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\twocolumn[

\Large 
\centerline {Second Annual Report for SuperCam:}
\large
\centerline {A SuperHeterodyne Camera for the Heinrich Hertz Telescope}
\centerline {AST-0421499}
\normalsize

\bigskip
]

\section{Summary}

SuperCam is a 64 pixel, superheterodyne camera designed for operation in the
astrophysically important 870~$\mu$m atmospheric window.  SuperCam
will be used to answer fundamental questions about the physics and
chemistry of molecular clouds in the Galaxy and their direct relation
to star and planet formation.  The advent of such a system will
provide an order of magnitude increase in mapping speed over what is
now available and revolutionize how observational astronomy is
performed in this important wavelength regime. Initially, SuperCam will be used
on the 10~m Heinrich Hertz Telescope (HHT) on Mt. Graham, Arizona to conduct a 
Galactic Plane survey in the $^{12}$CO and $^{13}$CO J=3$\rightarrow$2 lines. A long 
history of water vapor measurements ($>$10 years) indicate the 870~$\mu$m atmospheric window
is available ~50\% of the time from the HHT. Discussions are underway for taking 
SuperCam to a comparable Atacama telescope to conduct the southern portion of the 
Galactic Plane survey.   

Each component of Supercam has been modularized in units of {\bf
  1$\times$8 rows} of the full heterodyne array.  Thus, the mixer blocks, bias 
electronics, IF processors, and spectrometers are quantized into modules that
correspond to 8 detector ``pixels''  each.  This quantization is crucial to the realization
of large heterodyne arrays and has been successfully completed by
the SuperCam team. 

In 2005 we successfully tested a one pixel prototype of SuperCam (see
Puetz et al. 2006). Using this as a starting point, in 2006 our team
completed the design of the optical, cryogenic, RF, and electronics
subsystems for the array.  A single-pixel end-to-end test of all SuperCam
technologies was successfully conducted in August, 2006.  
All the subsystems required to perform an
end-to-end lab test of the first 1x8 row of SuperCam have been built
and tested. 
{\bf With the completion of this test in January 2007, all technological components of
SuperCam will have been demonstrated.  From that point on, we will fabricate the remaining ``rows'' of the array and perform final integration and testing at the HHT.  The project is on
schedule for completion by the end of 2007.  }

The following sections discuss the status of each component of SuperCam in more detail. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.47,angle=0]{../figures/supercam-b33.eps}
\caption{\label{beams} {\small The 64 beams of SuperCam overlaid upon
    the Horsehead Nebula (B33).  Each beam will measure a
    high-resolution spectrum (right).    }}
\end{figure}



\section{Optical System}

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.6,angle=0]{figures/apex-room.eps}
\caption{\label{optics} {\small SuperCam integrated in the HHT Apex Room. }}
\end{figure}

\subsection{ Telescope Matching }

SuperCam will initially reside on the HHT.  The
goal of the instrument's optical system is to produce an 8x8 array of
diffraction limited beams on the sky with $<$1\% overlap in their
power patterns (Figure~\ref{beams}). This can be achieved if the separation
between pixels in the instrument focal plane is 2.5$\;f\; \lambda$.  A
faster beam makes the focal plane more compact, but produces more
rapidly diverging beams.  The lower limit to the focal ratio is set by
the requirement that the emerging beams clear the cryostat window at
the 3$\omega$ level ($\omega$ is the Gaussian beam waist). For our
application an f/5 beam is a good compromise, yielding a pixel
separation of 11~mm in the focal plane. Re-imaging optics are
necessary to reduce the f/13.8 beam of the HHT to f/5. In
Figure~\ref{optics} is a 3D CAD drawing of a re-imaging system that
fulfills these design requirements and fits comfortably within the
apex room (located just behind the primary) of the HHT where SuperCam
will reside. The design was first made using Zemax and then optimized
with the more sophisticated ASAP optical design package.  

