HERA

The HEterodyne Receiver Array is expected to be available for most of next summer. The 9 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) which couple to separate SIS mixers. The 18 mixers feed 18 independent IF chains. Each set of 9 mixers is pumped by a seperate 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 (of the single polarization system) are listed in Tab. 1, and an updated user manual (version 1.7) 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. Although the tuning is still only known for a few frequencies, (the 3 CO isotopes at 230.5, 220.4, and 219.6 GHz; CS at 244.9 GHz; HCN at 265.9 GHz; HCO$^+$ at 267.6 GHz; DCN and HC$^{15}$N at 217.2 and 259 GHz; H$_2$CO at 225.7 GHz; H30$\alpha$ at 231.9 GHz), HERA proposals for any frequency within the nominal tuning range of 210 - 276 GHz will be considered, but we cannot guarantee at this moment that these proposals can actually be done. In any case, HERA observers should send the list of their frequencies to Granada as early as possible.

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. WILMA will be availble after successful completion of the current tests.

HERA is 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. Karl Schuster (schuster@iram.fr), the HERA project scientist, or Albrecht Sievers, the astronomer in charge of HERA (sievers@iram.es), may also be contacted.

Given the risk involved in the upgrade and considering that some relevant observational parameters are not yet well known for the full array, we request HERA proposers to use the parameters of the present single polarization array when estimating integration times. As mentioned above, the times scheduled for the successful proposals may then be adjusted, once the parameters of the full array are known.