JKT Autoguider

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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

Autoguider controller

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 CAMAC manual.

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 commands are:

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

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.

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 years ago.

Autoguider Tube

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 its housing.

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 tube casing.

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 is sent?

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 is seen?

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.

This list is by no means extensive, but covers most of the problems I have encountered over the years. ejm

Last edited: Sept 2002 ejm