Two bolometer arrays will be provided at APEX.

LABOCA

Description

LABOCA (LArge BOlometer CAmera) will be a bolometer array operating in the atmospheric window at 870 micron (345 GHz). It will have 295 channels arranged in a hexagonal layout consisting of a center channel and 9 concentric hexagonals (Fig #1).The APEX beam size at this wavelength is 18" and the total field of view for LABOCA is 11.4'. Note that the array is undersampled on the sky; the separation between channels in one row is twice the beam size (36"). Therefore it is necessary to use special observing techniques such as scanning or jiggling to produce fully sampled maps (see below).

Fig#1 Mechanical layout of LABOCA

OBSERVING METHODS

To obtain fully sampled maps it is necessary to move the array on the sky during observations. For large maps this can be done by scanning in one direction and stepping in the other, and for small maps (about the size of the array) by making pointed observations in a pattern designed to fill in the gaps between the channels (so called jiggling).

The first LABOCA observations will most likely done by scanning in Az and stepping in El. Thus we will only discuss this scanning method. The observations can be done with a slow scanning speed (a third of the beamwidth/second) simultaneously chopping with the secondary mirror, or by fast scanning (> 80"/s) without chopping with the secondary. The latter observing method is used by SIMBA at SEST. Note that since the maps are taken in Az and El at a given time, the maps rotate on the sky, and maps taken at different times of the same object have different scanning directions on the sky.

SENSITIVITY

In order to estimate the sensitivity of each channel on the sky, we will use the values for the bolometer array SIMBA at SEST, which was manufactured using techniques similar to LABOCA. The sensitivity/channel of SIMBA is roughly 100 mJy s^1/2. APEX is a 12m telescope compared to 15m for SEST, but the aperture efficiency of APEX will be higher because of its better surface accuracy. Therefore we expect to have a similar sensitivity of LABOCA on the sky. Typical zenith opacities at Chajnantor at 870 micron is about 0.1. For observations at 60 degrees elevation this gives a factor of e ^{tau/sin 60°}= 1.12. The channel sensitivity of LABOCA is therefore estimated to 110 mJy s^1/2 outside the atmosphere at typical elevations for observations.

TIME ESTIMATES FOR FULLY SAMPLED MAPS WITH SCANNING IN Az AND STEPPING IN El.

We will here give expressions to estimate the noise in the maps and the time it will take to produce fully sampled maps by scanning the telescope in Az and stepping in elevation (Fig #2). The scan speed in Az is V_Az in "/s and the stepsize, , in elevation is one third of the beamsize for a fully sampled map. The telescope will need some time, t_turn, to turn around for each subscan. The mapsize is AZ x EL. The number of steps in El in a map should be odd in order to assure that a full beam passes over the map centre. The total time to produce such a map is therefore:



The rms noise per beam in the map depends on the channel sensitivity (noise equivalent flux density (NEFD) in mJy s^1/2), the number of channels in the array (N_chan) and the time it takes to scan over one beam.

To further reduce the noise in the final maps, several maps can be added together. The number of maps needed to achieve a final rms is:

In order to have a map of the size of the array covered by all channels (this is the MINIMUM MAPSIZE) it is necessary to scan by twice the size of the array in Az and to step in elevation by the size of the array, in the case of LABOCA about 23' by 11.4' in Az and El.

An observing time calculator is provided in order to calculate the time it will take to observe an area of a certain size to a specific noise level.

Note that a single map should not take more than 1.5h to complete. Otherwise the map may suffer from field rotation and possibly also bad baselines. Adjust the scan speed for small maps, and large maps can be divided into several smaller maps which are later mosaiced to produce the final large map.

Fig#2 Schematic of a typical LABOCA map.
The different parameters shown here determine the time needed to complete it.

EXAMPLES OF MAPS

For studies of deep fields it is necessary to make long integrations using fully sampled maps with sizes similar to the size of the array (minimum sized maps). The map size is therefore 23' by 11.4'. With a scanning speed of 36"/s and a of 6" such a map will take 1.3h to complete, and it will have an rms of 5.2 mJy/beam. By adding together 28 such maps, it is possible to reach an rms noise of less than 1 mJy/beam in 36h.
For studies of star forming regions in the Milky Way it is of interest to map large regions. With LABOCA it is possible to map a field of one square degree in 3.7h with a scanning speed of 180"/s and a of 6". The rms noise in the map will be 12 mJy/beam. The square degree field should be divided into three or four parts mapped separately in order to produce maps with observing times less than 1.5h, or the scan speed can be increased.

350 micron array

APEX will also have a 37-channel bolometer array operating at 350 micron (850 GHz). It will have a typical hexagonal bolometer design.