The dewar and optical system will be mounted in a common frame capable
of being positioned in {\tt x} and {\tt y}, permitting the beams to be accurately
centered on the secondary. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{figures/DewarMount.eps}
\caption{\label{mount} {\small SuperCam dewar and Virginia Diodes 
LO (with 1x8 power splitter)  in their combined support frame. }}
\end{figure}

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.66,angle=0]{figures/lo-splitter.eps} 
\includegraphics[scale=0.66,angle=0]{figures/lo-splitter-zoom.eps}
\caption{\label{splitter} {\small 1x8 LO power splitter block, and
    zoom-in on a single waveguide junction}}
\end{figure}



\begin{figure*}[!htb]
\centering
\includegraphics[scale=0.55,angle=0]{figures/1x8_prototype_sm.eps}
%\includegraphics[scale=0.5,angle=0]{figures/mixerblock-zoom.eps}
\caption{\label{mixerarray} {\small (left) Photograph of completed 1x8 mixer
    array. }}
% (right) Zoom-in on the SIS device channel. }}
\end{figure*}


\begin{figure*}[!htb]
\centering
\includegraphics[scale=0.65,angle=0]{figures/mixer-block-cad.eps}
\includegraphics[scale=0.79,angle=0]{figures/mixers-assembled.eps}
\caption{\label{blockcad} {\small (left)  1x8 SuperCam mixer block
    exploded view.  (right) Fully assembled focal plane array. }}
\end{figure*}
                                                    
\subsection{Local Oscillator (LO) Injection}

Several local oscillator (LO) injection schemes were considered,
including waveguide cross-guide couplers. The large number of pixels
makes waveguide coupling very cumbersome and difficult to
fabricate. We are fortunate that high-powered LO chains ($>$1mW) using
commercially available frequency multipliers permit simple,
quasi-optical coupling into the signal beam with a dielectric beam
splitter. In a ``standard'' quasi-optical beam splitter Mylar is often
used as the dielectric. In such instances of order 1\% of the LO power
is injected into the cryostat, with the balance being terminated in an
absorbing load.  In order to conserve LO power, SuperCam will employ a
precision polished silicon etalon whose thickness is tuned for it to
serve as a Fabry-Perot diplexer at the frequency of interest. Such
diplexers are capable of injecting $\sim$50\% of the LO power into the
cryostat while passing $\sim$98\% of the sky signal (Mueller \& Waldman
1994). The primary science goal of SuperCam is to map the Galactic
Plane in $^{12}$CO and $^{13}$CO. A silicon etalon will be fabricated
for each of these line frequencies. 


An 8$\times$8 way waveguide, corporate power divider will be machined
(using the Kern micro-milling machine purchased largely through this
grant) to produce an independent LO beam for each pixel. One row of
the LO power splitter is shown in Figure~\ref{splitter}. The measured output power
from each horn is in excellent agreement with theory. After
encountering the silicon diplexer, the combined LO and signal beams
enter the cryostat through an AR-coated, crystalline quartz vacuum
window. They then pass through a cooled Goretex GR infrared filter before
entering an 8x8 array of feed horns.  

\section{ Mixer Arrays}
                                                  
\subsection{Design Approach}

SuperCam will utilize eight, 1$\times$8 rows of integrated mixer
arrays. For large ($>$25 pixels) format arrays like SuperCam, the
relative ease of assembly and low-cost of single-ended, DSB mixers
make them preferable over single-sideband
approaches. Figure~\ref{mixerarray} is a
photograph of the bottom half of a 1x8 mixer array that was machined
in 1 day using the Kern micro-milling machine at the UofA. The mixer
module is shown assembled with waveguide feed horns, IF MMIC modules,
and DC bias board. 3D renderings of an exploded view of a mixer module
and the fully assembled array are shown in Figure~\ref{blockcad}.


The signal/LO beams are gathered by the feed horns and launched into
half-height rectangular waveguide. The waveguide carries the energy
around a 90 degree bend to the SIS junction (fabricated by Arthur
Lichtenberger at the University of Virginia) and fixed backshort. The first single pixel
mixer prototype (Puetz et al 2006) utilized the same quartz substrate
SIS devices used in Desert STAR. In the final version of SuperCam, the
SIS junctions will be fabricated with beam leads on thin silicon (SOI)
substrates, making device mounting more straightforward and
reliable. SuperCam test devices (see Figure~\ref{SOI}), with everything but
the SIS device itself, were delivered and tested for form and fit in
the summer of 2006. These tests were successful, showing that a SOI
device can be mounted and wire bonded easily into the
block. Fabrication of SOI SIS devices is underway.  The first
Supercam device wafer was fabricated at the Univ. of Virginia in early
December 2006 up to the final thinning and patterning of the backside
of the wafer.  The layout was designed
so that a full thickness portion of the wafer can be diced off and
electrically dip tested in liquid helium before proceeding to the
final thinning and chip definition process. The electrical
characteristic of a typical mixer from this dip test is shown in
Figure~\ref{IV}.   Delivery of the devices to Arizona is expected in January
2007, once the final thinning and chip definition process have been completed. 
The 4-6 GHz intermediate frequency (IF)
output of each junction is then wire bonded to the input of a custom,
low-noise, MMIC amplifier module designed by Sandy Weinreb and his
group.  A close-up of the module design and performance data are
provided in Figure~\ref{MMIC}.

