LABOCA Calibration

Intensity calibration

• Zenith opacities

  The opacity of the sky is usually determined by skydip measurements. However, as of now, all official BoA releases underestimate the opacity resulting from the skydip reduction. The most likely reason for this is the assumption that the sky temperature equals the ambient temperature. This assumption overestimates the sky temperature and therefore underestimates the opacity. All opacities estimated by BoA are too low by a factor of about 1.3. For a more reliable opacity estimate, one has to combine the skydips, the taumeter measurements, and the calibrator fluxes.
  
  
  • LABOCA sky opacities search interface.

• Absolute calibration scale

  The raw data coming from LABOCA just have the detector voltage output as intensity. In order to obtain the intensity in astronomical units, the data have to be multiplied with a conversion factor. This conversion factor depends on the LABOCA bias voltage, which - after several tests - was fixed to 400mV. For this bias the conversion factor is:
  f = 6.3±0.5 Jy/μV
  This factor has been determined on the primary calibrators Mars, Uranus, and Neptune during the commissioning period.

• LABOCA calibrators

  
  
  • LABOCA primary and secondary calibration factors search interface.

  Values in the table below based on the file $BOA_HOME_LABOCA/secondary-fluxes.py, dated 2009-08-23.

  Source	RA(2000)	Dec(2000)	Flux[Jy]
  LKHALF101	04:30:14.40	+35:16:24.1
  HLTAU	04:31:38.45	+18:13:59.0	2.0 ± 0.16
  CRL618	04:42:53.6	+36:06:53.7	4.8 ± 0.5
  V883-ORI	05:38:19.0	-07:02:20.0	1.36 ± 0.3
  N2071IR	05:47:04.85	+00:21:47.1	9.1 ± 0.8
  ALF-ORI	05:55:10.28	+07:24:25.4
  VYCma	07:22:58.33	-25:46:03.2	1.53 ± 0.14
  CW-LEO	09:47:57.38	+13:16:43.6	4.1 ± 0.3
  B13134	13:16:43.15	-62:58:31.6	12.9 ± 1.3
  IRAS16293	16:32:22.9	-24:28:35.6	16.1 ± 1.3
  HD-163296	17:56:21.29	-21:57:21.9
  G5.89	18:00:30.38	-24:04:00.5	27.6 ± 0.2
  G10.62	18:10:28.66	-19:55:49.7	33.0 ± 1.8
  G34.3	18:53:18.50	+01:14:58.6	55.3 ± 3.7
  G45.1	19:13:22.07	+10:50:53.4	8.0 ± 0.6
  K3-50A	20:01:45.69	+33:32:43.5	14.7 ± 1.4
  CRL2688	21:02:18.80	+36:41:37.7	5.5 ± 0.9

  Pointing & Focus

  • LABOCA Pointing Model

    We have established a pointing model for LABOCA, based on measurements of planets, secondary calibrators, and bright QSOs. The rms of this model is 2″ in azimuth and 4″ in elevations.
    During an observing run, sources as weak as 600mJy can be used under good weather conditions to verify the telescope pointing.

  • Focus

    The focus settings in the three subreflector displacement axes (x, y, and z) have been verified by various measurements on Venus, Mars, Jupiter, and Saturn, and are found to be stable within ± 0.2 mm.

  Beam shape & angular resolution

  From beam maps on Mars the angular size of the telescope beam was measured for individual bolometers. After deconvolution by the planet and pixel size we found:

  FWHM = 18.6″ ± 1.0″

  The shape of the main beam is an almost circular Gaussian. At intensities below 1% of the peak, the beam starts to deviate from a Gaussian, with the first error beam pattern at a relative intensity of 0.5%. The support structure of the subreflector becomes visible at the 0.1% level.

  Array parameters

  For a multi-channel receiver, the array parameters include the relative offsets of the individual channels as well as their relative gains. Both can be estimated from fully sampled maps (for all channels) on strong compact objects.
  We have summarized the array parameters for all observing periods from LABOCA commissioning until now. The channel offsets normally are expected to change only if physical work is done at the rerceiver itself (repair, upgrade, or maintenance). However, the relative gains, as well as the properties of individual channels (like cross-talk, excess noise, etc.) can also change with every warm-up of the instrument. The gains depend also on the bias voltages of the amplifier electronics boxes, which may change with time.

  • LABOCA Array Parameters