MeerKLASS Glossary

SCANNING STRATEGY - In MeerKLASS, we operate the dish array in back-and-forth SCANs at constant elevation, for a total of around 90 to 100 minutes. Within each scan, we perform approximately 38 swings (about 2.5 min each). We perform the constant elevation scans during rising and setting sky. As the sky rotates this creates a raster-like scanning pattern in RA and DEC - see orange/red shaded areas in Fig. 1. We repeat scans multiple times (up to 26) over one survey area (BOX/PATCH). The survey speed is 5/7 arcmin / second (L/UHF band). We integrate data every 2 seconds. Each scan covers approximately 300 square degrees. Data is only taken 30 minutes after sunset / before sunrise, to minimise atmospheric and ionospheric disturbance.

SINGLE DISH DATA - auto-correlation data from each of the dish elements of the array, i.e. the diagonal elements of the interferometric visibilities.

CALIBRATOR SCAN - We perform a calibrator scan before and after each constant elevation scan to a bright calibrator and observe calibrator and 4 points around this for absolute, bandpass calibration and beam calibration.

NOISE DIODE - We fire the noise diode every 20 seconds for a duration of about 0.6 seconds. Data which includes the noise diode has the label ‘ON’. We only use data without noise diode, i.e. with the label ‘OFF’ in our scientific analysis.

TIMESTAMPS - We take data every 2 seconds which we also refer to as timestamps.

BLOCK - One observation is referred to as a data block. This is about 2 - 2.5 hours and includes calibrators. Each block has a unique MeerKAT numerical reference. These are the individual files available to download from the SARAO archive. Sky PATCH - The scan area covered by one observation block. It is about 300 deg2 of sky area and approximately rectangular. We cover each sky patch by about 26 blocks, half (about 13) when the patch is rising and half when it is setting. When setting up observations, each patch is defined with a rectangular shape in ra, dec coordinates. Due to the shape of the scans, the area actually covered will always be larger than the target rectangle. Sky patches overlap, so the total sky area covered is less than the sum of each patch. When analysing an individual patch, we trim the edges because of homogeneous coverage. So the sky area used for cosmology from an individual patch is less than the total sky covered by such patch.

BOX - (interchangeably used as PATCH). This was the internal name given to each sky patch starting from 2024 observations: BOX1, BOX2, etc. There are a few sky patches that used a different naming (e.g. DESI1, DESI2, WiggleZ). The sky location of our MeerKLASS BOXes are marked with coloured rectangles in Fig. 1.

SCAN - A scan refers to the constant elevation data taken during one observational block. Often BLOCK and SCAN are used interchangeably, though SCAN technically refers to the science data without the calibration data.

MAP - Once calibrated, data is projected into sky coordinates (RA/DEC). Data from one frequency channel is referred to as a map.

CUBE - Once calibrated, data is projected into sky coordinates (RA/DEC). Data from all frequency channels is referred to as a cube.

OTF - On the Fly Mapping - This refers to the processing pipeline which uses the full visibility measurement set from the constant elevation scans to process the interferometric data into images. The images have high angular resolution (~15 arcsec).

EPOCH - this term is used by the OTF team interchangeably with BLOCK/SCAN.

TILE - in the OTF imaging pipeline, the sky area is divided into smaller tiles which are processed individually into images before combining into a large image. Typically, one tile covers an sky area of order of 10 square degrees.