The module produces $\sim$35dB gain at a noise temperature of $\sim$5K over
SuperCam's IF range (nominally 4-6 GHz). Besides having excellent
performance, the MMIC module has a power dissipation of $<$9mW which
permits it to be integrated directly into the mixer block, adjacent to
the SIS junction as shown in Figure~\ref{mixerarray}. The outputs of the MMIC modules
are wire bonded to a multilayer PC board that carries the signals via
microstrip to a 1x8 array of miniature, push-on (GPPO) connectors
located on the back of the module. DC bias for the SIS junctions, MMIC
modules, and electromagnets (required to suppress the Josephson
effect) are provided by multi-pin edge connectors and conveyed to the
components through underlying layers in the PC board. All electrical
connections to the 1x8 mixer subarray are made through connectors on
the rear of the unit, allowing it to be removed without disturbing the
cryogenic wiring harness. 


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{figures/soi-device.eps}
\caption{\label{SOI} {\small SuperCam SOI substrate test device with beam leads.  }}
\end{figure}



\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{figures/soi-ivcurve.eps}
\caption{\label{IV} {\small  Sample I-V curve of a Supercam SOI
    SIS junction at the Univ. of Virginia.   The electrical
    characteristics are excellent; measured junction parameters are very close to the design.  }}
\end{figure}

                               
\subsection{ Prototype Mixer Tests}

In July 2005 a prototype, single pixel SuperCam mixer was tested. The
mixer, shown in Figure~\ref{proto} is a DesertSTAR spare that was
modified to accept a SuperCam style MMIC module. The RF design of the
modified mixer is operationally identical to that of a SuperCam
mixer. Key concerns were whether the presence of the MMIC module so
close to the SIS junction would cause unwanted heating (which could
reduce device sensitivity) or instabilities. However, neither of these
problems materialized and the mixer worked well the first time.  Plots
of the mixer performance are shown in Figure~\ref{protoperf}. Even with an
un-optimized IF impedance match, the mixer was low-noise and stable. A
paper describing the test results in detail has been published in the
Journal of Infrared and Millimeter Waves (Puetz et al. 2006). 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.65,angle=0]{figures/proto1.eps}
\includegraphics[scale=0.8,angle=0]{figures/mmic-perf.eps}
\caption{\label{MMIC} {\small (TOP) 4-6 GHz MMIC Module.   (BOTTOM)
    MMIC performance plots show $>$30 dB of gain from 1-14~GHz. }}
\end{figure}

\begin{figure}[!hbt]
\centering
%\includegraphics[scale=0.7,angle=0]{figures/proto1.eps}
\includegraphics[scale=0.7,angle=0]{figures/proto2.eps}
\caption{\label{proto} {\small Photograph of the single-pixel prototype SuperCam mixer }}
\end{figure}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{figures/protoperf.eps}
\caption{\label{protoperf} {\small Performance curves for the prototype
    SuperCam mixer using a spare DesertSTAR SIS junction. }}
\end{figure}

\section{ Cryostat}

The SuperCam cryostat is shown with its LO system in
Figure~\ref{mount}.  The cryostat has a 150mm diameter AR coated, crystalline quartz
window, is 20 inches on a side, and utilizes two closed-cycle
refrigerators: a Sumitomo RDK-415D to cool the mixer array and a CTI
350 to take the heat load of the 64 stainless steel coaxial (0.085 in)
cables. Laboratory measurements indicate the system has a 50\%
margin in thermal load capacity. IF connections to the outside are
made via 64 SMA style, hermetic, bulk-head connectors. Twelve
multi-pin connectors are used to convey DC bias signals in and out of
the cryostat. The cryostat is now wired for conducting tests with the
first 1x8 row of the array. 
 

