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The JKT uses a old Thorn autoguider (designed by Dave Thorn at
RGO in the early 80's) based on a image dissecting photomultiplier tube
(PMT). The system consists of three parts. These being:
The autoguider controller
The autoguider head PSU
The autoguider tube
This is housed in the instrument rack in the room at the top of the stairs.
The controller is built using an old 6800 microprocessor evaluation system
called an Exorciser. This consists of a mother board with PSU and
plug in boards. Most of these are the basic microprocessor system components
comprising of the MPU, RAM, ROM (holding the autoguider firmware written
in FORTH) and the serial and parallel I/O boards. Specialised boards are
also installed that process the signals from the autoguider head, generate
the scanning currents, handle the centroiding and output error correction
data that is passed to the TCS over an RS232 serial link. A 3340 communications
module in a CAMAC crate in the CLIP centre (Address: B4 C1 N8) handles
this data. It should be noted that commands sent to the autoguider from
the TCS are also via this module. See JKT
The autoguider controller was originally operated using the group of
push buttons below the small TV monitors in the control desk, but the buttons
have now been replaced with commands sent from the TCS. However, the autoguider
is still switched on/off using one of these buttons. The important TCS
At this point, although corrections are being sent to the TCS, the loop
still needs to be closed. The TCS command: AUTOGUIDE ON
will do this.
FIELD Perform a scan of the autoguider
tube for a guide star
ACQUIRE Move the A&G box X and Y probe (or the telescope)
to centre the star on the PMT photo cathode
GUIDE Perform centroiding on the
guide star and send corrections to the TCS
The autoguider controller is generally very reliable, but there is a
spare chassis available kept in the room behind the rack in case of a break
down. To change controllers, just unplug the connectors on the rear panel,
side out the chassis and replace it with the spare.
For more information, refer to INT Technical manuals: 70,
71 and 72 kept in the INT electronics workshop.
Autoguider Head PSU
This is the blue box mounted on the rear of the telescope electronics rack.
The PSU generates the High Voltage supply for the tube as well as the LT
supplies for the autoguider head pre-amplifier.
There is a light sensitive detector mounted on the box which if operated
will fold-back (or close down) the HV to the tube. The photomultiplier
tube is very expensive and can be permanently damaged through over-illumination.
For this reason, the autoguider controller is interlocked with the dome
lights. Switching on the autoguider (or the TV) will kill the main lights
in the dome. n.b. Even moonlight flooding into the dome can effect
the autoguider and cut back tube sensitivity due to the HV supply being
folded back to prevent damage.
The head PSU is very reliable, but there are plenty of spares units
available since the Thorn autoguiders were decommissioned on the INT several
The tube is mounted on the X-Y slide carriage in the JAG. The connections
to and from the tube; HV, LT, Pulses and X-Y Scan run to a connector panel
mounted on the side of the JAG and use the same style of connectors. If
the tube needs to be removed, slacken off the screws on the clamp ring,
remove the connections from the PMT end cover then unscrew it (CCW) from
The tube is an image dissecting type of photomultiplier. The diameter
at the front end of the tube is reduced to locate the X-Y scanning coil
bobbin. The scanning coil is mounted at the front of the tube casing, so
enabling a tube to be changed if necessary. In practice however, it is
much safer to exchange the complete unit.
The X and Y scanning connections to the coil are made using lemos on
a fibre block on the casing. The tube also contains a head pre-amplifier
mounted under a cover at the rear end of the case. All connections (with
the exception the scanning coil drives) are made at the end cover of the
Things to check out if there
are autoguider problems
Autoguider appears to be working, but the CCD images are elongated?
For the autoguider to work, a link must be made between the TCS (DEC ALPHA)
and ICS (Sparc) computers. The problem could simply be the TEL-D Drama
task has crashed. The solution here is to type: startobssys in
the pink window which will restart the observing system. A missing black
TCS Info Display window on the ICS is also a good indication that the TEL-D
task has crashed.
Another cause could be that autoguider corrections are not getting through
to the TCS. Autoguider corrections are shown on the bottom line of window
2 of the TCS Info Display (NOT the ICS copy). If the data is stationary
and not updating, re-boot the TCS and the autoguider controller (by switching
it off then on again).
If the problem still persists, run down the TCS then change the 3340 CAMAC
module (Address: B4 C1 N8) in the CLIP centre. If you need to change
this module, check that the DIL switches on the replacement are the same
settings as on the original. The telescope will need re-zerosetting
after this has been done.
No guide stars are found in the Autoguider window?
Has the telescope been zeroset and calibrated correctly? It would be sensible
to re-zeroset the telescope at least and check that a known bright star
can be acquired in the TV camera.
Has the JAG autoguider X-Y probe lost position? Refer to Taming
the JAG for information on how to recover from this situation.
A dense autoguider ND or COLOUR filter could be attenuating the light path
or a more common problem, an autoguider filter can lose position and block
the light path totally. If this is suspected, refer to the document: Taming
the JAG for information on how to recover from this situation.
No scan performed in the small window after a FIELD command
Even with no light falling on the tube, it is possible to see a change
in intensity during scanning within the small window at the top of the
autoguider display. It runs from left to right horizontally and vertically
down the window. If scanning occurs only in one direction, suspect a broken
cable connection on the lemos going to the JAG autoguider connector panel.
This has occured several times in the past.
IMPORTANT: The polarity of the X/Y scanning is critical.
If the connections are reversed, the autoguider will "anti-guide"!
Looks like the autoguider is scanning, but no build up of light pixels
This list is by no means extensive, but covers most of the problems I have
encountered over the years. ejm
A broken (or intermittent) connection within the TNC co-axial connector
which goes to the SIG (pulses) socket on the JAG connector panel or the
autoguider signal cable which runs between the telescope connector panel
(N) and the head PSU.
A faulty head pre-amp within the tube casing or the head PSU.
No HV supply getting to the tube. Check the HV co-axial cable going to
the JAG and the head PSU.
Last edited: Sept 2002 ejm