The tracking programs keep track of two source positions: the MAIN position and the REF position . The MAIN position is (almost always) the location on the sky that the observer is interested in, while the REF(reference) position is defined for the sake of data collection programs that require observations of an off-source position, such as position switched observations. The observer commands the telescope to track the MAIN position using the command MAIN; the reference position is tracked when the observer enters the REF command.
Source coordinates may be entered in the AZ-EL, RA-DEC, or L-B coordinate
systems using the keywords listed in Table 3.2. When coordinates are entered,
they refer to the MAIN or REF positions depending upon which of these two
positions are being tracked when the command is made. Thus, to enter the
coordinates of the Orion KL Nebula into the MAIN position, the observer should
enter:
MAIN (selects the MAIN position)
EPOCH 2000.00 RA 053246.8 DEC -52428.0 (Coordinate Input)
Since it is often convenient to initialize the REF position to be identical to
the MAIN position, the special keyword SAME
has been defined for this purpose.
Entering the command SAME at any time will cause the position currently stored
in the MAIN position to be entered into the REF position.
Many astronomical positions of interest are given in terms of offsets from
some nominal RA-DEC position. Thus, the tracking programs permit the user to
specify offsets to the current MAIN or REF position in any (or all) of the
coordinate systems it recognizes. This capability is especially useful for
entering a REF position that is offset by some particular amount from the MAIN
position. For example, to enter a reference position that is offset by 0.5
degrees in azimuth from the MAIN position, the following sequence of commands
could be entered:
SAME (sets the REF coordinates to be identical to MAIN)
REF (points the telescope to the
REF position= MAIN position
AZOFF 0.5 (adds a 0.5 degree azimuth offset to REF)
The position offset commands are summarized in Table 3.2.
For the focal plane array receivers in use at FCRAO, the
telescope tracks by default on a centrally located pixel within the
array. For SEQUOIA, this pixel is either horn 11 if the dewar is upright
or horn 6 if the dewar is flipped to track a particular
coordinate system. In this way, the identical field is covered by the
array regardless of whether the dewar is flipped or not.
This default and routine mode is enabled with the
command
QSELECT ALL
Most projects should use the QSELECT ALL mode for observing.
To track on a given pixel within the array, the user issues the QSELECT
command with the argument of which horn/pixel to track. This enters
the appropriate stored offsets relative to the reference pixel
of the array. For example,
to track pixel 3 within the array,
QSELECT 3
Note that if
the dewar flips, then the two fields will not be equivalent. The status
of the QSELECT command is shown on SCREEN 15. It is
a wise practice to check this status when taking over from the
previous observer.
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The user can manually put azimuthal and elevation offsets into the commanded position with the use of the AZ,EL digiswitches located at the bottom of the control panel. The leftmost digiswitch gives the sign of the offset while the rightmost switch allows one to adjust the value of the offset. The value of the digiswitch settings are displayed on Screen 1. Manually setting offsets is useful when pointing the telescope at the beginning of a run. This mode of operation is discouraged since there are equivalent software commands AZPC,ELPC. typically used by enabled with the PCMAN command.
A final convenience for observers is the use of the NAME
command with the
source catalogs maintained by the
tracking programs. Two catalogs, the FCRAO
catalog and the USER catalog are present.
The USER catalog, as the name
implies, permits the observer to define a set of source positions and refer to
them by their source name, rather than having to input source coordinates from
scratch each time the source is to be observed. (See description of CAT
utility program in Chapter 5 for further information about the USER catalog).
To switch the pointer from one catalog to another, one uses the CATA command.
For example, to enter a source coordinate for the
Orion KL Nebula from the FCRAO catalog,
the observer should enter:
CATA FCRAO (selects FCRAO catalog)
NAME ORIONKLN (selects Orion KL coordinates)
NAME may also be used to simply enter the name of the source that you wish to
observe, for purposes of including the source name in the header information
of any scans obtained on the source. When NAME is used, the tracking programs
first search the catalog last selected (FCRAO or USER) for a source of that
name, and if one is found then its coordinates are entered. If no source is
found, then only the source name is changed.