MPIfR Bolometer arrays

The bolometer arrays, MAMBO-1 (37 pixels) and MAMBO-2 (117 pixels), are provided by the Max-Planck-Institut für Radioastronomie. They consist of concentric hexagonal rings of horns centered on the central horn. Spacing between horns is $\simeq 23''$. Each pixel has a HPBW of 11$''$. We expect that MAMBO-2 will be normally used, but MAMBO-1 is kept as a backup.

The effective sensitivity of MAMBO-1 for onoff and mapping observations is $\sim35$ mJys$^\frac{1}{2}$. For MAMBO-2 effective sensitivities of $\sim\!40$ mJys$^\frac{1}{2}$ (ON/OFF mode) and $\sim\!45$ mJys$^\frac{1}{2}$ (mapping mode) were measured. Since in the mapping mode all beams cover the inner region of the map area, MAMBO-2 turns out to be more sensitive if areas of $2'$ and larger are to be mapped (see the Time Estimator). The sensitivities apply to bolometric winter conditions ( $\tau(\small {250{\rm GHz}})\sim$ 0.25, elevation 45 deg, and application of skynoise filtering algorithms). In cases where skynoise filtering algorithms are not or not fully effective (e.g. extended source structure, atmosphere not sufficiently stable), the effective sensitivity is typically about a factor of 2 worse. Such projects should only be observed under atmospheric conditions with low skynoise (i.e. stable atmosphere, no clouds, little turbulence), unless the expected signal is about 1 Jy/beam or stronger.

The bolometer arrays are mostly used in two basic observing modes, ON/OFF and mapping. Previous experience with MAMBO-2 shows that the ON/OFF reaches typically an rms noise of $\sim2.3$ mJy in 10 min of total observing time (about 200 sec of ON source, or about 400 sec on sky integration time) under stable conditions. Up to 30 percent lower noise may be obtained in perfect weather. In this observing mode, the noise integrates down with time $t$ as $\sqrt{t}$ to rms noise levels at least 0.5 mJy.

In the mapping mode, the telescope is scanned in azimuth (also the direction of the wobbler throw) in such a way that all pixels see the source once. A typical single map³ with MAMBO-2 covering a fully and homogeneously sampled area of $150''\times150''$ (scanning speed: $5''$per sec, raster step: $8''$) reaches an rms of 2.8 mJy/beam in 1.9 hours if skynoise filtering is effective. If sky noise filtering is not effective (see Time Estimator), much more time may be needed. The area actually scanned ( $7.3'\times6.5'$) must be larger than the map size by the wobbler throw and the array size ($4'$) if extended emssion is to be properly restored. Shorter scans may lead to problems in restoring extended structure. Mosaicing is also possible to map larger areas. Under many circumstances, maps may be co-added to reach lower noise levels, but this may require vey sophisticated data reduction (please contact the experts). If maps with an rms $\lower.5ex\hbox{$\; \buildrel < \over \sim \;$}1$ mJy are proposed, the proposers must indicate how they plan to reach this ambitious goal.

The bolometers are used with the wobbling secondary mirror (wobbling at a rate of 2 Hz in azimuth). The orientation of the beams on the sky changes with hour angle due to parallactic and Nasmyth rotations, as the array is fixed in Nasmyth coordinates. Bolometer proposals participating in the pool have their observations (maps and ONOFFs) pre-reduced by a data quality monitor which runs scripts in the newly developed MOPSIC. This package, complete with all necessary scripts, is also installed for off-line data analysis in Granada and Grenoble. It is also available for distribution from the IRAM Data Base for Pooled Observations or directly from R. Zylka (zylka@iram.fr). The older software packages (NIC [7] and MOPSI[8]) are still available.

Bolometer proposals will be pooled together like in previous semesters along with suitable heterodyne proposals as long as the respective PIs agree. The web-based time estimator handles well the usual bolometer observing modes, and its use is again strongly recommended. The time estimator uses rather precise estimates of the various overheads which will be applied to all bolometer proposals. If exceptionally low noise levels are requested which may be reachable only in a perfectly stable perfect winter atmosphere, the proposers must clearly say so in their time estimate paragraph. Such proposals will however be particularly scrutinized. On the other extreme, if only strong sources are observed and moderate weather conditions are sufficient, the proposal may be used as a backup in the observing pool. The proposal should point out this circumstance, as it may affect the chance that the proposal is accepted and observed.