Release of Spectral-Line Five-Points

At JCMT pointing is done by using a five-point routine, although some radio telescopes prefer to do it by cross-scans. The five-point scan measures the signal strength at the nominal tracking position, as well as at four points spaced half a beam-width away north, south, east, and west of the center. These data are then fed into a least squares fitting program with the constraint that the beam-size is known, or in case of extended sources, that the convolved beam-size is known. This method works surprisingly well, even in cases where the source is off by more than half a beam-width. The pointing can be checked this way with all of our common user receivers, because they all have a continuum mode.

At high frequencies (even B-band), the number of continuum sources are rather limited for our line receivers. The continuum sensitivity is low, and the bulk of the pointing sources, e.g. blazars, are faint. We have therefore decided to also provide a pointing mode, which allows pointing in spectral line configuration, i.e. to use AGB-stars for pointing. Pointing on point-like spectral line sources is by no means

a new concept, most radio telescopes have used this technique for years, especially on strong maser sources (H2O and SiO masers). Although some efforts were used several years ago to provide JCMT with the ability to do spectral-line five-points, it was not until last year that the software was finally implemented and debugged by J. Scobbie. At the same time we also released a new version of the JCMT pointing catalogue.

The spectral-line five-point mode was officially released in January 1996. It has not been used very much, presumably because observers are not aware that it exists, but it is very simple. It looks identical to a normal five-point. The only difference is that instead of choosing the continuum backend, you will have to use the DAS with 125 MHz bandwidth and non-continuous calibration. The last section of the pointing catalogue has all the necessary information on stars suitable for spectral line pointing, and what the program needs is the radial velocity and the linewidth over which to integrate. It is therefore necessary to be in velocity mode, or if not, to set the correct velocity, because the integration range is determined relative to the center. The software then takes the integrated line intensities, as well as errors, and uses the same least squares fitting routine that is used for continuum pointing.

The only drawback with spectral-line five-points is that one has to be tuned to one of the main CO transitions, J=2-1, 3-2, or 4-3. In some cases it is possible to use CO isotopomers or molecules like CS and HCN, but these sources are currently not flagged in the catalogue. Unless you, or your support scientist, is very familiar with the chemistry of AGB-stars, don't try it. It should also be mentioned that some AGB- stars have somewhat uncertain coordinates, and we tried to identify and correct the positions of all the stars that we found, but there may still be a few discrepancies in the catalogue.

Please try it out, especially if you have a RxC2 program, because in the C-band there are hardly any pointing sources except planets and a few strong compact H II regions and 'protostellar' sources. Further information about spectral line fivepoints can be found on the JCMT web pages.

Goeran Sandell, JCMT