Camera Control

  The controls for the two Westinghouse TV's , Cassegrain and Finder are located in Bay 4, Rack 1 directly beneath the small TV monitors at the top which all act as raw video displays. The Grinnell monitor itself is located directly underneath the monitors in Bay 4, Rack 2. Its picture can also be routed to a monitor in Bay 1. The selector switches which allow TV pictures from all focal stations and the finder to be sent to the monitor are located directly below, alongside an array of feature-selection buttons which are described in more detail further on. The final piece of hardware in the TV system is a joystick controller box which normally resides on the telescope Control Desk. Figure shows the location of the TV camera controls on the INT Control Desk (bay 4).

Control of the flipmirror as part of the A&G box is made through both the ADAM instrument control software environment and the telescope User Interface, from which Guide and TV probe move requests can be made. The TV flip mirror should be in place when it is necessary to view a new field or when ready to guide. Note the possibility to use colour filters with the camera: the finder filters numbered 1, 2, 3 and 4 are respectively CLEAR, B, V and R.

  
Figure: TV Acquisition Camera controls

The Grinnell Camera is controlled with several groups of switches and push buttons: A group of three rotary switches serve to define the setup of image integration and contrast. The main Grinnell functions are controlled with a 33 block of buttons just above the rotatary switches. Note the facilities to archive the camera image and to obtain a hard copy. A further set of push buttons serve to select functions from a menu which enable on-line image analysis.

The three rotary switches below the panel buttons act as follows:

• INTEGRATION

  The absolute position of this switch determines the number of frames of integration N=2.

• BACKGROUND

  Makes the background blacker or whiter. There is no absolute position, the effect is determined by the direction and number of steps moved. However, after a complete RESET of the TV control System, the background always has the value 2048.

• CONTRAST

  As for the BACKGROUND switch but it controls the contrast i.e. softens or enhances.

The main 33 block of the main Grinnell controls have the following functions, from top to bottom and left to right:

• AVERAGE

  Recursively filters on-target integrations to increase signal-to-noise ratio. The number of on-target integrations (N=2) are selected by the INTEGRATION rotary switch. The filtering is done in the TV system microprocessor; the image is effectively smoothed over 2 frames, where S equals 2 or 3. The signal-to-noise ratio is proportional to and the effective time constant is 40(2N) milliseconds. Note that when using AVERAGE, for a given S/N ratio a smaller number of on-target integrations is required than with SUM, and less charge on the cathode is accumulated. Because AVERAGE gives a higher S/N ratio, at the expense of a longer effective time constant, it should be used in preference to SUM except for quick looks with short integrations, e.g. acquisition of an object just below the DIRECT mode limit.

• SUM

  ON-target summation of N=2 frames (each of 40 msec), selected via the INTEGRATION rotary switch. The signal-to-noise (neglecting dark current) is proportional to N. On-target integration is potentially dangerous because the photocathode may be damaged by excessive build-up of charge, so care is needed. In all but special cases AVERAGE is to be prefered: it is usually better to use moderate on-target integrations with filtering rather than very long integrations.

• DIRECT

  Real time digitisation at normal frame rates. Normal Frame integration time is 40ms.

• FILL

  Fills in the alternate lines of a stationary (FROZEN) picture. Considerably improves the appearance of faint, extended objects.

• FREEZE

  Freezes the current picture.

• ARCHIVE

  Saves TV frames in the memory of the 3210 Instrument Computer.

• OVERLAY-ONE

  Enables/disables the first graphics overlay plane (i.e. the boundary, the orientation and the scale).

• OVERLAY-TWO

  Enables/disables the second graphics overlay plane (i.e. the integration, tau, background and contrast information).

• CROSS HAIR

  This puts a crosswire on the screen.

To the left of the rotary switches are three more push buttons:MENU , SELECT and RESET. Pressing MENU will freeze the image and cause the first of a two-page image processing Menu to be displayed. A description of the available menu functions is given in Section towards the end of this chapter. A cursor will appear, which can be positioned on a selected Menu item by repeated depressions of the SELECT button. Alternatively, the joystick may be used for the selection of a Menu function. The joystick controller box also contains the buttons to enable/disable the movement of any displayed cursor. Press the ENTER button on the joystick controller to enter the menu selection. Pressing the RESET button will completely reset the system: all overlays etc. will be lost.