# Spectral line observations¶

Spectral line observations can either take place exploiting proper schedules (see dedicated documents on how the schedules must be written or how to produce them with the schedulecreator tool) or by means of command-line operations. Here this command-line scenario is described.

## Line rest frequency¶

The spectral line rest frequency must be provided to the system using the following command:

> restFrequency=[value]


[value] is given in MHz and is a multiple argument: it can list a different value for each section - as long as XARCOS is the backend in use (not all the backends allow this sub-tuning). Specifying a single value assigns the rest frequency to all the sections.

e.g. > restFrequency=22235.8

used with the 7-feed K-band receiver implies that all the 14 sections are meant to observe the same line. Instead, providing different values allows to tune the sections on different lines (notice the semicolon used as a separator):

e.g. > restFrequency=22235.8;22235.8;22080.0,22080.0,...,

notice that all the N values (these being 14 for the K-band MF) must be explicitly provided or errors will rise.

The specified values will hold until different ones are commanded, or until a new general setup command is entered.

## Target velocity parameters¶

When spectral line observations are required, targets must be provided with proper indications on their radial velocity, i.e.:

• Radial velocity
• Reference frame
• Definition

in order for the system to compute, at runtime, the frequency drift due to the line of-sight doppler contribution.

Once a target has been defined, for example commanding a sidereal positioning, its velocity can be provided by means of a manual command in the operatorInput panel, i.e.:

> radialVelocity=[vrad],[vref],[vdef]


Where

• [vrad] (radial velocity) is in km/sec if vdef is not Z

• [vref] (reference frame) can be one of the following:

• BARY: Solar System BARYCENTRE
• LSRK: Kinematic Local Standard of Rest
• LSRD: Dynamical Local Standard of Rest
• LGRP: Local Group
• GALCEN: Galactic Centre
• TOPOCEN: TOPOCENTRIC (observer’s frame)
• [vdef] (velocity definition) can either be:

• RD: Radio Definition
• OP: Optical Definition
• Z: stands for Redshift

The wildcard code * can be provided for all the required arguments and will be considered as “keep the present values”.

The specified velocity parameters are valid until a new target is commanded.

Note

The radialVelocity command overrides any other velocity value that might have been differently expressed (e.g. if the target was selected from the source catalogue with the track command).

## Commanding the doppler computation¶

Once the above information have been provided to the system, the computation of the line observed frequency can take place. Use the command:

> fTrack=[dev]


It collects all the required data from the antenna, the back-end and the front-end, plus the information provided by the user (see the above radialVelocity and restFrequency commands), then it tunes the telescope devices in order to centre the line(s) in each section bandwidth. The command lets the user select which device [dev] is asked to perform the tuning:

• LO: only the front-end local oscillator is moved
• ALL: first of all the front-end local oscillator is tuned, then the back-end - if it allows such a sub-tuning - also performs a further frequency adjustment, in order to centre the line(s) in the various sections. This option is useful in case multiple rest frequencies are to be observed, yet a complete success cannot be guaranteed as the dopplered frequencies might turn out to be incompatible with the section bandwidths and the LO ranges. In case at least one line lies outside the RF band of the receiver or the back-end input bandwidth, an error rises.