INT TV system

The INT TV system comprises of five units. These being:

Westinghouse TV camera with image intensifier
Westinghouse camera controller crate
PROBELL video switching crate
GRINNELL frame store
RGO MMS 6800 microprocessor crate

Westinghouse camera

The Westinghouse image intensified camera is mounted inside the CASS A&G box. A mirror can move into the beam at 45^o for DIRECT viewing or using another probe can look at reflected light off the IDS slit jaws for SLIT VIEWING. It is generally used in this mode. There is also a photo-cathode shield which protects the camera when not in use.

The TV camera can be used in two modes:

Direct mode: The TV image displayed comes directly from the camera without video processing.
Integrating or Average (Summing) mode: The TV image is passed to the Grinnell system where successive frames are built up in a frame store memory, thus much fainter objects can be resolved. The number of frame integrations is selectable as are the contrast and black levels to get the best resolution.

On the control desk (bay 4) there is a bank of small monitors. Monitor 3 (CASS) looks at the raw image direct from the TV camera.

If the CASS and DIRECT buttons are pushed, the raw image can also be displayed on the large TV monitor (in bay 3). 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.

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 below the small monitor switches the camera on/off; controls the gain and has the facility for reversing the field scanning. For the INT CASS TV, these switches are always left in the UP position.

Westinghouse TV controller

This 3U crate is located in bay 8 in the CLIP centre. 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 CASS TV camera is changed, it will be necessary to do this. Instructions are as follows:

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.
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).
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
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.

PROBEL video switch

This is simply a video switching matrix to select between different video input sources (cameras) and various monitors. The video matrix selection is done using the two banks of switches on the control desk (bay 3). The PROBELL switching crate is located at the top of bay 7 in the CLIP centre.

Due to the decline of the use of TV monitors in the INT it is seldom used now, but is still functional.

Full details are in: INT Technical manuals: 37 38 39

GRINNELL frame store

This large crate is located at the bottom of bay 8 in the CLIP centre. 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 crate.

A full description of the Grinnell frame store is beyond the scope of the document, but information is available in: INT Technical manual: 61

RGO MMS 6800 microprocessor crate

This is located at top of bay 8 (rear) and controls the various functions of the Grinnell frame store. It also generates the menu and overlays that are displayed on the Grinnell TV monitor. The bank of push buttons on the control desk 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 can be found in the folder: Phase 1 MMS TV information

Last edited: Aug 2002 ejm