\section{IF Processors}

The 4-6~GHz IF signal emerging from the SuperCam dewar must be filtered, downconverted to
baseband, and amplified to 0~dBm to be properly conditioned for the
input of the spectrometer system.   Sander Weinreb and his graduate
students Joe Bardin and Glenn Jones have designed and constructed a
fully-tested 2-channel prototype and the first 1$\times$8 modular row
of IF processors for SuperCam.   The IF processor provides each channel
an initial 48 dB of gain, a variable (0-31 dB) digital attenuator, a
selectable 256~MHz or 512~MHz bandpass filter, a mixer conversion to
baseband, low-pass filter and 50 dB of baseband gain.   An additional 
circuit provides total-power measurement of the IF power for telescope
pointing and continuum measurements.   A picture of the 8-channel IF
processor module and a sample 512 MHz bandpass is shown in
Figure~\ref{IFproc}.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{figures/IFproc2.eps}
\includegraphics[scale=0.8,angle=0]{figures/IFproc1.eps}
\caption{\label{IFproc} {\small 8-channel IF processor module and
    sample bandpass}}
\end{figure}

\section{FFT-in-FPGA Spectrometer}

Science drivers for the SuperCam spectrometer stipulate two principal
requirements. Sufficient kinematic resolution of molecular cloud
components in CO emission will only be achieved with frequency
resolution of finer than 512~KHz per channel, with 256~KHz
preferred. The divergence of the velocity field of the inner Galaxy
requires a bandwidth of 256~MHz or greater.  
{\bf Rapid advances in digital logic have made the spectrometer solution
even more powerful than envisioned originally. }  Though we had
originally baselined a 64$\times$128~MHz 2-bit autocorrelator system
from Spaceborne Inc., recent gains in high-speed ADCs and FPGAs have
made a direct-digitization spectrometer economically feasible.  In
this scenario, the entire IF signal is digitized in real time with high 8-bit
fidelity (using multiple Atmel 1Gs ADCs) and a Fast Fourier Transform
is then performed by a (Virtex4) FPGA.   
Omnisys Inc. was consigned to construct a prototype spectrometer card
capable of processing a total of 2~GHz of bandwidth, software-selectable
between 8-input mode (256~MHz per input), 4 input mode (512~MHz each), and
2-input mode (1~GHz each). 
Initially, we will power-combine in pairs the beams
from an entire row to a single spectrometer card, yielding 256~MHz of
bandwidth per beam, and thus requiring 8 spectrometer cards in
total.   A future upgrade path will be to
install 8 more boards, for 512~MHz of bandwidth per channel.  
The performance of this spectrometer card
has proven to be exceptional (Figure~\ref{specperf}).  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.7,angle=0]{figures/speccard.eps}
\caption{\label{speccard} {\small SuperCam spectrometer card as
    constructed by Omnisys Inc.  The board features 2~GHz of
    bandwidth, software selectable between two 1~GHz inputs, four
    512~MHz inputs, or eight (power combined) 256~MHz inputs. }}
\end{figure}

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{figures/specperf.eps}
\caption{\label{specperf} {\small Allan variance of the prototype
    SuperCam spectrometer is $>$2000 seconds with a reference noise
    source as its input.  }}
\end{figure}

\begin{figure*}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{figures/supercam-bias-noshield.eps}
\hfill
\includegraphics[scale=0.78,angle=0]{figures/supercam-bias-shielded.eps}
\caption{\label{bias} {\small (LEFT) 1$\times$8 bias electronics card
    exposed in its test box.  (RIGHT) Bias electronics card with top
    shield installed and controlling Single Board Computer attached.}}
\end{figure*}


\section{Electronics}

The control electronics for the biasing of the SIS detector array, and
its attendant electromagnets and MMIC Low Noise Amplifiers is a
consolidation of the successful electronics implementations used for
previous DesertSTAR and PoleSTAR heterodyne arrays.  A single
6U electronics bias board controls all functions of an entire 8-pixel
row of Supercam (Figure~\ref{bias}).  Operation of the bias board
prototype has been completely validated.   All 1$\times$8 preamp
boards for the full 8$\times$8 array have been provided by
UMass/Amherst; a picture of one preamp board in a test box is shown in
Figure~\ref{preamp}. 
Remaining work on the bias electronics involves the fabrication of the final
bias electronics boards, acquisition and installation of electronics
components, integration into the final  electromagnetically-shielded
rack-mounted case, and the completion of array control software.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{figures/supercam-preamp.eps}
\caption{\label{preamp} {\small 1$\times$8 preamp board from UMass/Amherst mounted in its test box. }}
\end{figure}


