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One LABOCA beam is 18”.6 wide and the field of view (FoV) of the complete array covers 11’.4. The array undersamples the sky, with a two beam distance between adjacent pixels, therefore to produce fully sampled maps it is necessary to use scanning observing modes. Moreover, in order to get the sensitivity of LABOCA, the astronomical signal needs to be modulated by scanning with the array across the source, this way producing a signal whose frequencies are mostly above the frequencies of the atmospheric disturbances.
The APEX control system currently supports two basic scanning modes: on-the-fly maps (OTF) and spiral scanning patterns.

On-the-fly maps (OTF)
OTF scans are rectangular scanning patterns with a constant scanning speed, in horizontal or equatorial coordinates. The OTF pattern is typically used to map sky areas much larger than the field of view (FoV, 11.4 arcminutes) of LABOCA.
OTF  patterns  have  been  tested  for maps  on  the  scales  of  the FoV  and for long  slews across the plane of the Milky Way, up to a few degrees.

Spirals
For compact objects or pointing and flux calibrations, the spiral scanning pattern provides the faster method to obtain a fully sampled coverage of the FoV of LABOCA at the required scanning speed. In this mode the telescope scans with a constant angular speed along a spiral, in horizontal or equatorial coordinates. The picture below shows the path of a single bolometer for a spiral scan, plotted over the measured footprint on sky of all functional bolometers of the array.



A single spiral is typically much smaller than the FoV and can be repeated on a raster pattern to increase the sampling density. The picture below shows the path of a single bolometer for a four point raster of spirals, plotted over the measured footprint on sky of all functional bolometers of the array.




Spirals vs. OTF
Small OTFs (FoV of LABOCA) give very comparable results to the raster-spirals but the overheads are much larger at scanning speed of 2’/s. For larger OTFs the relative overheads decrease.




The maximum telescope scanning speed for LABOCA is limited by the time resolution in the position information given by the APEX control system and is about 4’/s. The minimum scanning speed required for a sufficient source modulation depends on the atmospheric stability and on the source structure and is typically about 30”/s. As spirals are done with a constant angular speed, the linear scanning velocity is not constant but increases with time (i.e., with increasing radius). We have selected two spiral modes with 20 and 35 seconds integration time which both produce fully sampled maps for the full FoV with scanning velocities limited between 1’/s and 4’/s. These spirals are the preferred observing modes for pointing scans on sources with flux up to a few Jy. For fainter sources these basic spiral patterns are combined with a raster mapping mode (raster-spirals) with 4 pointings resulting in an even denser sampling of the maps and longer integration time. The raster-spirals lead to excellent results for sources smaller than the FoV of LABOCA and are even suitable for integrations of very faint sources. The advantage of spirals compared to the OTFs is that the scanned area on the sky is only slightly larger than the FoV and the most of the integration time is spent on the central 11 arcminutes. A second advantage is that the overheads from the APEX control system are much smaller for the spirals compared to the OTFs because the time consuming turns of the telescope at the edges of the OTFs do no occur for spiral maps: in a 80 s raster-spirals the overheads are about 25%, in a comparable 80 s OTF of 10 arcminutes the overheads are about 150%.

Pointing
The standard pointing procedure consists of one subscan in spiral observing mode and results in a fully sampled map of the field of view of LABOCA. The pointing offsets relative to the pointing model are computed via a bidimensional Gaussian fit to the source position in the map using a BoA pipeline script. Note that this pointing procedure is not limited to pointing scans of the central channel of the array but works independently of the reference pixel, thus allowing pointing scans centered on the  most  sensitive  part  of  the  array.

Focus
The focus scan is the same as for the spectroscopy receivers. By default we use 10 subscans in 5 different subreflector positions with 5 seconds of integration time each.This is the only observing mode without scanning the telescope across the source, therefore we are currently restricted to sources brighter than the atmosphere (Mars, Venus, Saturn and Jupiter).

Skydips
The attenuation of the astronomical signals due to the atmospheric opacity is determined with skydips. These scans measure the power of the atmospheric emission as a function of the airmass while tipping the telescope from high to low elevation.
A skydip procedure consists of two steps: a hot-sky calibration scan, to provide an absolute measurement of the sky temperature and continuous tip in elevation.

Further details on the LABOCA observing modes are accessible here.