lamp fails during the night, the telescope operator can now use the ZEROSET TARGET AZ (or ALT) command to set the telescope to a known position and continue observing. The replacement of an encoder lamp is now considered to be a day time job.

To replace a Baldwin Encoder lamp can take at least one hour. In the case of the azimuth encoder, working in a hazardous area makes the job more difficult.

The azimuth fine/coarse absolute encoders and synchros are located within a cylindrical protection cover on the right-hand side of the encoder cradle which itself is situated between the two motor cradles. Access to this area is via a wooden staircase that leads upto the mid-level of the cable twister. One needs to crouch under the ring girder azimuth bearing to gain access to the drive gear. This area is particularly messy due to oil from the azimuth bearing spreading onto the floor grids. Great care must be taken to avoid slipping over and injuring oneself! Put on a boiler suit and take a good supply of cardboard for sitting on. There is a short ladder in the area for climbing up to the drive gear.
The altitude encoders are mounted on the right-hand side of the motor and are enclosed within an identical protection cover. Access to the elevation drive gear is via a swing out ladder mounted on the Drive Side tyne and through a hatch below the Nasmyth platform.


Before disassembly, record the value of the coarse absolute encoder using the CAMTEST program. To run this, proceed as follows. Open up a TCS window on the telescope operator's X-Terminal: Username: engineer
Password: ********     (No way! Look under camtest in WHT CAMAC manual)

After a few seconds, a window will appear and the prompt CAMTESTwill return in the lower window. If you need help, type 'help' at the prompt.

For the altitude encoder, enter in the order B C N A F :

EXEC 6 2 13 0 0 /REPEAT=x`
Where x = the number to repeated cycles

For the azimuth encoder, enter in the order B C N A F :

EXEC 6 2 15 0 0 /REPEAT=x
A hex number will be returned which gives the current values for both the coarse and fine encoders.

e.g. 00293439    In this case: 0029 = the coarse encoder value and 3439 = the fine encoder value.

n.b. If a fine encoder lamp has blown, a random value will be displayed in the four least significant digits which does NOT CHANGE when the telescope is moving. The problem will appear as a significant pointing error and it will be impossible to locate an object on the direct viewing TV display.

Record these values in the logbook and note as accurately as possible, the positions of the fine and course synchro indicators on the engineering control desk. The synchros and course encoder will need to be reset to these values as gaining access to the fine encoder lamp involves disconnecting the shaft linkage between the two encoders.

IMPORTANT Enter in the telescope logbook and on the engineering control desk notice board before you start:



  1. Remove the power cable and encoder cables from the ENCODER LINE DRIVER BOX.
  2. Remove the three bolts securing the protection cover located on the rear of the unit and lift off the cover to reveal the coarse encoder and the coarse and fine synchro's.
  3. Remove the two cable clamps securing the coarse encoder and synchro cables.
  4. Remove the second part of the cover by sliding it off towards you, thus revealing the fine encoder.
  5. To gain access to the fine encoder, you need to remove the coarse encoder and synchro assembly. This is achieved by un-doing the flexible shaft coupling between the two encoders with a 3/32 allen key and removing the six nuts at the bottom of the pillars at the base of the encoder assembly. The gearbox complete with the coarse encoder and synchros can now be removed.
  6. Slacken off the lower grub screw in the flexible shaft coupling so that the assembly is removed, but the coupling stays attached to the gearbox input shaft. If the coupling is left on the fine encoder shaft, it will be impossible to remove the cover to gain access to the lamp.
  7. Remove the cover from the fine encoder and undo the small screw holding in the lamp (don't lose this, there are NO spares).
  8. Disconnect the flying leads to the lamp.
  9. Replace the lamp making sure that no finger contact is made to the glass bulb as this can reduce life expectancy. Be careful to select the correct lamp from the stores. The type number is 55153.
  10. Reconnect the flying leads and plug in the encoder and power cables to the encoder line driver box. Check the lamp illuminates before re-installing. n.b. New lamps have been found to be faulty!
  11. Re-assembly is a reversal of the previous steps. However, before locking the flexible shaft coupling between the two encoders, the coarse encoder and synchros must first be reset to their original positions. To do this, a second person in the control room is needed who can monitor the synchro indicators and run the CAMTEST program (use the 2 way radios) .

