Telescope Command Reference Manual
 

3 Glossary
 

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Altitude: angle measured positive from the horizon toward the zenith, with a range of -900 to +900. On the 2.3 metre telescope, Altitude is a display variable.

Aperture: A location in the focal plane known to the 2.3 metre telescope system, which it uses for tracking. The tracking coordinates are the coordinates of the object which appears in the currently selected aperture. Changing the selected aperture (with the APERTURE command) causes the telescope to move to place the object in the new aperture, but the tracking coordinates DO NOT change.

If the field rotates throughout observing, it will appear to rotate about the currently selected aperture, even when this is not the centre of the field. Observing an object which is not located in the current aperture will result in poor tracking.

Apparent Coordinates: The geocentric apparent place of the current object is calculated by taking the J2000 coordinates, applying precession to today and nutation for today, and correcting for annual aberration for today. The apparent place comprises Apparent_RA and Apparent_Dec.

Apparent_Dec: Declination portion of apparent Coordinates. Apparent_Dec is a display variable.

Apparent_RA: Right Ascension portion of apparent Coordinates. Apparent_RA is a display variable.

Azimuth: On MSSSO telescopes, azimuth is measured positive eastwards from north (i.e. like a traditional compass bearing). On the 2.3 metre telescope, Azimuth is a display variable, with a range of -180? to +340?.

Base Coordinates: are the coordinates obtained from the file coordinates after applying precession (and possible coordinate system transformation) from the file equinox to the track equinox, proper motion from the file epoch to now, and correcting for annual parallax. The base coordinates comprise the base RA, the base Dec and the track equinox. They may be changed to a new position during tracking with the TRACK/ZERO command if desired.

When a new object is first acquired, before any offsets have been added, the telescope points to the base coordinates. That is, the tracking coordinates are equal to the base coordinates.

Base_Dec: Declination portion of base coordinates. Base_Dec is a display variable.

Base_RA: Right Ascension portion of base coordinates. Base_RA is a display variable.

CFILE: Shorthand for telescope Control file

Configuration Variable: The telescope configuration is set by modifying a number of configuration variables. This can be done directly from the keyboard with the CONFIGURE command, or the variables may be loaded from a control file with the CFILE command. To see the current telescope configuration, use the DISPLAY CONFIGURATION command.

Each telescope has its own set of configuration variables. To see a list of the variables appropriate to your telescope and the values each may take, see the chapter ‘Control of Telescope Configuration’ in your telescope Observer’s manual.

Control File: Telescope Control files (also known as CFILEs) may contain telescope configuration information, aperture definitions and instrument rotator settings (on alt-azimuth telescopes), oscillating secondary mirror control settings (where relevant) and pointing correction coefficients. [Observers’ Control files do not normally contain pointing correction coefficients; these are contained in a system default Control file.]

When many aspects of telescope configuration are to be changed, it is quicker to load them from a Control file with the CFILE command than it is to enter successive CONFIGURE commands. For more information on the contents of a telescope Control file see the chapter ‘Control of Telescope Configuration’ in your telescope Observer’s manual.

Coordinate File: A file containing celestial coordinate specifications, one object per line. Coordinate files are used by the TRACK command. For more information on the contents of a coordinate file see §2.3.

Default_File_Equinox: The equinox which is assumed by the system when coordinates are supplied to it without any equinox specification. It then becomes the file equinox.

Default_File_Equinox is a configuration variable. For more information on the syntax of Equinox strings, see §2.1.4

Default_Humidity: This value is used in refraction calculations when the humidity sensor is faulty or disabled. The closer you set this value to the actual value of relative humidity, the better your pointing and tracking will be. Default_Humidity is a configuration variable.

Default_Pressure: This value is used in refraction calculations when the barometric pressure sensor is faulty or disabled. The closer you set this value to the actual value of atmospheric pressure, the better your pointing and tracking will be. Default_Pressure is a configuration variable.

Default_Temperature: This value is used in refraction calculations when the temperature sensor is faulty or disabled. The closer you set this value to the actual value of temperature, the better your pointing and tracking will be. Default_Temperature is a configuration variable.

Delta_Dec: The offset in Declination. Delta_Dec is a display variable.

Delta_RA: The offset in Right Ascension. Delta_RA is a display variable.

