The JCMT Gould Belt Survey: a quantitative comparison between SCUBA-2 data reduction methods

Abstract

Performing ground-based submillimetre observations is a difficult task as the measurements are subject to absorption and emission from water vapour in the Earth's atmosphere and time variation in weather and instrument stability. Removing these features and other artifacts from the data is a vital process which affects the characteristics of the recovered astronomical structure we seek to study. In this paper, we explore two data reduction methods for data taken with the Submillimetre Common-User Bolometer Array-2 (SCUBA-2) at the James Clerk Maxwell Telescope (JCMT). The JCMT Legacy Reduction 1 (JCMT LR1) and The Gould Belt Legacy Survey Legacy Release 1 (GBS LR1) reduction both use the same software (Starlink ) but differ in their choice of data reduction parameters. We find that the JCMT LR1 reduction is suitable for determining whether or not compact emission is present in a given region and the GBS LR1 reduction is tuned in a robust way to uncover more extended emission, which better serves more in-depth physical analyses of star-forming regions. Using the GBS LR1 method, we find that compact sources are recovered well, even at a peak brightness of only 3 times the noise, whereas the reconstruction of larger objects requires much care when drawing boundaries around the expected astronomical signal in the data reduction process. Incorrect boundaries can lead to false structure identification or it can cause structure to be missed. In the JCMT LR1 reduction, the extent of the true structure of objects larger than a point source is never fully recovered.

Alma Resolves the Torus of NGC 1068: Continuum and Molecular Line Emission

“…A careful comparison requires not only consideration of the differing beamsizes at each wavelength, but also proper treatment of the full beam profile, which, for the JCMT, includes a significant amount of flux in the "secondary" beam at 450 μm (Di Francesco et al 2008;Dempsey et al 2013). Using the procedure outlined in Pattle et al (2015) and Rumble et al (2015), D. Rumble et al(2017, in preparation) compiled temperature estimates for all of the GBS clouds based on the Legacy Release 1 reductions (see Section3.1 and Mairs et al 2015). Here, we examine the temperature map derived for IC 5146 to determine whether or not our assumption of a constant temperature of 15K for the clumps is reasonable.…”

“…The data reduction used for the maps presented here follows the GBS Legacy Release 1 methodology (GBS LR1) using the JCMT's Starlink software (Currie et al 2014), 34 which is discussed by Mairs et al (2015). The data presented here were reduced using an iterative map-making technique (makemap in SMURF), 35 and gridded to 2″ pixels at 450 μm and 3″ pixels at 850 μm.…”

The JCMT Gould Belt Survey: A First Look at the Auriga–California Molecular Cloud with SCUBA-2

“…Sources between 2 5 and 7 5 will be detected, but both the flux and the size are underestimated because Fourier components with scales greater than 5′ are removed by the filtering process. Detection of sources larger than 7 5 is dependent on the mask used for reduction (see Mairs et al 2015 for more details).…”

The JCMT Gould Belt Survey: first results from SCUBA-2 observations of the Cepheus Flare region

“…The data were reduced using an iterative map-making technique (makemap in smurf, Chapin et al 2013), and gridded to 3 ′′ pixels at 850 µm and 2 ′′ pixels at 450 µm, as part of the Legacy Release 1 (LR1) GBLS data set (see Mairs et al 2015). The iterations were halted when the map pixels, on average, changed by <0.1% of the estimated map rms.…”