  12. Turn the shaft with the flexible coupling in place, (but not tightened) till the synchro indicators on the engineering control desk are back to their original settings.
  13. With the CAMTEST program running in repeat mode, carefully (and very slowly) rotate the flexible shaft coupling until the value for the coarse encoder reads as before. There may be a small amount of hysterisis, so find the point where the value changes above and below the correct value.
  14. Carefully position the shaft to the mid-point and lock the lower grub screw of the coupling.
  15. Re-assemble the protection covers and cable clamps. When finished, move the telescope to a position in Computer Mode and go into the ENCODER INFO page and check that all encoder values both absolute and incremental are in close agreement and up-dating.
Exit from CAMTEST before running the telescope control software.


This will show as a gross error in telescope pointing. Run CAMTEST as previously described. Check the CAMAC value to see if the most significant bits are changing with the telescope moving slowly in Engineering Mode, if not, it's almost certain the COARSE encoder's lamp has blown. Proceed as follows:
  1. Locate the encoder assembly on the appropriate telescope axis and remove the encoder and power leads from the Encoder Line Driver Box.
  2. Un-screw the three bolts and remove the cylindrical protection cover and un-plug the 25 way `D' connector from the coarse encoder.
  3. Remove the cover from the encoder and the lamp locking screw (don't lose this, there are no spares) and un-plug the lamp's flying leads.
  4. Re-connect the leads from the replacement lamp and re-fit the cables to the Encoder Line Driver Box. Check the lamp illuminates before re-installing. n.b. New lamps have been found to be faulty!
  5. Re-assemble in the reverse order. Check the CAMAC encoder value changes when moving the telescope in Engineering Mode. A full check in Computer Mode can then be carried out by verifying that all encoder values are in step when the telescope is tracking, by looking at the ENCODER INFO page.


If the absolute encoding is suspect, the following commands can be issued from the TCS. These being:

Zeroset Absolute

The purpose of these commands is to force the incremental encoder starting point values to assume the absolute encoder's outputs. They should be used when it is noticed that the absolute and incremental encoder values are NOT in good agreement.

Before using these commands, you should check that the ABSOLUTE encoder values are correct. Simple checks like watching the numbers increment (INFO page 1 on the Telescope Info Display monitor) whilst the telescope is moving in Engineering Mode are useful when a missing bit is suspected. Seeing the telescope line-up with the dome shutters is strong evidence that the values are correct.

Zeroset Zenith

These are the commands to use when a problem with the ABSOLUTE encoder is suspected. With the servo systems powered up, switch the telescope to Engineering Mode and push the ZENITH PARK button on the console. The telescope will move automatically to the zenith and rotate in azimuth until the `railway lines' align up with the tracks on the walkway. When in position, the zenith park lamp will illuminate and one or both of the above commands can be entered on the Operator's terminal. If the zenith park button does not operate from the control room, it is possible the keyswitch on the gallery control box has been switched to LOCAL. Either switch to REMOTE or operate the ZP button from the gallery controls.

Zeroset To

These commands are used to set the encoder starting points to an arbitrary value. You would need to know the position of the telescope by some external means before using any of these. The last command is used the set the rotator position. If the telescope is moved in declination and a star appears to keep parallel to the spectrograph slit, a ZEROSET ROT TO 0 or 180 would be appropriate. Similarly, if the star remained parallel to the slit when the telescope was moving only in hour angle (RA), ZEROSET ROT TO 90 or 270 could be used.

Zeroset Target

These are the ONLY commands that use the Zeroset Two Zone Module and target and should be used when an absolute encoder has failed

n.b. This method is identical to the system as used on the INT.

Last updated: 9th Sept 2002  ejm