Beam maps are a vital tool for characterizing a telescope. They provide information about the optics of the telescope and receivers, and especially at high frequencies they are very sensitive to changes in the surface accuracy of the dish, both as a function of elevation and time, because for a telescope like JCMT we have constantly tried to improve the surface accuracy. For continuum mapping in the sub-mm beam maps of a planet are often necessary, if one wants to accurately calibrate the data. Sub-mm seeing (refraction) can often substantially broaden the beam resulting in large calibration errors if one does not take a beam map of a planet under similar conditions. In day time, when the telescope heats up, the thermal expansion introduces spherical aberration, which significantly broadens the in-focus beams and results in rather large error lobes in the antenna beam pattern.

The main source for beam maps has been from the work done in engineering and commissioning runs as well as my own PATT runs, but I have also searched through PATT runs to find additional maps. The archive is by no means complete, and work is still ongoing to supply additional maps, but at 800 and 450 mm it gives a reasonable review of the history of the telescope.

THE BEAM MAP ARCHIVE IS NOW AVAILABLE THROUGH ANONYMOUS FTP.

This is what you need to do to find out current beam sizes for JCMT or to retrieve a beam map:

FTP>connect jach.hawaii.edu

FTP>login anonymous

give your full e-mail address as password

Once you are logged on to JCMT, set the directory to pub/jcmt/beams, i.e. cd pub/jcmt/beams and a .message file will automatically display additional information. If the file scrolls over your screen too fast just copy it over to your own directory by the command `get .message'.

In the beams directory you will also find a JCMT INTERNAL REPORT, which describes the beam map archive, both as a latex file as well as a postscript file (beam_maps.tex, beam_maps.ps)

The actual beam maps reside in subdirectories below the beams directory. Currently you will find the following subdirectories: rxa2, rxb3i, and rxc2 (which contain beam maps obtained with our common user heterodyne receivers), as well as 2000, 1300, 1100, 850, 800, 750, 450 and 350 (which contain beam maps for the diffraction limited beams of UKT14 below 1.1 mm). The name convention here is the nominal filter wavelength expressed in microns. In addition we also have beam map directories for fully open and intermediate apertures for UKT14 for wavelengths shorter than 1.1 mm. These are named 850_65, 800_65, 750_65, 450_65, 350_65, 450_47, 350_47, 450_35, 350_35 etc. Each sub-directory contains a .message file, which is automatically displayed when you go into the directory. For example pub/jcmt/beams/450 contains diffraction limited (27 mm aperture) beam maps obtained with UKT14 as well as an index that characterises each beam map. This index is stored in the .message file. For directories which contains a lot of maps, it is clearly easiest to just copy over the .message file, because this may contain all the information you really want to know.

Goeran Sandell, JAC