JKT TV system
The JKT TV system comprises of four units. These being:
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Westinghouse TV camera with image intensifier
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Westinghouse camera controller crate
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GRINNELL frame store
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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:
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Direct mode: The TV image displayed comes directly from the
camera without video processing.
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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. The number of frame integrations is selectable as are the
contrast and black levels to get the best resolution.
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:
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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.
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