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Using SPC

startup
SPC is available on the Unix machines at Epping, Parkes, Narrabri and Mopra. On any of these machines, it can be started up by typing spc from any account.

commands
Commands in SPC are activated by typing a command name (case-insensitive, minimum-match) in response to the command prompt. Command names are written in this manual so as to indicate the minimum number of letters required, for example INFORMATION. A short list of useful commands is given with the prompt; the full set of commands is given in part III.

defaults
Control of SPC commands is via questions and answers. Defaults (generally indicated before or with the question) are accepted by hitting <CR>. The last answer to a question becomes the default value unless otherwise indicated.

short commands
Some of the questions can be avoided by putting a full stop ``.'' after the command. For example, the command PL. avoids most plot setup questions, using the previous setups as defaults.

trouble
If you type a dash (-) as a file name (in DIRECTORY, LOAD, or WRITE) you return to the command prompt. Typing a dash or a zero in response to a LIST request also returns you to the command prompt. Typing ^C during execution of some commands (PLOT, NH3FIT, BASELINE, etc.) does the same thing. This can be useful if you've given a wrong answer to a question or launched a long but useless task (e.g. plotting 111 spectra).

help
VMS-style on-line help is available using the HELP command.

files
SPC does not alter SPECTRA (.SPC) files; it is even safe to load data from an open SPC file at Parkes, i.e. during the observation. Use the command DIRECTORY to list available sequence numbers in a data file to check what's there.

loading spectra
Use the LOAD command to load spectra into registers. You must give a list of sequence numbers to read from the input file, and an equal number of registers to hold the spectra.

registers
There are 111 registers available in SPC. All commands are performed on these registers. Data in the original files loaded are not altered, although SDF files may be appended to.

saving spectra
Reduced spectra are saved using WRITE. These SDF files are written sequentially, and successive writes to an existing file are appended to the old file. When loading a spectrum back into SPC from a SDF file, the ``sequence'' number required by LOAD is the sequence in which that spectrum was written to the SDF file; the sequence number from the original SPC file is retained as a header item. Use DIRECTORY to review what is contained in a SDF file.

quadrants
A register consists of 8260 real numbers. The first 8196 are spectral channels; the remaining 64 numbers are header items. The 8196 spectral channels may be composed of up to four quadrants representing the four possible IFs available simultaneously at Parkes. The quadrant are thereafter distinguished by channel range; e.g. 1-1024 = Q1, 1025-2048 = Q2, etc. The number of quadrants, channels per quadrant, velocities, etc. are contained in the header items. You can inspect (INFORMATION, STATISTICS), edit (EDIT, SHIFT, COPY) and plot (PLOT, XYPLOT) both header items and data.

S, R, Q
With SPC files, you may load either the average or individual checkpoint blocks (CPBs). A Reference, Signal or Quotient spectrum may also be selected, if present for that sequence number. This data structure does not apply to non-SPC files; what you saved is what you get.

frequency and velocity
The frequency scale in data from the telescope is referred to the observer's or topocentric frame of reference, sometimes referred to as sky frequency. The velocity scales are computed using the radio convention, $v_\mathrm{radio} = c (1 - \nu / \nu_0)$, where $\nu_0$ is the rest frequency of the spectral line of interest.

SPC has a command, RVEL, to switch between the topocentric, geocentric, barycentric, LSR (kinematic) and LSR (dynamic) reference frames (the LSR-K frame being the one commonly used). The reference frame and the Doppler shift computed and used by SPC to rescale the frequency axis is recorded for each register and is reported by the INFO command. This information is recorded in output SDF files and recovered again when they are read.

Unfortunately, the telescope identification in the RPFITS header is often incorrect and this affects the geocentric (diurnal) velocity correction. The INFO command displays the telescope identification and if necessary this may be corrected with the EDIT command. The telescope identification code is recorded in SPC header item 17 as follows:

• 0: Parkes 64m,
• 0.1: Parkes 64m MultiBeam, beam no. 1,
• 0.2: Parkes 64m MultiBeam, beam no. 2, etc.,
• 1: ATCA 22m,
• 7: Mopra 22m,
• 8: Hobart, Mt. Pleasant 26m,
• 9: Ceduna 30m,
• 10: SEST.

Only Parkes, Ceduna and SEST are likely to be identified correctly, Mopra and Hobart are likely to be misidentified without user intervention.

Note that the station coordinates of these antennas are hard-coded in SPC.

log
SPC writes a file SPC.LOG containing all the information written to the terminal during your reduction session. On exit from SPC this file is printed if requested, otherwise it is deleted. It may be saved on disk only by exiting via ^Y. You can clear your log file, or print it without exiting, by using the command PRINT.

lists
Register or sequence lists may be specified using a shorthand notation where a dash indicates a continuous range of integers and a comma or a space may be used as a delimiter. For example,

1-3,12 20-18 = 1 2 3 12 20 19 18

If the first item in a list is zero or a dash, the subroutine returns to the command prompt. This is often useful for getting out of a bad situation.

pairs
When a range or range list is required (usually channel ranges), any delimiter is accepted except an integer. If an odd number of integers are input, the last range is made to be the odd integer. For example,

$$\left. \begin{array}{l} 1-512,1024 \\ 1\ 512\ 1024\ 1024... ... 1-512-1024 \\ \end{array} \right\} = (1-512)\ (1024-1024)$$

Pairs also have the zero convention; zero or dash = return.

plots
Plots can be directed to any device that PGPLOT has a driver for. The device for output is chosen during routines that involve plotting. However, some environment variables need to be set up to facilitate sending plots to printing and display devices; see, environment, below.

environment
Certain environment variables need to be defined in the SPC startup script as explained in the installation instructions.