HERA

The HEterodyne Receiver Array is expected to be available for most of next summer. The 9 dual-polarization pixels are arranged in the form of a center-filled square and are separated by $24''$. Each beam is split into two linear polarizations (after a successful upgrade in March 2005) which couple to separate SIS mixers. The 18 mixers feed 18 independent IF chains. Each set of 9 mixers is pumped by a separate local oscillator system. The same positions can thus be observed simultaneously at any two frequencies inside the HERA tuning range (210-276 GHz). A derotator optical assembly can be set to keep the 9 pixel pattern stationary in the equatorial or horizontal coordinates. Receiver characteristics are listed in Tab. 1, and an updated user manual (version 1.9) is available on our web page. Frequency tuning of HERA, although fully under remote control and automatic, is substantially more complicated than for the observatory's other SIS receivers. A new tuning tool has been developed which speeds up considerably the DSB and SSB tuning of the 18 mixers. Despite this good progress, there may still be some difficult frequency spots. HERA observers are therefore advised to send a list of their frequencies to Granada at least 2 weeks ahead of their run. Recent observations have shown that the noise temperature of the pixels of the second polarization array varies across the 1 GHz IF band. The highest noise occurs towards the band edges which are, unfortunately, picked up when HERA is connected with VESPA whose narrow observing band is located close to the lower edge of the 1 GHz band. Therefore, while not as dramatic for wide band observations with centered IF band, the system noise in narrow mode is considerably higher (factor 1.5 - 2) as compared to the first polarization array. The problem will be tackled during the next 6 months and improvements will be announced on the HERA page on our Spanish web site. HERA can be connected to three sets of backends:

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VESPA with the following combinations of nominal resolution (KHz) and maximum bandwidth (MHz): 20/40, 40/80, 80/160, 320/320, 1250/640. The maximum bandwidth can actually be split into two individual bands for each of the 18 detectors at most resolutions. These individual bands can be shifted separately up to $\pm200$ MHz offsets from the sky frequency (see also the sections on backends below).

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a low spectral resolution (4 MHz channel spacing) filter spectrometer covering the full IF bandwidth of 1 GHz. Nine units (one per HERA pixel) are available. Note that only one polarization of the full array is thus connectable to these filter banks.

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WILMA with a 1 GHz wide band for each of the 18 detectors. The bands have 512 spectral channels spaced out by 2 MHz.

HERA will be operational in two basic spectroscopic observing modes: (i) raster maps of areas typically not smaller than $1'$, in position, wobbler, or frequency switching modes, and (ii) on-the-fly maps of moderate size (typically $2' - 10'$). Extragalactic proposals should take into account the current limitations of OTF line maps, as described in the User Manual, due to baseline instabilities induced by residual calibration errors. HERA proposers should use the web-based Time Estimator. For details about observing with HERA, consult the User manual. The HERA project scientist, Karl Schuster (schusteriram.fr), or Albrecht Sievers (sieversiram.es), the astronomer in charge of HERA, may also be contacted.