\typeout{------- receivers --------}
\subsection{Receivers}

This section gives all the technical details of observations with 
the bolometer and the multi--beam receiver HERA. The new single pixel
spectroscopic receiver EMIR is described above.
%in section~\ref{ss:emir}.

%30m telescope that the typical user will have to know. 
%A concise summary of telescope characteristics is published on the
%\htmladdnormallink{IRAM web pages}
%{\wwwes/mainWiki/TelescopeSystemSummary}.

\subsubsection{HERA}

A full description of HERA {\bf HE}terodyne {\bf R}eceiver {\bf A}rray
and its observing modes is given in the \htmladdnormallink{HERA
manual}{\wwwes/otherDocuments/manuals/HERA_manual_v20.pdf}.
Here we only give a short summary.

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 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 for the first polarization, and 210-242 for the second
polarization).

A derotator optical assembly can be set to keep the 9 pixel pattern
stationary in the equatorial or horizontal coordinates. Receiver
characteristics are listed on the \htmladdnormallink{30m web site}
{\wwwes/mainWiki/HeraforAstronomers}.

Recent observations have shown that the noise temperature of the
pixels of the second polarization array may vary 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 important for wide band
observations with centered IF band, the system noise in narrow mode
is higher (factor 1.5 -- 2) as compared to the first
polarization array. We do not recommend
to use the second polarization for frequencies $>241$\,GHz.



HERA can  be connected to three sets of backends:
\begin{itemize}%{$\rhd$}
\item[$\rhd$] 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).
\item[$\rhd$] 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.
\item[$\rhd$] 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 available after successful completion of the current tests.
\end{itemize}

HERA is operational in two basic spectroscopic observing modes:
{\it (i)} raster maps\footnote{\ As long as the NCS raster command is
  not operational, the raster pattern has to be traced out with the
  help of a SIC loop.}
 of areas typically not smaller than $ 1'$, in
position, wobbler, or frequency switching modes, and {\it (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 HERA User Manual, due
to baseline instabilities induced by residual calibration errors.
 HERA proposers should use the web--based
\htmladdnormallink{Time Estimator}{\wwwes/obstime/time_estimator.html}.
For details about observing with HERA, consult the User manual.
The HERA project scientist, Karl Schuster (schuster\mbox{@}iram.fr),
or Albrecht Sievers (sievers\mbox{@}iram.es), the astronomer
in charge of HERA, may also be contacted.

Accepted HERA proposals will be pooled together in order to make more
efficient use of stable 1.3mm observing conditions (see section~\ref{pools}. 
Questions concerning the HERA pool organization can be directed to 
the scheduler (thum\mbox{@}iram.fr) or the HERA Pool Coordinator,
Helmut Wiesemeyer (wiesemeyer\mbox{@}iram.es).

%\subsubsection{EMIR}
%see the special section above


\subsubsection{MPIfR Bolometer arrays}

The bolometer arrays, \Mone\ (37 pixels) and \Mtwo\ (117 pixels),
are provided by the Max--Planck--Institut f\"ur Radioastronomie. They
consist of concentric hexagonal rings of horns
centered on the central horn. Spacing between horns is $\simeq 20''$.
Each pixel has a HPBW of 11$''$.
We expect that \Mtwo\ will be normally used, but \Mone\ is kept as
a backup.

