Summary of JCMT Instrumentation for Semester 95A

Semester 95B (1 August 1995 - 31 January 1996) JCMT instrument availability and sensitivities are summarized below. Additional details can be found in 'The James Clerk Maxwell Telescope: A Guide for the Prospective User', which is available over the e-mail fileserver or through the JCMT Section of the Royal Observatory Edinburgh, by contacting the JCMT Group at the Herzberg Institute of Astrophysics in Canada or the NFRA at Dwingeloo, The Netherlands, or from the Joint Astronomy Centre in Hawaii. The current version is dated January 25, 1995. The e-mail fileserver system exists to provide instrumental data, both archival and current, and other information. To get acquainted with the latter send the one-line message "help" to the Internet address:

JCMT_INFO@JACH.HAWAII.EDU

The User Guide also may be browsed on the Internet hypertext-based World-Wide Web, at URL /JCMT/home.html which carries a variety of other information too.

Spectral Line Observations

Three SIS mixer receivers form the core of the heterodyne program, A2, B3i and C2. One other SIS receiver (G) has been available on occasion via collaboration with the MPE, Garching group. A summary of the properties of this instrumentation is given below, followed by a few additional details.

Table 1. Summary of spectral line observational data.

            Freq.    IF     B/W   Trx            Efficiency          Tel.    HPBW

          (GHz)   (GHz)  (MHz)  (K)   Ap.   Beam   fss   losses    (")

  A2    205-285  1.50    600    95   0.57   0.69    0.80    0.91     19.7

  B3i   300-380  1.50    700   180   0.49   0.58  0.70    0.91     14.3

  C2   450-505  3.94   1000  250   0.31   0.52    0.70    0.83     10.8

  G     660-692  1.50    800   800  (0.20)  0.25   (0.60)   0.65      7.0

A new SIS version of Receiver G first saw service in March 1994 and the data given above are based on information from that time.

Users should note that because receiver C2 uses an EIP counter to phaselock the Gunn local oscillator, the resolution achievable is limited by the Gunn's intrinsic phase stability, with the consequence that lines observed with C2 are likely to be broadened instrumentally by an additional 0.5 km/sec, and thus C2 may NOT be suitable for the observation of sources requiring velocity resolution better than a few tenths of a km/sec. Note that the line frequency itself is not affected by the phase 'jitter'. Until late 1994 A2 had a similar EIP system; this has since been replaced with a true phaselock system adapted from the old receiver B2. In the case of B3i a much more agile phaselock system is used that can suppress the Gunn oscillator's inherent phase noise. Thus for B3i the resolution is limited only by the spectrometer.

(a) Receiver A2

A2 is a single-channel receiver which was commissioned in March 1992. Its excellent low-noise performance results in a total SSB system temperature (Tsys) of better than 400 K at the zenith across most of the band under normal conditions.

(b) Receiver B3i

B3i (also a single-channel device) is one of the best receivers available in this band in the world. The DSB receiver temperature response is not constant with frequency, and ranges from a best value of near 160 K at 355 GHz (even better values have been recorded below 310 GHz), up to about 265 K at 330 GHz. On the sky, SSB system temperatures below 600 K have been obtained under good conditions.

(c) Receiver C2

C2 is a single-channel receiver which covers frequencies from about 450 to 505 GHz. In May 1993 it successfully completed commissioning at the JCMT, and has been used for PATT observations since that time, when atmospheric conditions have permitted. Generally this is when the opacity of the atmosphere measured at 225 GHz is less than 0.1 at 461 GHz, and less than 0.06 at 492 GHz.

Two Gunn oscillators (overlapping at 475 GHz) are used to cover the complete frequency band; changing Gunns during an observing night is not encouraged. DSB receiver temperatures of 190 and 220 K are typical for C2 near 460 and 490 GHz respectively. Under excellent conditions total SSB system temperatures can be less than 1000 and 2000 K respectively at these frequencies. Typical system temperatures are usually 2000 and 3000 K respectively, or better, under good observing conditions.

