Calibration
- RxB3 is calibrated in the normal way for JCMT heterodyne
receivers, with chopper wheels providing a three-load calibration using
the sky, ambient and cold loads. The normal calibration preocedure is the
"backend cal" which does a channel-by-channel calibration in the DAS.
The result of such a calibration, as plotted in a SPECX scan, is an antenna
temperature scale commonly denoted by T(A)*. Line strengths in this
scale have been corrected for atmospheric transmission and losses associated
with telescope inefficiences and rearward scatter. Conversion to a
source radiation temperature requires further corrections for the forward
spillover and scattering efficiency eta(fss) (see below) and geometric
coupling of the main beam on to the source.
For the most reliable and repeatable calibrations it is better to tune
in SSB mode. In DSB mode the calibration algorithm assumes equal gain
in both the image and signal sidebands, which may not be true for either
or both mixers.
Comparison with RxB3i, Standards, and Relative Performance of
Mixers A&B
- There is not yet a complete set of standard lines available
for RxB3. This is partly because the increased "parameter space" associated
with a dual channel receiver operating in either SSB or DSB has made the
collection of data time-consuming.
Users interested in comparisons of RxB3 and RxB3i line strengths should be
aware of the 'standard' utility. Typing 'standard' at any JCMT UNIX machine
will invoke a menu-driven utility that prints and (optionally) plots all
the available measurements of a selected standard line source observed with
B3i and B3. These data are also accessible through the JCMT Home Page
on the Web.
During the first several months (i.e. Dec-June '97) of operation, Mixer
A systematically produced line strengths (i.e. T(A)* values) that were
10-15% on average higher than produced by Mixer B. When averaged
together in SSB operation, the lines were about 15% stronger than
comparable RxB3i measurements. This difference from RxB3i is at least
partly attributable to higher main beam efficiencies in RxB3.
During this period there was also an indication that both mixers produced
stronger lines in DSB mode than in SSB, although this comparison may be
distorted by the statistics of a small number of measurements. There
does seem to be repeatable evidence that, for the CO/HCN line pair observed
in DSB, the sideband gain ratio is not equal to 1 for the two sidebands.
Experience with our other intrinsically DSB receivers clearly has shown
that the sideband gain ratio changes from tuning to tuning. It's not yet
clear how repeatable the gain ratios are for RxB3 or how they change with
frequency. Most observers are choosing to observe with the receiver in
SSB mode because of more accurate calibrations and better Tsys values.
In June '97 it was discovered that the Observatory's IF switch that
downconverts RxB3's 4 GHz IF to the 1.5 GHz required by the DAS was probably
operating in a partially saturated mode during the first 6 months of RxB3's
time on
the telescope. We also made some refinements at that time to the effective
RxB3 load values used in the calibration software.
Since these changes line strengths measured with the two mixers are now
similar and the SSB and DSB measurements yield strengths that are closer
together.
Beamwidths, Pointing and Focus
- The differential pointing offsets between the beams of the two mixers
has been repeatedly measured to be less than 2 arc seconds. No significant
differences have been found between the foci of the two beams.
Efficiencies and beam widths
These values are at 345 GHz.
Efficiency Chan. A Chan. B
eta(beam) 0.62±0.03 0.62±0.03
eta(tel) 0.88±0.03 0.88±0.03
eta(fss)* 0.82±0.04 0.78±0.04
eta(apert)
HPBW 13.7±0.4" 14.0±0.4"
* Assuming Tr(full moon) = 352K.
The most recent beam maps are consistent with round beams.
Beamwidth measurements (courtesy G. Sandell) show no
significant differences between beam sizes or shapes in DSB and SSB
modes. Calculated FWHM beamwidths assuming diffraction-limited
theory, based on beam maps taje June 1998 are as follows
Mixer Frequency(GHz) Deconvolved HPBW"
B 330 14.6±0.04
B 345 14.0±0.04
B 362.8 13.3±0.04
A 330 14.3±0.03
A 345 13.7±0.03
A 362.8 13.0±0.03
Beam maps taken on Mars at an LO frequency of 341 GHz are shown in
Fig
2a and Fig
2b . Mars had a diameter at the
time of 8 arcsec. Contour levels in these maps are 2%, 4%, 8%,
12%.... of the peak, and the shaded area represents a 14 arcsec
diameter disc.
Original Text by Lorne Avery, 1 July 1998