Display Variable: A variable which may be displayed on the observer’s display screen. The selection of variables to be displayed on the screen is controlled using a display definition file. The current value of any display variable may be examined with the VIEW command, whether or not that variable currently appears on the display screen. See the chapter ‘Telescope Display’ in your telescope Observer’s manual to see a list of valid display variable names.

Display Definition File: A file controlling the layout and appearance of the telescope display screen. In particular, it specifies which display variables are to be displayed and where. The format for a display definition file is different for each telescope. For information on the file format, see the chapter ‘Telescope Display’ in your telescope Observer’s manual.

Effective_Wavelength: Wavelength (in nanometres) used in atmospheric refraction calculations. Set this value to one appropriate to your instrument. Effective_Wavelength is a configuration variable.

Ephemeris File: File containing time-stamped celestial coordinates used for tracking objects with high proper motion, such as comets. This file type is used with the TRACK/EPHEMERIS command. The telescope interpolates between the coordinates to track the object. Unlike normal coordinate files, each ephemeris file can contain coordinates for only one object. For more information on the contents of ephemeris files, see §2.4.

File Coordinates: The current object's coordinates as they were read from the coordinate file with the TRACK command, or were entered directly with the TRACK/COORDINATE command. They comprise the file RA, file Dec, file equinox and file epoch.

File_Dec: Declination portion of file coordinates. File_Dec is a display variable.

File_RA: Right Ascension portion of file coordinates. File_RA is a display variable.

File_Epoch: The epoch which is used as the zero point for proper motion calculations. This is only specified in input coordinates when it is necessary to distinguish between this epoch and the file equinox. When not specified, it is set equal to the file equinox.

File_Epoch is a display variable. See §2.1.4 for more information on the syntax of Equinox strings.

File_Equinox: The epoch of the equinox and equator of the file coordinates for FK4 or FK5 mean places, or the string "APPARENT" for apparent place. If it is not specified in an input coordinate string, the control system sets it to the current default file equinox.

File_Equinox is a display variable. See §2.1.4 for more information on the syntax of Equinox strings.

J2000 Coordinates: These are the current tracking coordinates after precession from the track equinox to J2000.0 and transformation to the FK5 coordinate system. J2000 coordinates comprise the J2000 RA and the J2000 Dec.

J2000_Dec: Declination portion of J2000 coordinates. J2000_Dec is a display variable.

J2000_RA: Right Ascension portion of J2000 coordinates. J2000_RA is a display variable.

Offsets: The difference difference between the base coordinates and the current tracking coordinates. When a new object is acquired, the offsets are zero. Offsets accumulate when the jog/offset buttons at the console are pressed, and when differential tracking rates are established with the RATE command. They may also be modified with the OFFSET command. The offsets in RA and Dec are Delta RA and Delta Dec respectively.

Oscsec: Shorthand for the Oscillating Secondary mirror of the 2.3 metre telescope. It is a servo-controlled Cassegrain secondary designed for beam-switching and synchronous detection at IR wavelengths. The frequency and amplitude of the chop and the direction of the chop axis can all be controlled.

Osc_Chop_Mode: The action of the oscillating secondary mirror. It has seven operating modes, as follows:

 
Beam_A mirror is stationary at its Beam A position.
Beam_B mirror is stationary at its Beam B position.
Axis mirror is stationary at its neutral position.
Centre mirror is stationary mid-way between beam A and beam B.
Chop mirror oscillates between Beam A and Beam B.
Ext_Freq frequency of oscillation is controlled by an external source.
Ext_Waveform mirror position is controlled by an external source.
Osc_Reference: The control mode of the Oscsec. It can take one of the following values:  
Not_In_Use  the Oscsec is shut down and is ignored by the telescope control software.
Position_Angle Chop axis is controlled to hold constant position angle on the sky.
Coupled Chop  axis is controlled to hold a constant offset from the cassegrain rotator position.
Stationary Chop axis is controlled, and may be moved by setting a new angle with the OSC command. It does not otherwise move.

Parallactic_Angle: The angle between the hour circle and the vertical circle measured from north through east to the upward vertical, measured 0? to 360?. On the 2.3 metre telescope, Parallactic_Angle is a display variable.