(d) Receiver 'G'

The new SIS version of receiver G was first used at the JCMT in March 1994. Excellent results were obtained. A Gunn oscillator gives continuous coverage across the 650-692 GHz band, which includes both the CO (6-5) and 13CO (6-5) transitions. Observations with G are possible if the zenith opacity at 225 GHz is less than about 0.08.

Receiver 'G' has been available via a collaborative agreement between the MPE group in Garching and the JAC, and observers interested in using it should first contact Dr. L. Tacconi (e-mail LINDA at MPE- GARCHING.MPG.EDU for further information). Because of the collaborative nature of the use of the receiver, it is usually offered for a limited time period only. If the usual pattern of availability is followed it seems unlikely that receiver G will be offered in semester 95B however, and it is likely to be replaced subsequently by the upper band of receiver W (see below).

(e) Forthcoming spectral line receivers.

B3i will ultimately be replaced by a dual-channel receiver (B3) to be delivered in summer 1995 for commissioning in July. It should be available for use in semester 95B. The performance of B3's mixers should be better than that of B3i, with a DSB receiver temperature of typically 100 K from 325 - 360 GHz. The instantaneous bandwidth should be greater, also, but the tuning range is likely to be somewhat less than that of B3i.

Early in semester 95B, a dual-mixer dual-waveband SIS system (Receiver W; 'W' is for 'wide-band optics') for the C- and D-band windows should be commissioned (current estimates place its delivery to Hilo at late summer 1995). Receiver W will likely replace both C2 and G. The goals are for W to cover the ranges 430 - 510 GHz and 625 - 710 GHz. The sensitivity of W should easily exceed that of C2; no estimates can be given yet for the D- band window.

(f) Spectrometer Backends

The Digital Autocorrelation Spectrometer (DAS) has 2048 delay channels having a total maximum bandwidth of 920 MHz in each of two inputs. It is capable of a wide range of configurations, with spectral resolutions of between 0.14 and 1.5 MHz. The widest bandwidth modes are useful only for receivers (such as C2) with sufficient IF bandwidth. 750- and 760-MHz configurations are available to make use of the full IF bandwidths of receivers A2 and B3i. In some cases it is possible to observe several lines simultaneously with good resolution.

The AOSC is an acousto-optical spectrometer which offers a resolution of about 330 kHz and a total bandwidth of 500 MHz for a single IF channel. The AOSC serves as a backup for the DAS. Receiver G has its own dedicated AOS.

Further details regarding both spectrometers are given in the User's Guide.

(g) Approximate rms sensitivities after 30 minutes' integration

Below is a table of the calculated rms noise in Kelvin after a total observation time of 30 minutes (this assumes 15 minutes on source, 15 minutes on a reference position) at selected line frequencies, for three different atmospheric transmissions. In the first case, a 'typical good' value is used for the system temperature, based on the observed distribution of values over the last six months. In parentheses, the expected values of the rms noise are given for 'exceptional' (corresponding to a water vapour pressure of 0.5 mm in all cases), and 'marginal' weather conditions. In the latter case the highest value of water vapour pressure (given in terms of zenith optical depth at 225 GHz; 'tmax' below) at which we recommend observations for the given frequency is used. The behaviour of atmospheric transmission with water vapour pressure impacts strongly on the system temperatures of receivers B3, C2 and G, and conditions worse than 'marginal' render observations unrewarding.

Table 2. Summary of spectral line sensitivities.

Freq. Rec.  Trx    Tsys     dv      tmax        Rms noise (K)
  
(GHz)          (K)    (K)   (MHz)            Typical (good, poor)

230      A2     95    400    0.38    0.20    0.035 (0.026, 0.059)

266              140    600    0.38    0.20    0.052 (0.036, 0.093)
  
331      B3i   265   1500    0.38    0.10    0.130 (0.087, 0.250)

345              180    800    0.38    0.15    0.070 (0.057, 0.220)

461      C2    190   3000    0.38    0.08    0.260 (0.150, 0.860)

492              220   3800    0.38    0.06    0.330 (0.240, 1.020)

661      G    1100  17000    1.00    0.08    0.820 (0.430, 3.400)

692              800  18000    1.00    0.08    0.840 (0.400, 4.500)

Most spectral- line observations are carried out either in position-switched or beam-switched mode. In the first case, the reference position is given relative to the source position. In the second instance, the secondary mirror position is 'chopped' at a rate of 1 Hz, with a throw of typically 1 to 3 arcmin.

