LABOCA Calibration

On this page we give an overview over the various topics which are covered under "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.
  We are currently working on a web interface to provide the correct opacity information in an easy and interactive way. Until then, we provide here a file which contains the properly estimated opacities for the period May-December 2007, and which can be read by BoA. The four columns contain scan-number, date (human-readable), date (MJD), and opacity. This file was created and kindly provided by A. Weiss (MPIfR).
  • Opacity file: Laboca_opacities_2007.dat

• 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.

• Secondary calibrators

  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	2.3 ± 0.16
  CRL618	04:42:53.6	+36:06:53.7	5.2 ± 0.5
  V883-ORI	05:38:19.0	-07:02:20.0	1.4 ± 0.3
  N2071IR	05:47:04.85	+00:21:47.1	8.8 ± 0.8
  VYCma	07:22:58.33	-25:46:03.2	1.5 ± 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	12.5 ± 1.3
  IRAS16293	16:32:22.9	-24:28:35.6	15.6 ± 1.3
  G5.89	18:00:30.38	-24:04:00.5	27.6 ± 0.2
  G10.62	18:10:28.66	-19:55:49.7	32.2 ± 1.8
  G34.3	18:53:18.50	+01:14:58.6	53.5 ± 3.7
  G45.1	19:13:22.07	+10:50:53.4	7.8 ± 0.6
  K3-50A	20:01:45.69	+33:32:43.5	14.6 ± 1.4
  CRL2688	21:02:18.80	+36:41:37.7	5.4 ± 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:

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.
During the commissioning phase, several of these maps (so-called beam maps) were observed on the planets Mars and Saturn. After averaging, a final array parameter file was produced. These parameters are implemented in the APEX control system and in the Boa reduction software. The accuracy in position is below 1.5″ and the gain accuracy better than 5% for each bolometer.

• Array parameter file: LABOCA-centred.rcp