2·3m Telescope Control for DBS Observers

If you wish to perform observations with the target away from this point, then proceed to follow these steps:

If you have any problems with the above procedure, you might try defining your own new aperture from scratch. This is a slightly more involved procedure requiring you to re-acquire the same object at either end of the slit and with the rotator at different orientations. For further details see CALIBRATE APERTURE in the MSSSO Telescopes Command Reference Manual.

Rotator selection

The DBS rotator is selected as the active rotator by the same command that sets the tertiary mirror position. Simply type
TEL$ CONFIGURE FOCAL_STATION NASMYTH_A
Before being able to use the rotator, you may also have to specify one of the modes of operation as described below.

Moving the rotator

Rotator position is set with respect to the current reference with the ROTATOR command. See the MSSSO Telescopes Command Reference Manual in the console room for details. For example, set the rotator to a position of 37° with
TEL$ ROTATOR 37

Modes of operation

Rotator positions may be set with respect to different reference positions depending on the selected mode of operation. The configuration variable ROTATOR_REFERENCE selects the mode of operations and may be set to one of the values VERTICAL_ANGLE, POSITION_ANGLE, STATIONARY or NOT_IN_USE. e.g.,
TEL$ CONFIGURE ROTATOR_REFERENCE VERTICAL_ANGLE
or
TEL$ ROTATOR/REFERENCE=VERTICAL_ANGLE


ROTATOR_REFERENCE = NOT_IN_USE
This mode causes the control system to completely ignore the rotator system. No test is made for satisfactory TCL transmissions to/from the rotator system, the status and position are not read and no control information is output. No limit testing is performed. The telescope functions normally but without rotators. The LED numeric display at the console indicates zero. (Electrical faults in the rotator system will continue to cause the console annunciator SERVO FAULT to light unless technical staff have disabled the rotator system signalling in the servo cubicle.)

ROTATOR_REFERENCE = STATIONARY
In this mode the selected rotator is driven to the requested physical rotator angle and left stationary. A target value for the rotator angle is entered using the command ROTATOR   and the rotator jog pushbuttons operate directly on this reference rotator angle.

ROTATOR_REFERENCE = POSITION_ANGLE
This is the mode used to keep the instrument focal plane stationary on the celestial sphere i.e., to remove diurnal field rotation. When the telescope is tracking, the rotator is continuously driven so that the slit or other selected orientation is maintained at the nominated position angle. The command ROTATOR   enters the numeric data into the display variable POSITION_ANGLE directly and the rotator pushbuttons afford jog control of position angle. ROTATOR 0 is oriented North and measured from North through East. (See Figure).

ROTATOR_REFERENCE = VERTICAL_ANGLE
This is the mode used to maintain the field at a constant alignment to the vertical, for example, to keep the slit of a spectrograph vertical on the sky so that atmospheric dispersion is transverse to the spectrographic dispersion. When the telescope is tracking the rotator is continuously driven so that this alignment with the nominated vertical angle is maintained. The command ROTATOR   enters the numeric data into the display variable VERTICAL_ANGLE directly and the rotator pushbuttons afford jog control of vertical angle. ROTATOR 0 is oriented towards the zenith and measured counterclockwise from upward vertical. (See Figure).

For the STATIONARY, POSITION_ANGLE and VERTICAL_ANGLE modes, the console LED numeric display is set to 1, 2 or 3 according to which rotator reference is selected. It is steady whilst the rotator is tracking and is flashed whilst the rotator is slewing.

Selecting rotator orientation

The configuration variable ROTATOR_ORIENTATION specifies which particular direction in the focal plane of the instrument is to be maintained in the alignment specified by the chosen mode of operation. There are four possibilities. For full details refer to the MSSSO Telescopes Command Reference Manual. For the purposes of using the DBS with the long slit, only ROTATOR_ORIENTATION=SLIT is relevant. Set this option with the command
TEL$ CONFIGURE ROTATOR_ORIENTATION SLIT
or
TEL$ ROTATOR/ORIENTATION=SLIT


ROTATOR_ORIENTATION = SLIT
This maintains the length of the slit in the requested orientation, which is not the same as maintaining the rotator at the same orientation since the centre of the slit need not be coincident with the rotator centre. (See Figure). The system uses the orientation parameter of the aperture specified by the configuration variable ROTATOR_AP1 as the orientation angle. If ROTATOR_AP1 is a spectrograph slit, as is the case if you have selected aperture 1, or defined you own aperture, then this setting can be used to keep the slit at constant position angle on the sky, or in the vertical so as to counter atmospheric dispersion.

