Getting Started

Documentation

Glish

Learn More

Programming

Contact Us

Version 1.9 Build 803

News	FAQ
Search	Home

Doppler tracking (DOPPLER)

A new doppler tracking table is added to store source-based doppler tracking information for each SPECTRAL_WINDOW, as discussed in Section 3.10.2.

In spectral-line observing, both the on-line telescope and back-end processor control systems and the off-line data reduction package require information specifying the Doppler tracking parameters used during observing or required for post-processing. There are many different possibilities for how Doppler tracking is implemented in practice; these depend on the instrument and science goals of the observing. The MSv2 data format attempts to represent this information in a sufficiently general format that is not instrument-dependent, and covers all Doppler tracking strategies.

In general, Doppler tracking parameters can be sub-divided into those that are dependent on the astronomical source or field, and those that capture the frequency tracking strategy used at the telescope and/or back-end processor. The source-based parameters include:

(a) Celestial coordinates of the source or field position used for Doppler tracking.

(b) Properties of the associated transitions (1..N_lines).

(i) Transition name (e.g. CO J=2-1, HI etc.).

(ii) Systemic velocity (V_sys).

(iii) Rest frequency ($\nu_{0}^{}$).

(c) Options for frequency tracking during observing include:

(i) No tracking on-line; shift spectra in post-processing.

(ii) Coarse tracking on-line; refine shift of spectra in post-processing.

(iii) Fine tracking on-line; no refinement in post-processing.

(iv) Shift spectra in post-processing to a new line or new frame, not tracked during observing, but lying in the observed frequency band.

In MSv2, the source properties (a,b) are recorded in the SOURCE sub-table, which is accordingly not optional if Doppler tracking is employed. The transition information is SPECTRAL_WINDOW_ID dependent, as reflected in the SOURCE sub-table key. The SPECTRAL_WINDOW_ID is part of the primary key in the MAIN table, via DATA_DESC_ID, and can accordingly be used in any sub-table key without introducing a circular dependency. If only the field center is tracked, then a SOURCE entry for the field center should be added, named accordingly. The transition list in the SOURCE table should include only those transitions of scientific interest to the observer, not all possible transitions in the frequency band. With increasing observing bandwidths, multiple transition entries will become more common however.

The frequency tracking strategies (c) are recorded in the SPECTRAL_WINDOW, FREQ_OFFSET and DOPPLER sub-tables. An underlying principle is that spectral windows which can be calibrated together should share a common SPECTRAL_WINDOW_ID wherever possible. For minimal or no on-line tracking (c-i,ii), the frequency in the SPECTRAL_WINDOW table should be labelled as topocentric, and will be fixed. Option (c-ii) is accommodated by adding supplementary offset in the FREQ_OFFSET sub-table as required. This table can also be used to accomodate antenna-based and other frequency offsets in a flexible manner. Both (c-i,ii) have DOPPLER_ID = -1.

For (c-iii), the frequency axis can be labelling in a frame in which it is constant, to within meaningful scientific precision. In this case, the DOPPLER_ID index points to the DOPPLER entry describing the transition information used in the on-line tracking. This can be source-dependent, as the DOPPLER key includes a SOURCE_ID. For Doppler tracking in software (c-iv), the DOPPLER_ID key should be adjusted to reflect the resultant frequency shift, in the same way as described for on-line Doppler tracking.