JKT TV system

The JKT TV system comprises of four units. These being:

  1. Westinghouse TV camera with image intensifier
  2. Westinghouse camera controller crate
  3. GRINNELL frame store
  4. RGO MMS 6800 microprocessor crate

Westinghouse camera

The Westinghouse image intensified camera is mounted on the JAG. A mirror which can move into the beam at 45o directs light into the camera. There is also a photo-cathode shield (TV shutter) which protects the camera when not in use.

The TV camera can be used in two modes:

On the control desk there are two small monitors. The left hand screen looks at the raw image direct from the TV camera (the right hand monitor is for the autoguider).

If the DIRECT button is pushed, the raw image can also be displayed on the large TV monitor. Pushing the INTEGRATE or AVERAGE (SUM) buttons will pass the image to the Grinnell frame store for processing. The depth of the image is dependant on the number of frames selected for integration.

Generally, the TV camera is only used these days to acquire stars when zerosetting and calibrating the telescope at the beginning of the night. It is also occasionally used to check targets for the CCD camera when using finding charts.

The camera is very expensive and the imaging tubes are no longer available. For this reason, great care must be taken. Many of the ING cameras have burnt photo cathodes due to being left switched on with the gain turned up or forgetting to turn the gain down when the telescope is slewing across the sky. To protect the TV camera from gross illumination, the TV controller is interlocked with the dome lights. Switching on the TV will kill the main lights in the dome.

A remote TV control box mounted in the control desk switches the camera on/off; controls the gain and has the facility for reversing the field scanning. This is useful as the observer can change the N-S or E- W aspect of the sky field when comparing the TV image to finding charts.

Westinghouse TV controller

This 3U crate is located in the instrument rack in the JKT control room annex at the top of the stairs. It contains the circuits to drive the camera. An important adjustment that needs doing occasionally is the electronic focus. No matter how accurately the camera is focused on the telescope, if the image is not in focus electronically, it will never be sharp. If the JAG TV camera is changed, it will be necessary to do this. Instructions are as follows:
  1. Drop the front panel and locate the TV Focus Current board and the Focus Current Adjust preset potentiometer. The board is the third one up from the right.
  2. Turn on the camera and check that the dome lights go out. After a few seconds the red DELAY led will extinguish. Turn back the GAIN control fully CCW to turn off the RESET led (if on).
  3. Slowly bring up the gain until pixel activity (scintillation) is seen in the middle of the TV monitor. What you are looking for are small perfectly focused pixel events.
  4. Using a trimming tool, turn the Focus Current pot +/- a small amount to achieve the best pixel focus. n.b. This job is made much easier if a second person can look at the monitor and tell the engineer when the best pixel focus has been obtained.
As the Westinghouse cameras are being phased out from the WHT, more controllers and camera heads are becoming available as spares.

GRINNELL frame store

This large crate is located at the bottom of the instrument rack. Its purpose is to build up an image over a selected number of TV frames when the TV camera is used in INTEGRATING or AVERAGE mode. Using these modes, the TV can resolve much fainter targets than when working in DIRECT mode.

A video connection is made to the Grinnell frame store from the Westinghouse controller and control over the frame store functions are through parallel I/O data connections coming from the RGO MMS 6800 microprocessor chassis mounted above it.

A full description of the Grinnell is beyond the scope of the document, but information is available in the JKT technical manuals.

RGO MMS 6800 microprocessor crate

This controls the various functions of the Grinnell frame store and also generates the menu and overlays that are displayed on the Grinnell TV monitor. The bank of push buttons associated with the Grinnell system; Integration frame number selection and contrast/black level controls are connected to this unit.

At times, the Grinnell TV monitor can get corrupted data. If this occurs, switch off both the MMS controller and the Grinnell frame store, wait a few seconds then switch them on again. This usually cures most problems.

Problems with the Integrating TV system are fortunately rare, but can be very difficult to fix (an understatement)! The Grinnell and MMS system are heavily interactive and rely on each other to function.

A full description of the system and the Grinnell frame store can be found in: JKT Technical manuals:  20 and 21

Last edited: June 2002  ejm