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\twocolumn[

\Large 
\centerline {A Submillimeter-wave Superheterodyne Camera (SuperCam)}
\centerline {for the Heinrich Hertz Telescope}
\normalsize

\bigskip

We propose to develop the world's most powerful, submillimeter-wave 
heterodyne array receiver for use at the Cassegrain
focus of the Heinrich Hertz Telescope (HHT).
The array will operate in the astrophysically rich, 
870~$\mu$m atmospheric window. At this wavelength the HHT has the
highest aperture efficiency of any submillimeter telescope in the 
world and excellent atmospheric transmission more than 50\% of the
time. The proposed Superheterodyne Camera (SuperCam) will
be an 8x8, integrated receiver array fabricated using leading-edge 
mixer, local oscillator, low-noise amplifier, cryogenic, and digital 
signal processing technologies. When combined with the HHT, the array wil 
be an extremely powerful instrument for probing the history of 
star formation in our Galaxy and the distant Universe.  Indeed, SuperCam
will be an order of magnitude larger than any existing spectroscopic
imaging array at submillimeter-wavelengths and will put the HHT in the
forefront of research and tecnology in this wavelength regime for 
years to come.
\bigskip
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]


\section*{Scientific Justification}

\vspace{-0.6cm}
\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.46,angle=0]{figures/2mass.eps}
\caption{\label{beams} {\small The power of SuperCam:  a definitive
    chemical and kinematic survey of star forming clouds over 270
    square degrees of the sky can be performed in 16
    full days.  The corresponding survey with contemporary single
    pixel receivers would take 4-5 years. }}
\end{figure}


The submillimeter-wave portion of the electromagnetic
spectrum contains the answers to many questions 
concerning the origin and evolution of the Universe, 
galaxies, stars, and planets. Over the past decade,
advances in telescope and detector technology have for the
first time made this regime available to a number of
groundbased observers. The the Heinrich Hertz Telescope (HHT),
the Caltech Submillimeter Observatory (CSO), the James Clerk 
Maxwell Telescope (JCMT), and, in the near future, the
SMA provide the largest apertures for observing at these wavelengths.
Due to the prevailing physical conditions in the interstellar medium, 
the 870~$\mu$m atmospheric window is one of the richest in the 
submillimeter portion of the spectrum. This window also has the highest
atmospheric transmission of any submillimeter band. A large format 
array designed for this wavelength range would make excellent use of 
available telescope time and provide a new perspective on stellar, 
chemical, and galaxy evolution in both present and past epochs (Figure 1).
Over the past few years, SORAL has constructed a small, currently 3 pixel, 
prototype 870~$\mu$m array to test the technologies for the proposed 
larger format system. The prototype array will go into regular operation 
on the HHT in November 2003 and be expanded to 7 pixels next summer.
The sensitivity of the array is excellent, with individual mixers 
having a DSB noise temperature of $\sim55$~K. This prototype array
is now the largest in existence at these frequencies. 
The experience gained with this
system puts the UofA in a unique position to carry array development
to the next stage of evolution.

\vspace{-0.3cm}
\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.24,angle=0]{figures/supercam-smt64.eps}
\caption{\label{beams} {\small The 64 beams of SuperCam overlaid upon
    the Horsehead Nebula (B33).  Each beam will measure a
    high-resolution spectrum, shown at right. }}
\end{figure}


\section*{Instrument Description}

SuperCam will be located in the Apex room of the HHT, just behind the
primary. A pair of retractable, off-axis parabolic mirrors will transform
the incoming light from f\#/13.8 to f\#/4.  Scalar feedhorns will efficiently
couple the f\#/4 beams into eight, 1x8 waveguide mixer arrays. The
separation between adjacent pixels will be $2 f \lambda$. When projected on to
the sky, the 64 pixel array will produce the beam footprint shown in
Figure 2. Each diffraction limited beam will have a
full-width-half-maximum width of $\sim22''$. State-of-the-art
Superconductor-Insulator-Superconductor (SIS) mixing devices will be used
in a balanced arrangement to provide the lowest possible noise performance
for each array pixel. This arrangement advantageously allows
100\% of the available local oscillator (LO)
power to be injected into each mixer using an efficient waveguide
coupler. The downconverted, intermediate frequency (IF) output of each
mixer is then amplified before further processing. The mixer arrays, LO
power splitter, and 1st stage IF amplifiers all reside in a closed-cycle
cryostat. The mixers and LO power splitter are cooled to $\sim4$~K by a
closed-cycle JT refrigerator. The 1st stage IF amplifiers are cooled to
$\sim15$~K by a second closed-cycle refrigerator. A 3D CAD drawing of the
SuperCam array cryostat and reimaging optics is shown in Figure 3. A block
diagram of the instrument is provided in Figure 4. The 64~IF outputs from
the array cryostat enter an IF processor where they are further amplified
and downconverted to a baseband frequency appropriate for the
autocorrelator spectrometer (ACS). The ACS is composed of 8 commercially
available correlator chips, each capable of producing power spectra over a
1.5~GHz bandwidth. For our application, each 1.5~GHz spectrum is divided
into eight, 187~MHz sub-bands. One sub-band is assigned to each array
pixel. At the design center frequency of the instrument (345~GHz), each
array pixel will have 163~\kms of velocity coverage at a velocity
resolution of 0.64~\kms. This velocity coverage and resolution are an
excellent match to the vast majority of Galactic
astronomy projects. For investigations requiring greater bandwidth (e.g.
surveys of external galaxies), the number of pixels can be traded for
increased bandwidth. The computer controlled bias system for the proposed
instrument is essentially identical to that developed for Desert STAR.

As part of the proposed effort, we will augment the existing capabilities
of Steward Observatory by purchasing a high performance micromilling
machine, which will be used to frabricate all the
required mixer and optical components. This machine will continue to be a
valuable asset to the department and University after the proposed project
is completed. All components for SuperCam (LO's, low-noise amplifiers,
correlators, SIS mixing devices, closed-cycle cryostats) are available
commercially or through subcontracts to other universities.

With the implementation of SuperCam, the HHT will have a new, powerful
capability for exploring the origin of stars, galaxies, and planetary
systems like our own. We look forward both to the exciting science that
will come from SuperCam and the challenge of making it a reality.


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.35,angle=0]{figures/345SuperCam-New.eps}
\caption{\label{beams} {\small 3D CAD image of the optical arrangment
    of SuperCam within the Apex room of the HHT. }}
\end{figure}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.16,angle=0]{figures/345SuperCam.10.6.03.eps}
\caption{\label{beams} {\small Block Diagram of SuperCam subsystems. }}
\end{figure}


\end{document}