Note:

In 2006 the 460/490 GHz window of RxW was converted to 345 GHz. The information below describes the pre-conversion RxW. A description of the new 345 GHz Gunn can be found here

Contents

• Introduction
• Current status
• Performance figures
• Sample Spectra

For TSS's: How to tune W - (a) basic , (b) complicated, (c) extra tips for D-band if things go wrong and (d) results from tuning tests.
Representative spectra for C-band and D-band.
Relative pointing between RxB and RxW

Introduction

Receiver W, developed at MRAO in Cambridge, is used to observe high-frequency spectral lines at the JCMT. The receiver has four SIS mixers: two (built by MRAO) for observations at "C band" (around 650 microns wavelength), and two (built by SRON) for "D band" (450 microns). The latter have been replaced with more sensitive versions in the past few months. All mixers use SRON junctions. The pairs of mixers receive orthogonally-polarised radiation, so that one or the other mixer, or both mixers, at either C- or D-band may be used. Note that since only one local oscillator is provided, both mixers observe the same frequency range. It is not possible to observe at C- and D-band simultaneously.

The tuning range at C-band is from about 430 to 510 GHz, and that for D band about 620 to 710 GHz. The C-band coverage considerably increases the frequency regime accessible around the CO 4-3 and CI lines at 461 and 492 GHz, compared with that for the older receiver C2. D-band observations including the J=6-5 transition of CO and its isotopomers are possible with W/D.

The atmospheric window covered by the C-band is shown here. The atmospheric transmission for a pwv of 0.5mm is shown together with the positions of a selection of spectral lines likely to be of interest. Over much of the C-band the SSB receiver temperature is around 330 K or possibly better. The atmospheric transmission in this wavelength range is sharply divided by deep absorption bands into several narrow "windows". A similar plot for D-band is shown here. Typical SSB receiver temperatures for the new tunerless mixers are about 700 K for the latter. Although the transmission in this window is somewhat lower overall than for the C-band, there are no major absorption bands within the D-band.

The receiver is normally operated in single-sideband (SSB) mode with the unwanted sideband terminated on a cold load. However, dual-sideband (DSB) operation is possible. The tuning is partly automatic. The LO chain is tuned manually by the operator, although the mixers and diplexers are tuned remotely, with subsequent optimisation from controls in the cabin. Switching between C- and D- band also is manual. The new D-band mixers are tunerless, and operationally the receiver is now simpler at D-band than at C-band. Generally tuning from scratch by the telescope operator takes 15-20 minutes.

Observers should be aware that observing at these higher frequencies is quite demanding, and be prepared to allow considerably more time for careful pointing and focus determinations; in addition, it will help us too if observations of standard targets are made whenever possible. Also, when the planets once again become available, it will be very helpful for users to obtain efficiency measurements.

Here is a detailed description of receiver W if you want to know more.

Current status of W

Receiver was delivered to Hilo in Summer 1998, with commissioning beginning in earnest at the JCMT in August. D-band performance was much improved by the installation of new tunerless devices in March 2000. Although use has been limited so far, the receiver (picture of RxW in the cabin here) has performed very well to date, with frequency coverage and sensitivity close to those anticipated in both frequency windows. Those "standard" lines which have been observed generally show very good agreement in both intensity and line shape with the limited available data. A key exception seems to be the situation around CO 6-5 at 691.6 GHz, where lines shapes and intensities for the new mixers do not agree very well with those limited data for the older mixers. There are few such direct comparisons available, but, for example, the old and new ¹³CO 6-5 line profiles and intensities do agree very well.

The commissioning of W/C and especially W/D remains incomplete, mostly the result of the shortage of the very good ("band 1") observing conditions required for useful observations with Receiver W. The new tunerless devices continue to undergo characterization.

Although there is much to be done before the receiver can be said to be fully commissioned, it is available for astronomical observing on a routine basis. In view of the incomplete commissioning observers using W are requested to measure efficiencies as well as check calibration on known sources. This will help us improve the characterisation of W. As new information becomes available, it will be placed on the Web at this location.

Current performance

Some typical values for SSB receiver temperatures measured recently, and other parameters, are given in the tables below. Receiver temperature values appear to be critically dependent on correct suppression of Josephson currents and elimination of magnetic flux trapping, but those below appear to be typical.

Improved efficiency measurements await the return of suitable planets (Mars, Saturn, Uranus), which are all presently involved in a major conjunction on the far side of the Sun. Only a single measurement of the relative pointing between the channels at D band has been made since the upgrade to the tunerless mixers. The C-band offset has not been remeasured recently. However from earlier data the relative pointing offset is believed to be close to the C band values.

Some sample spectra

• OMC1 CO 4-3; this was essentially the "first light" spectrum after initial alignment and setup.
• OMC1 CO 6-5
• OMC1 ¹³CO 6-5
• W75N CI (neutral carbon at 492 GHz)
• CO 4-3 in NGC253 (a nearby active galaxy)
• CO 4-3 in IC342 (another nearby galaxy)
• H21alpha recombination line maser in MWC349 (cf. Thum et al., 1994)
• HDO at 464 GHz in OMC1
• HDO in W3(OH) (rightmost line - cf. JCMT Newsletter Sept 96)