\section{ Data Pipeline}

A schematic of the SuperCam data pipeline is shown in Figure~\ref{data}. The
spectrometer will have a dedicated, 1~Gb ethernet connection to a
data acquisition/spectrometer (DAS) control PC.  The DAS PC will have
two ethernet cards and a RAID5 array for local data storage and fault
tolerance.   Five $>$500 GB drives will provide $>$2.5 TB storage.  During
the Galactic Plane survey the data rate will be 400 GB/day. The DAS
disk array will provide at least a week of raw storage at 90\% duty
cycle.  

The DAS PC will stream the raw data to a background process that will 
be responsible for regridding the data into a more manageable format; 
approximately 200 GB for the entire Supercam Galactic plane survey.
The data processing task will also be responsible for determining the
quality of the data and flagging bad OTF scans that need to be
repeated.   Quality will be assessed by evaluating the RMS
noise after subtracting the baseline from each spectrum  and comparing
with that expected from T$_{sys}$.  The data processing task
will spool pre-processed data images to the telescope control
computer(s) for the observer to see, upon request.  The HHT telescope
control computer in return will send data acquisition requests to the
DAS PC, thus closing the communications loop. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.75,angle=0]{figures/pipeline.eps}
\caption{\label{data} {\small Schematic of SuperCam data pipeline }}
\end{figure}

\section{Integration and Testing}

Integration and testing of the Supercam array is second in importance only to its
fabrication!    The one-pixel Supercam prototypes underwent a complete
end-to-end test, from Supercam test mixer with integrated MMIC LNA, to
IF processor, to Omnisys FFT spectrometer.    In addition to the tests
performed earlier in Puetz et al. (2006), the entire chain of IF
technologies were validated and assessed, with ``first spectral
light'' and stability tests of the IF chain system performed
(Figure~\ref{end2endresults}).   A picture of the overall lab
configuration is shown in Figure~\ref{end2end}.   The same analysis
will be performed for the 1$\times$8 mixer blocks in the SuperCam
cryostat in January 2007, and again for the full 8$\times$8 mixer
array near the end of 2007.   At the conclusion of lab tests,
integration of the optical system to the HHT telescope and data system
to the HHT telexcope control system will be perfomed, so that first
light will be achieved during the winter of 2007-8.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.8,angle=0]{figures/spec-line.eps}
\includegraphics[scale=0.75,angle=0]{figures/specperf2.eps}
\caption{\label{end2endresults} {\small Sample results from the end-to-end
    SuperCam test in August 2006:  (TOP) 
    First
    spectral light of an injected line at 346 GHz as detected by the
    SuperCam FFT spectrometer.  (BOTTOM) Allan-variance of the
    combined Caltech
IF processor and Omnisys FFT spectrometer.  Classical spectroscopic Allan time is
650~seconds. }}
\end{figure}


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.53,angle=0]{figures/end2end.eps}
\caption{\label{end2end} {\small Lab setup of the August 2006 end-to-end test of key SuperCam
    technologies, from an integrated SuperCam mixer module in a test dewar, to
    prototype IF processor and FFT spectrometer.   }}
\end{figure}

\twocolumn[

\section*{References}

\begin{enumerate}

\item Puetz, P., Hedden, A., Gensheimer, P., Golish, D., Groppi, C.E.,
  Kulesa,  C., Narayanan, G., Lichtenberger, A., Kooi, J.W., Wadefalk,
  N., Weinreb, S., \& Walker, C.K.,  ``345 ~GHz Prototype SIS Mixer
  with Integrated MMIC LNA'',  {\it  International Journal of Infrared
  and Millimeter Waves}, 2006, in press

\medskip

Reprint at:  

{\tt http://soral.as.arizona.edu/supercam/puetz2006.pdf}

\item Mueller, E., \& Waldman, J., ``Power and Spatial Mode
  Measurements of Sideband Generated, Spatially Filtered,
  Submillimeter Radiation'', {\it IEEE Transactions on Microwave
    Theory and Techniques}, 42, 1891.

\medskip

Reprint at: 

{\tt http://stl.uml.edu/PubLib/Mueller,\%20Power\%20and\%20Spatial.pdf}


\end{enumerate}


]

\end{document}