The effective sensitivity of both bolometers for onoff observations is
%\textdefsummerwinter{39}{$\sim35$}
$\sim 40$ mJy\,s$^\frac{1}{2}$ and
$\sim45$ mJy\,s$^\frac{1}{2}$ for mapping. The {\it rms},
in mJy,  of a \Mtwo\  map is typically
$$ rms = 0.4 f \sqrt{v_{scan} \Delta s} $$
where $v_{scan}$, in arc\,sec/sec, is the velocity in the scanning
direction and $\Delta s$, in arc\,sec, is the step size in the
orthogonal direction. The factor $f$ is 1 (2) for sources of size
$<30''\ (>60'')$. It is assumed that the map is made large enough
that all beams cover the source.
%Since in the mapping mode all beams cover the inner region of the map
%area, \Mtwo\ 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
%\textdefsummerwinter{conditions (stable atmosphere}{winter conditions}
conditions (stable atmosphere),
($\tau(\small{250{\rm GHz}})\sim$ %\textdefsummerwinter{0.3}{0.25},
0.3, 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. 
%For  those projects,  only atmospheric conditions with
%low skynoise (i.e. stable atmosphere, no clouds,
%little turbulence) are recommened unless the
%expected signal is about 1 Jy/beam or stronger.
The principal investigators of accepted proposals will be requested to
specify in the pool database which minimum atmospheric conditions
their observations need. 


The bolometer arrays are mostly used in two basic observing modes, ON/OFF and
mapping. Previous experience with \Mtwo\ 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 below 0.4 mJy.

In the mapping mode, the telescope is scanning in the
direction of the wobbler throw (default: azimuth) in such a way
that all pixels see the source once.
% and fully sample the beam.
A typical single map\footnote{\ see also the Technical report by
D.~Teyssier and A.~Sievers on a special fast mapping mode
(IRAM Newsletter No.~41, p.~12, Aug.~1999).}
with \Mtwo\ 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.
Much more time is needed (see Time Estimator) if sky noise filtering cannot be
used.
The area actually scanned ($8.0'\times6.5'$) must be larger than the map size
(add the wobbler throw and the array size ($4'$), the source extent,
and some allowance for baseline determination)
if the EHK--algorithm is used to restore properly extended emission.
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 very sophisticated data reduction
%(please contact the experts).
%Attempts to reach map noise levels below 1 mJy
%are still fraught with poorly understood problems
If maps with an rms$\simlt1$ mJy are proposed, the proposers should
contact R.~Zylka (zylka\mbox{@}iram.fr).

%Another note of caution: mapping of extended sources cannot rely on the
%skynoise reduction algorithm (simple subtraction of correlated sky-noise)
%presently available, and the noise level reached may be at least
%twice as high as that quoted above.

The bolometers are used with the wobbling   secondary mirror
(wobbling at a rate of 2 Hz).
%The wobbling direction which used to be
%fixed in azimuth, can now be freely chosen within some limits (see
%IRAM Newsletter No.~61). This allows in virtually all cases to adapt
%the wobbling/scanning direction to the source under study.
%Nevertheless, t
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 and the wobbler direction is fixed with
respect to azimuth during a scan.
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\mbox{@}iram.fr).
%removed CT 16-Feb-2007: not on GILDAS, 30m decumentation any more
%The older software packages  (NIC \cite{R12} and MOPSI \cite{R13})
%are still available, but %will not be updated.
%Time estimators for planning ON/OFF or mapping observations are also available
%\cite{R12,TE2}.
%cannot process data obtained with the NCS.

Bolometer proposals will be pooled  together like in previous
semesters along with suitable heterodyne proposals as long as the respective
PIs agree.
%Their time requests should be based on
%``bolometric  conditions'',
%like requests using SIS receivers,
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 %\textdefsummerwinter{(quasi winter)}{(perfect winter)}
(quasi 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 affects positively the
chance that the proposal is accepted and observed.

%\subsubsection{The 2mm bolometer}
%DIABOLO which currently employs a linear 3 pixel array at 2mm is designed
%for detection of weak and somewhat extended sources. The instrument which
%was used successfully at the 30m telescope last winter, is
%described by X.~D\'esert
%elsewhere in this newsletter. The level of integration
%of the instrument into the 30m telescope's standard control and acquisition
%system is minimal, however, complicating the use of this instrument.
%Obervers interested in using DIABOLO should contact
%X.~D\'esert at the Observatoire de Grenoble for arranging some form of
%collaboration.