In addition to control of the telescope from the console, control is also available at the telescope focal station by means of the rotator handset. There are three handsets, one each for the rotators at the three telescope foci. Only the handset at the currently selected rotator may be used to control the telescope motion and the focusser; the other two provide only manual control of their respective rotators.

Image of rotator handset.
The pushbutton/annunciator POWER (lamptest) at the top of the handset is illuminated whenever the rotator is ON in manual or automatic modes. It can also be pressed at any time to test all of the annunciator lamps or to see whether TELESCOPE SYSTEMS have been switched on. A brightness control knob at the bottom is used to set the correct level of illumination for all the annunciators on the handset.

At the top of the handset two pushbuttons give control of the mode of operation of the rotator:

Pressing the blue MANUAL pushbutton brings the rotator on in manual mode (provided TELESCOPE SYSTEMS have been switched on) and after a short delay ROTATOR READY is signalled by the green annunciator at the top right. The rotator may now be controlled manually as described below.

Pressing the green AUTO/OFF pushbutton brings the rotator on in automatic mode (provided TELESCOPE SYSTEMS have been switched on and the rotator in question has been selected). After a short delay ROTATOR READY is signalled by the green annunciator at the top right and the rotator is now controllable from the computer. If that rotator was not the one selected, it simply switches off.

A group of annunciators down the left-hand side of the handset indicate the state of the rotator and telescope system hardware.

To actually move the rotator, make sure the handset is in MANUAL mode, hold down the CONTROL button and press ROTATOR CW or ROTATOR CCW for clockwise and counter-clockwise rotation respectively. When finished return the rotator to automated control by pressing AUTO/OFF.

Observers should avoid repeated switching between automatic and manual modes whilst the telescope is tracking, because it will drop out of the tracking state and it is possible for large impulses to be imparted to the rotator drive gears. If you switch to manual at the handset whilst the telescope is tracking, allow about 10 seconds before switching back to automatic so that the control program has time to halt the rotator smoothly.

The majority of telescope system logical configuration changes are made via the CONFIGURE command. Physical movement of actual telescope hardware is mostly controlled via the SWITCH command. The syntax for CONFIGURE is simply
TEL$ CONFIGURE variable-name value

The configuration variables and their possible values are listed below:

Configuration variable	Possible values
Observer_name	string variable set at login time only
CFfle	file specification for telescope control file
Instrument_ident	string
Secondary	Nasmyth, IR_Cassegrain
Focal_Station	Nasmyth_A, Nasmyth_B, Cassegrain, Stowed
Focus_Control	Manual, Automatic, Compensated
Windscreen Control	Open, Closed, Tracking, Vertical_Only Tracking, Off
Windscreen_Aperture	Normal, Wide_for_Finder
Windscreen_Normal-Clearance	(numeric value in metres)
Default_File_Equinox	string must be valid system equinox specification
Tracking_Equinox	string must be valid system equinox specification or the keyword FILE
Default_Temperature	temperature in degrees Celsius
Default_Pressure	atmospheric pressure in mBar
Default_Humidity	relative humidity in %
Effective_Wavelength	effective wavelength for detector and source in nm
Rotator_Reference	Not_In_Use, Position_Angle, Vertical_Angle, Stationary
Rotator_Orientation	Slit, Radial, Apertures, Angle
Rotator_Ap1	aperture ident for first aperture
Rotator_Ap2	aperture ident for second aperture
Rotator_Orientation_Angle	angle in degrees
Rotator_CCW_Limit	angle in degrees (software limit to rotator motion)
Rotator_CW_Limit	angle in degrees (software limit to rotator motion)

Physical movement of actual telescope hardware is mostly controlled via the SWITCH command. Changes to system configuration choices are made via the CONFIGURE command, though the division of duties between SWITCH and CONFIGURE is not always obvious. The syntax for SWITCH is simply
TEL$ SWITCH variable-name value

The switch variables and their possible values are listed below:

Switch variable	Possible values
Telescope_Systems	On, Off, Reset
Ventilation_Fans	On, Off
Shutter	* Closed
Mirror_Cover	Open, Closed
Flatfield_Illumination	On, Off

* The value `Open' also exists, but is available to the TCS only. It cannot be set from the user terminal for safety reasons.

Note: Flatfield_Illumination is the only system for which there is no physical control button on the console, and thus the only one you are likely to use.