JCMT Beam Efficiencies

Looking to the results of aperture and beam efficiency measurements from planet observations (derived following the recipes in Section 9.3 of the Userguide) in the database one can see a large scatter.

Below are figures showing the beam efficiencies derived from planet observations since 1998 with RxA3i, RxB3, and RxW for Mars, Jupiter, and Uranus. The blue points were obtained between HST 9h30 - 17h30 (extended observing), and the red points in intervals 2.5 hours before 9h30 and after 17h30 when the telescope surface and atmosphere (seeing) are known to be worse than during the night.

Surface changes as a function of HST were investigated in June-November 2004 and in February 2005 (when they were reduced by cooling of the centerbeams in the cabin).

While the plots from the database do not contain known bad spectra or Hetpol measurements the datafile used to make the figures below was not cleaned (maybe this will be done in the future), but this does not affect the results significantly.

In most cases the derived efficiencies are higher during the night than during the day (in particular for Uranus with RxB3), although there are many datapoints with lower values also during the night. This is consistent with results of SCUBA FCF data (see e.g. here for 850 micron results - similarly for 450 micron).
Because of the scatter only typical (rather than average) values for the efficiencies are given in the following table. For comparison this table also shows the results of Moon measurements.

These numbers can deviate somewhat from those in the Userguide. The origin of the latter values is not clear in all cases and may be coming from a single measurement.

Every observer may want to choose his or her appropriate values from the data. However it is obvious that single measurements made during one night cannot be considered reliable.

The planet efficiencies are derived interpolating beamsizes and planetary temperatures in the file scuba.dat from the program fluxes.
It is assumed that the beamsize is equal to the theoretical beamsize. However in reality it can be broadened by focus errors or bad seeing conditions. There can also be small beam size variations due to reflections in the receiver (not included in the used values). The RxA2 beam size followed a predicted 1/f curve but also had a sinus variation with an amplitude +/-0.5" and a period of ~20GHz on top.
Although pointing is usually done before efficiency measurements, errors (e.g. due to anomalous refraction - see e.g. Olmi (2001)) cause the efficiencies to be underestimated.
Also calibration problems can cause scatter and systematic deviations in the derived efficiencies due to errors in load temperatures and opacity effects (the calibration is less accurate when the opacity is high).
And of course errors (e.g. wrong chop frequency) will affect the results. See also Section 9.2 of the Userguide.

When the planet diameter is smaller than the beamwidth of the telescope, the derived efficiencies are the main beam efficiency. This is always valid for Uranus, and sometimes for Mars. For larger diameters one would expect the efficiency to increase to the Moon efficiency.
To investigate this for Mars, in the last figure the green circles indicate observations made when the Mars diameter is larger than 14" (the approximate HPBW for RxB3). The efficiencies derived for these datapoints might be larger than for other data, however the difference between day/night observing confuses any result (when the diameter of Mars is large it will be observed at night).
For RxA3 the efficiencies derived for Mars, Jupiter, Uranus and the Moon are approximately equal (with those from the Moon and Jupiter even smaller than those derived from the smaller sources Mars and Uranus).
Also for RxB3 the Jupiter and Uranus efficiencies are equal, which might be due to errors in the assumed physical temperatures.

RxA3 Mars

RxA3 Jupiter

RxA3 Uranus

RxB3 Mars

RxB3 Jupiter

RxB3 Uranus

RxW-C Mars

RxW-C Jupiter

RxW-C Uranus

RxW-D Mars

RxW-D Jupiter

RxW-D Uranus

RxB3 Mars