LABOCA calibration

Intensity calibration

Zenith opacities

The opacity of the sky is usually determined by sky-dip measurements. However, all official BoA releases underestimate the opacity resulting from the sky-dip 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 sky-dips, the τ-meter measurements, and the calibrator fluxes.

You can search in our database the sky opacities in the different observing runs with LABOCA. Please visit the 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 calibration sources

Check here the primary and secondary calibration factors search interface. Values in the table below based on the file $BOA_HOME_LABOCA/, dated 2009-08-23.

Source nameRA (J2000)DEC (J2000)Flux [Jy]± sigma
HLTAU04 31 38.45+18 13 59.002.0 ± 0.2
CRL61804 42 53.60+36 06 53.704.8 ± 0.5
V883-ORI05 38 19.00-07 02 20.001.4 ± 0.3
N2071IR05 47 04.85+00 21 47.109.1 ± 0.8
VYCma07 22 58.33-25 46 03.201.5 ± 0.1
CW-LEO09 47 57.38+13 16 43.604.1 ± 0.3
B1313413 16 43.15-62 58 31.6012.9 ± 1.3
IRAS1629316 32 22.90-24 28 35.6016.1 ± 1.3
G5.8918 00 30.37-24 04 00.4027.6 ± 0.2
G10.6218 10 28.66-19 55 49.7033.0 ± 1.8
G34.318 53 18.50+01 14 58.6055.3 ± 3.7
G45.119 13 22.07+10 50 53.408.0 ± 0.6
K3-50A20 01 45.69+33 32 43.5014.7 ± 1.4
CRL268821 02 18.80+36 41 37.705.5 ± 0.9

Pointing and 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 sub-reflector 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 and angular resolution

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

FWHM = 19.2″ ± 0.3″

The shape of the main beam is an almost circular Gaussian. At intensities below 3% of the peak, the beam starts to deviate from a Gaussian, with the first error beam pattern at a relative intensity of 1%. The support structure of the sub-reflector becomes visible at the 0.1% level.

The main beam area of the APEX/LABOCA beam is 420″² ± 11″², while the total beam area is 518″² ± 21″². This may lead to an overestimate of the total flux density of up to 20% for extended sources, and should be taken into account by the PI during the calibration of his/her data.

We provide a FITS file with the combined LABOCA beam pattern, based on these 30 Uranus beam-maps (courtesy of A. Weiß, MPIfR).

Note: A previous version of this section mentioned slightly different numbers. However, that numbers were obtained during the LABOCA commissioning, included less data sets (and only from Mars), and are therefore considered less reliable. The new numbers come from 30 Uranus beammaps obtained between 2007 and 2011.

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 summarised 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 receiver 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.