For relatively narrow line sources frequency switching is an attractive option. The chief advantage of this technique over either position- or beam-switching is that the telescope never leaves the source position. Thus its use reduces the rms noise by a factor of 1.4 over those values given in the above table for the same total integration time. Frequency switching is implemented in hardware for B3i (and presumably also B3), and via software ('slow frequency switching') for A2 and C2.

Spectral line mapping is usually done in a position- or frequency-switched mode, on a grid of positions. When the lines are strong (say, more than 2 K), it will be possible to use the on-the-fly mode of observing, in which the telescope is scanned at a rate corresponding to an integration time of not less than 5 seconds per point. This method is being developed, and is not offered for regular use at present.

Continuum Observations

(a) UKT14

Until displaced by SCUBA (see below) the UKT14 bolometer system will be available for observations with filters at 2, 1.3, 1.1, 0.85, 0.8, 0.75, 0.6, 0.45 and 0.35 mm. The aperture of the bolometer can be adjusted between 21 and 65 mm. Sensitivities range from typically 0.3 Jy/sqrt(Hz) through to 10 Jy/sqrt(Hz) under good photometric conditions.

The properties of UKT14 using the various available filters and apertures are given in the following table. The wavelength corresponds to the effective value for a precipitable water vapour of 1 mm, and a thermal blackbody (e.g. a planet). The value of the NEFD given is that which should be obtained under 'good' conditions for a 65 mm aperture; users should use this value to estimate time requirements. In parentheses, values for the 'best' and 'poor' atmospheric conditions are given to indicate typical ranges obtained. The aperture which gives a diffraction-limited beam is given in the table; photometry will normally be carried out with full aperture (65 mm). Beam maps are available via anonymous FTP, as noted above.

Photometry is carried out by chopping the secondary mirror at several Hz, with a throw of 1-2 arcmin in azimuth usually. Most mapping with UKT14 is done in a continuous scanning ('on-the-fly') mode.

(b) UKT14 polarimeter

So long as UKT14 is available, the Aberdeen/QMW polarimeter also will be offered as an optional accessory for it. The effective NEFD of the polarimeter/UKT14 combination is slightly worse than NEFD(p) = 2xNEFD/P, where P is the degree of polarisation of the source and NEFD is that for the filter/waveplate in question for UKT14 alone. Observations are possible at 1100, 800, and 450 microns. Additional information appears in the User's Guide.

(c) SCUBA

The submillimetre bolometer camera SCUBA is not expected to arrive in Hilo before late summer 1995. After undergoing tests in Hilo it will displace UKT14 on the left-hand Nasmyth platform, in order to undergo mechanical, electrical and cryogenic fitting. It will not be available for astronomical observations until some time later following a lengthy commissioning period.

Table 3. Properties of the UKT14 bolometer system.

Filter Wavelength  Centre  Band- Aperture  Beam-        NEFD       Notes

                               Freq.  width                 width     (65 mm aperture)

(mm)     (mm)       (GHz)   (GHz)   (mm)      (")          Jy.sqrt(sec)           

2.0      1920            150        40        65         27        0.8 (0.7, 1.2)      1

1.3      1260            233        64        65         19        0.5 (0.4, 1.0)

1.1      1090            264        74        65         19        0.4 (0.3, 1.0)

0.85      850            354        30        47         13        0.8 (0.7, 5.0)

0.8        790            384      103        47         13        0.5 (0.4, 5.0)

0.75      750            411        28        42        13         0.9 (0.7, 5.0)

0.6        625            480      119        36         11        5.0 (3.0,  * )      2

0.45      444            682        84        27          8         5.0 (3.0,  * )      3

0.35      351            864      113        21          6       10.0 (6.0,  * )      3

(1) At 2 mm UKT14 is poorly matched to the telescope.

(2) This filter is best avoided. It is difficult to obtain consistent calibrations, due to deep atmospheric absorption lines in the window.

(3) Observations are not possible under 'poor' conditions at these wavelengths.