Observing Modes

B3 may be used in any of the standard JCMT observing modes for heterodyne receivers - position switching, beam switching (usually at 1 Hz), raster mapping and slow frequency switching. Fast frequency switching is not available, however. To avoid timing errors in raster mapping with two channels, integration times of 3 seconds per point or longer (up to the maximum of 6 seconds) should be used.

Position Switching

In general, B3 has better baseline characteristics than its predecessor B3i. Large position-switch amplitudes may be possible, although, as always, it is best to switch in azimuth using the minimum amplitude needed to find a "quiet" region of sky off the source.

Frequency Switching

Frequency switching produces the best baseline cancellation when amplitudes are small - i.e. for narrow lines. To remove the standing wave corresponding to reflection between the receiver and the telescope's secondary mirror, a switch amplitude of 8.2 MHz is recommended. This results in line features in the unprocessed scans that are 16.4 MHz apart.

The frequency-switched scans may be processed using the SPECX utility command "fsw".

Bandwidths

Both mixers have front end bandwidths that are greater than 1 GHz. Currently Channel B is wider than A and can easily be used at 1.8 GHz (wide band 920 MHz setting). DAS bandwidths of 125, 250, 500, 760 and 920 MHz are available in either single or dual polarity mode. In dual polarity, for a given bandwidth, the channel separation is twice that obtained in single polarity mode.

1.8 GHz Bandwidth

Using a single mixer a wideband mode of 1.8GHz width (equivalent to 1500 km/s at 345 GHz) is available; see, for example Figure 1. In practice, because of end-effects, the useable bandwidth exceeds the maximum allowable by the DAS (1.8 GHz) with channel B but, because of end-effects due to a narrower second-stage amplifier, only 1.4 GHz in channel A. To use this mode you must tune single polarity. Generally, Channel B should be used because of its greater bandwidth. Choose WIDE from the DAS backend options, then 920 MHz. The resulting scan will be split into two 920 MHz portions and transmitted to the DAS using both 1.5 GHz channels. See the "Data Reduction with SPECX" part of this manual for notes on reducing data of this kind.

DAS Special Configuration Files

Because the IF is 4 GHz, the signal and image sidebands are separated by 8 GHz. This spacing permits simultaneous observations of a number of important lines in opposite sidebands. Among the more important line combinations are

CO 3-2/HCN 4-3 at 345.796/354.504 GHz

CH3OH 7(0)-6(0)/13CO 3-2 at 330.587/338.410 GHz

HCN 4-3/HNC 4-3, H2CO 5(0,5)-4(0,4) at 354.504/362.568, 362.735 GHz

CH3OH 7(0)-6(0)/SO 8(9)-7(8) at 330.587/346.528 GHz

It is possible for some combinations of line pairs and resolutions to use the versatility of the DAS to control the relative positions of the lines in the spectral scan. This can be done by using non-standard DAS configuration setup files invoked by the be] command.