Background information and TSS links

Receiver A3 is a single-channel, double-sideband system with completely automated tuning. Based on a 4-element SIS junction, A3 is useable for sky frequencies (i.e. after Doppler corrections) from 211 to 276 GHz, although true heterodyne performance has not been checked to the extremes of this range.

A3 can be used in any of the normal observing modes (i.e., position-, beam-, and frequency-switching, and raster mapping).

Current status

Notes for users

An important point to note is that the intensity of a line will depend to a greater or lesser degree on whether it is observed in the upper or lower sideband, since A3 is a double-sideband receiver (this effect is also quite pronounced in receiver B3, but the normal use for B3 is as a single-sideband instrument, where the calibration is much better-behaved). The most rextensive data which can be used to quantify this statement were taken during commissioning in December 1998, although recent data using HC₃N lines is perhaps as revealing. The differences can range from a few percent (around CO 2-1) up to more than 50%, and once again standard spectra should be obtained whenever possible for comparison. See the User Note on line strengths and sidebands for further information.

Basic parameters. Some of the more reliable values for beamwidth and aperture efficiency are given below for five common line frequencies, and compared with values obtained for receiver A2. Within the likely errors the numbers are the same, and indicate that values for efficiencies and beamwidths for A2 may be used until a more complete set are obtained for A3. See the A2 fact sheet for further information. An example of a beammap at 230.538 GHz/USB is shown here; contours are every 4%, beginning at 2% of the maximum.

Receiver performance and history. In the laboratory before leaving Victoria A3 achieved a DSB receiver temperature of 70-80 K across the useable band, as shown by the bright blue line in the plot below (click here for a better view), In general the receiver retained this performance after arriving at the JCMT with one important change: a degradation in noise temperature between about 245 and 255 GHz. Since the cryogenic components have been transplanted into the replacement dewar we have seen a further worsening of this situation; although the extent of the "hump" does not seem to have increased in frequency extent, the peak value may have more than doubled. If it is necessary to observe in this range, be especially careful to obtain calibration data to attempt to quantify the effect of the "hump" on the temperature scale.

The receiver performance has changed at times since first light. A plot of the receiver temperatures (Trec, DSB) from first light through the present covering LO frequencies from 226 to 236 GHz (i.e. excluding the "hump") is shown here (the color coding indicates different frequency settings). Aside from various extreme Trec values, obtained during abnormal conditions, this plot shows that it is quite normal for the Trec values to drift between warmups; the most recent history since the beginning of 2002 is shown on an expanded scale here shows this quite clearly. The latter plot covers an LO frequency range from 214 to 235.5 GHz, and it is clear that the frequent observations of CO in the LSB (in green) have consistently lower Trec values than those for C¹⁸O 2-1 at 219 GHz, say. In the latter plot data stop in late June due to the telescope heavy engineering period. Just prior to this period the Trec values show a steady decline from values which are somewhat high following work on the receiver in February (when A3 warmed up) to more normal values in June. Historically over the life of A3 such variations are not unusual.

Please address any comments, suggestions or requests to Per Friberg at the Joint Astronomy Centre.