The Multifrequency Snapshot Sky Survey (MSSS) is the first major observing program to be carried out with LOFAR during its ongoing commissioning phase. The primary goal of MSSS is to produce an accurate and detailed low frequency sky model, which will be used as the basis of calibrating images produced in the future by LOFAR. Along the way, the process of making MSSS possible is an efficient way of shaking down telescope operations!
This page provides an overview of the MSSS survey design, progress, processing flow, and expected output. Updates will be posted as the survey progresses. More detail about MSSS will be provided in a refereed journal article which is now in preparation.
MSSS covers two main frequency windows: one within the LBA range (covering a frequency span from 30 to 74 MHz) and the other in the HBA range (spanning 120 to 160 MHz). Each window is simultaneously observed in 8 bands of 2 MHz each. LOFAR's multi-pointing capability is utilized to observe six fields at the same time, in every MSSS observation. (In the LBA, one of the six simultaneous fields is always a standard calibrator.)
Although the full LOFAR array (core, remote, and even international stations) will be used during MSSS observations, the initial processing is limited to the core stations for MSSS-HBA, and 10-km baselines for MSSS-LBA. This ensures similar angular resolution between the two halves of the survey, with far less computing overhead than would be required to process all of the baselines. To ensure good uv-coverage while using the limited array for short observing blocks, each field is observed in snapshots (in MSSS-LBA, 9 snapshots per field, while MSSS-HBA includes 2 snapshots per field).
The nominal MSSS survey parameters are given in Table 1, and the expected survey output characteristics are summarized in Table 2. These values are subject to updates, as observations are processed and our understanding of the telescope's performance evolves.
|Field of view per field (FWHM, degrees)||5.77 @ 60 MHz||2.42 @ 150 MHz|
|Bandwidth (MHz) per field||16||16|
|Number of simultaneous fields||5 (+calibrator)||6|
|Time per field||9 x 11 min||2 x 7 min|
|Required number of fields||660||3616|
|Required on-source observing time (hr)||218||
|Survey Name (Telescope)||Frequency Range (MHz)||Sensitivity (mJy/beam)||Angular resolution (arcsec)||Sky area (sq deg)|
MSSS is an ideal platform for testing and verifying multiple operational modes. Several added-value aspects of MSSS have been utilized in order to test these modes (for example, cosmic-ray shower detection) in parallel without impacting the primary survey goals. These added-value modes will increase the scientific usefulness of MSSS.
MSSS test observing runs began in late 2011, and have become more frequent as we progressed from a pure testing phase into regular operational mode. The observing program focused on the MSSS-LBA component in 2012, and has switched to MSSS-HBA as of early 2013. The processing pipeline development was driven in large part by MSSS considerations. For a graphical indication of the observed survey area, please see the regularly updated map, which is located here for MSSS-HBA observing status and here for MSSS-LBA observing status.
The calibration strategy for MSSS is tied to regular observations of a grid of primary flux reference sources. Each MSSS snapshot is preceded by a flux calibration scan, which is used to set the amplitude scale in the initial processing stage. Once all of the snapshots for a given field are collected and flux calibrated, the "major cycle" of imaging and calibration begins. In this stage we build off of existing all-sky surveys to produce an independent wideband catalog of the low frequency LOFAR sky.
The output of the MSSS data processing pipeline will include full-polarization images, as well as source catalogs. Collectively these catalogs will form the initial LOFAR Global Sky Model (GSM), and are the primary MSSS goal. It is our intention to release these MSSS data products into the public domain at this website.