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:

  1. The autoguider controller
  2. The autoguider head PSU
  3. 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:

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

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