Joint Astronomy Centre

James Clerk Maxwell Telescope

B. Weferling

In 2003 a several of developments and problems relating to the SCUBA flux calibration occurred. This page will give an overview. The table at the end summarises the situation. This document will be updated upon final conclusion of the opacity calibration issues in 2003.

• Water Vapor Monitor: The Water Vapor monitor is in use at the JCMT since July 2000, but only during 2003 has it been developed from an instrument that merely delivers a rough on-site opacity estimate into the main device used for opacity corrections. The instrument is currently calibrated against the polynomial fits to the 225GHz-CSO-Dipper and completes a measurement along the line of sight of the JCMT every 1.2 seconds. These characteristics make the WVM the first choice for opacity correction. The WVM measurements will become part of the SCUBA data files and SURF and ORAC-DR will be enabled to use them in the reduction. These efforts will probably be completed in the spring 2004. For more details regarding the WVM please refer to the timeline below. The WVM-Data can now interactively be queried from the WWW. This interactive archive automatically recomputes the raw WVM-Data using the most recent atmospheric model. The original measurements, based on the model current at the time of the observations can be found in the Historical WVM archive - this archive contains data since April 15 2003. Furthermore is the whole of the WVM data now available for interested users from the JCMT ftp server (please contact B. Weferling for details.) Work on an improved atmospheric model for the WVM is ongoing in a collaboration with the Cavendish Laboratories. Details about the current empirical WVM calibration can be found in R. Phillips, B. Weferling, T. Jenness MNRAS 2004 (in preparation).

• 225GHz-Dipper: The 225GHz-Dipper which serves as the basis for the polynomial fits, was not working for most of the year. Because of a hardware failure there was no data at all from the instrument from January 14 to July 11. Despite the best efforts of the CSO to repair the device the instruments behavior and calibration remained questionable throughout the remainder of the year. Since July 12 polynomial fits have been produced where possible, but unless a proper calibration can be confirmed they will not become part of the automated reduction file csofit.dat and ORAC-DR. If it should not be possible to continue to use the instrument for the polynomial fits, the archive will be completed based on the 350µm-Dipper, which now also serves as the basis for the fits from January 13 to July 11.

• 350µm-Dipper The NRAO/CMU submillimeter (or 350µm) tipping radiometer hosted by the CSO has been recalibrated during 2003. The details will be published in B. Weferling, MNRAS 2004 (submitted, please contact me if you need the information sooner). With this the 350-µm-Dipper has become an equal of the 225GHz-Dipper. The polynomial fits January 14 through July 11 are based on the instrument. During that time fits based on the instrument had also been provided, but using the old calibration. The new ones are completely equivalent to the normal fits and can be used without any modification, they are part of csofit.dat and orac-dr. Still, to distinguish them the folders and files are marked '350'. The preliminary files which were based on the old calibration will be kept for reference and are accessible through a separate archive folder called "old_350" that you can find here. It might become necessary to continue to use the 350µm-Dipper as the basis for the polynomial fits for some time. The polynomial fits will remain one of the major tools for opacity calibration.

Much has happened during 2003 with regard to the opacity correction. To give you some orientation, here are in summary the major events:
• January 14: 225GHZ-Dipper stops working.

• March 12: The new WVM calibration (V1.0) is installed, it includes an elevation correction. The WVM gives now 225GHZ-Tau-Fits equivalent. The new model is a hybrid based on the previous model empirically corrected with the 225GHz-Tau-Fits. On the same day the WVM measurement cycle is switched from 6s to 1.2s.

• March 27: The increased WVM readout rate causes VAX problems and is reverted to 6s.

• April 2: The WVM software is completely overhauled and improved. Because of this the WVM is for a short time reverted back to the old atmospheric model.

• April 9: The new WVM software (by M. Rippa in C now under CVS). Atmospheric Model V1.0 is now used again.

• April 12: The atmospheric Model is improved for high opacities (V1.1).

• April 15: From this day onwards the nightly measurements of the WVM are stored as is in the Historical WVM archive.

• May 7: The interactive WVM archive comes online. The data can be queried by numerical and graphical output. The output is automatically updated using the most current atmospheric model.

• July 3: WVM cycle finally set to 1.2s. No problems.

• July 12: CSO-225GHz-Dipper back online. For the period since January 14 CSO-Fits are produced based on the 350-micron-Dipper. The performance of the 225-Dipper it turns out, after some more repairs, is not satisfactory. It is attempted to recalibrate the instrument. Times for which this should not be possible will get a CSO-Fits based on the 350-Dipper. This work is ongoing.

• October 31: The WVM measures the highest tau recorded so far at the JCMT: 0.5893.

• November 9: The WVM makes its 10-million-st measurement.

• December 8: The CSO and Jonathan's Weather page now publish 350-Dipper data with an improved calibration.