Some background: the Archibald et al (2002) paper describes how the CSO(225GHz) tau to SCUBA opacity terms were determined for the different SCUBA filters. It was assumed for commissioning and S2SRO that the new SCUBA-2 filters were sufficiently similar to the wide-band SCUBA filters that these terms could be used for extinction correction. For reference the SCUBA corrections were:
Tau(450μm) = 26.2 * (Tau(225GHz) - 0.014)
and
Tau(850μm) = 4.02 * (Tau(225GHz) - 0.001)
The JCMT Water vapour radiometer (WVM) now is calibrated to provide a higher-frequency opacity value which has been scaled to the CSO(225GHz) tau. The WVM (not the CSO 225GHz tipper) data was used for this analysis.
The new filter opacities as determined by the skydip data are as follows:
and
A follow-up post to this will show analysis of the difference applying the new corrections can make to data combined from multiple observations taken in differing extinction conditions.
It is worth noting that if an individual science map and corresponding calibrator observation is already reduced with the old factors (and your source and calibrator are at about the same airmass and if the tau did not change appreciably), any errors in extinction correction should be cancelled out in the calibration.
The new filter opacities as determined by the skydip data are as follows:
Tau(450μm) = 19.04 * (Tau(225GHz) - 0.018)
and
Tau(850μm) = 5.36 * (Tau(225GHz) - 0.006)
A follow-up post to this will show analysis of the difference applying the new corrections can make to data combined from multiple observations taken in differing extinction conditions.
It is worth noting that if an individual science map and corresponding calibrator observation is already reduced with the old factors (and your source and calibrator are at about the same airmass and if the tau did not change appreciably), any errors in extinction correction should be cancelled out in the calibration.
No comments:
Post a Comment