Position_Angle: The angle between the hour circle and the projection of the selected orientation on the sky, measured from north through east, with a range of 0? to 360?. On the 2.3 metre telescope only, Position_Angle is a display variable.

 Rotator_Orientation: The mode for specifying the selected orientation. It can take one of the following values:

 
 
Slit  selected orientation is the direction of a line drawn along the slit of a specified aperture.
Radial selected orientation is the direction of a line drawn from the rotator centre through the centre of a specified aperture.
Apertures selected orientation is the direction of a line drawn from the centre of a specified aperture through the centre of a second aperture.
Angle  selected orientation is the direction of a line specified directly as an angle from the instrument fiducial.
Rotator_Orientation is a configuration variable. For more information, see the chapter ‘Rotator Control’ in the 2.3 metre telescope Observer’s manual.

Rotator_Reference: The control mode of the instrument rotator. It can take one of the following values:

 
Not_In_Use the rotator is ignored by the telescope control software and it cannot be controlled from the console.
Stationary  the rotator is controlled, and may be moved with the console buttons or the ROTATOR command. It does not otherwise move.
Position_Angle  the rotator is controlled to maintain constant position angle-i.e. to remove field rotation.
Vertical_Angle the rotator is controlled to maintain constant angle to the zenith-i.e. constant vertical angle.
Rotator_Reference is a configuration variable. It may also be set with the ROTATOR/REFERENCE command.

Selected Orientation: An orientation in the focal plane which is used within the 2.3 metre telescope system for control of the instrument rotator. You define the selected orientation to be some direction of interest to you (e.g. the orientation of your aperture slit) and thereafter the telescope takes the projection onto the sky of this direction and uses it to calculate the telescope position angle and vertical angle.

There are a number of alternative ways of specifying the selected orientation (see the chapter ‘Rotator Control’ in the 2.3 metre telescope Observer’s manual for more information) but ultimately it is an angle in the focal plane measured from the instrument fiducial. Its value may be viewed on the Configuration Display as the value of ROTATOR_ORIENTATION_ANGLE.

Slit Coordinates: Slit coordinates are those used by the jog/offset buttons on the console of the 2.3 metre telescope when the SLIT coordinate selection button is pressed. They are also used by the OFFSET/SLIT and RATE/SLIT commands.

Currently, slit coordinates are defined by the Selected Orientation, which means that they may or may not relate to any actual aperture slit (depending on how the Selected Orientation was defined) and if they do relate to a slit, it may not be the currently selected aperture’s slit. It is our intention to change this in the future, so that slit coordinates will be defined by the slit orientation of the currently selected aperture.

For information on specifying the Selected Orientation, see the chapter ‘Rotator Control’ in the 2.3 metre telescope Observer’s manual.

Startup Command File: A DCL command file which is executed as the final stage of telescope system startup. Typically it is used to define programmable pushbuttons or set the telescope configuration. For information about how to set up a startup command file, see the description of the STARTUP command in §1.3.

Tracking Coordinates: When the telescope is tracking, the tracking coordinates indicate the telescope’s current sky position. They differ from the base coordinates by any offsets which have been added using the jog buttons or by telescope command. They comprise the tracking RA, the tracking Dec and the track equinox.

Tracking_Dec: Declination portion of the tracking coordinates. Tracking_Dec is a display variable.

Tracking_Equinox: A configuration variable used to specify the telescope’s track equinox. It may be a normal equinox specification string or it may be the special string "FILE", in which case the track equinox is set to the object’s file equinox.

See §2.1.4 for more information on the syntax of Equinox strings.

Tracking_RA: Right Ascension portion of the tracking coordinates. Tracking_RA is a display variable.

Track_Equinox: The equator and equinox of the mean place coordinate system that the telescope is using for calculations and display. A star’s input file coordinates are precessed from the file equinox to the track equinox as part of transforming them to base coordinates before commencing tracking. The track equinox is set to the value of the configuration variable tracking_equinox unless that variable has the value "FILE", in which case it takes the value of the file equinox.

Track_Equinox is a display variable. It appears on the top line of the display whenever the telescope is tracking.

Vertical_Angle: The angle between the vertical circle and the projection of the selected orientation on the sky, measured counter-clockwise from the upwards vertical. It has a range of 0? to 360?. On the 2.3 metre telescope only, Vertical_Angle is a display variable.