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\author{C.~K.~Walker\supit{a}, C.~A.~Kulesa\supit{a}, 
D.~Golish\supit{a}, A. Hedden\supit{a}, K.~Jacobs\supit{b}, J.~Stutzki\supit{b}, J.~R.~Gao\supit{c}, 
J.~Kooi\supit{d},  D.~Glaister\supit{e}, W.~Gully\supit{e}, 
I.~Mehdi\supit{f}, M.~Swain\supit{f}, P.~Siegel\supit{f} 
\skiplinehalf
\supit{a} University of Arizona, 933 N. Cherry Ave, Tucson, AZ, USA; \\
\supit{b} Universit\"at zu K\"oln, K\"oln, Germany;\\
\supit{c} SRON, National Institute for Space Research, Groningen, The Netherlands;\\
\supit{d} California Institute of Technology, Pasadena, CA, USA;\\
\supit{e} Ball Aerospace, Boulder CO, USA;\\
\supit{f} Jet Propulsion Laboratory, Pasadena, CA, USA;\\
}

%>>>> Further information about the authors, other than their 
%  institution and addresses, should be included as a footnote, 
%  which is facilitated by the \authorinfo{} command.

\authorinfo{Further information: Send correspondence to
  cwalker@as.arizona.edu or ckulesa@as.arizona.edu}
%%>>>> when using amstex, you need to use @@ instead of @
 

\title{Forecast for HEAT on Dome~A, Antarctica:\\ The High Elevation Antarctic Terahertz Telescope}

\begin{document}
\maketitle 

\begin{abstract}
We have proposed to develop a prototype 0.5-meter far-infrared telescope and
heterodyne receiver/spectrometer system for fully-automated remote
operation at the summit of Dome~A, the highest point on the Antarctic
plateau.  The unparalleled stability, exceptional dryness, low wind
and extreme cold make Dome~A a ground-based site without equal for
astronomy at infrared and submillimeter wavelengths.  HEAT, the High
Elevation Antarctic Terahertz Telescope, will operate in the 
atmospheric windows between 150 and 400~$\mu$m, in which the most crucial
astrophysical spectral diagnostics of the formation of galaxies,
stars, planets, and life are found.  At these wavelengths, HEAT will
have high aperture efficiency and excellent atmospheric transmission
most of the year.
The proposed superheterodyne receiver system will be comprised of
0.8~THz, 1.4~THz and 1.9~THz channels which will observe the pivotal
J=7$\rightarrow$6 line of CO, the J=2$\rightarrow$1 line of atomic
carbon, and the far-infrared fine structure lines of N$^+$ and C$^+$,
the brightest emission lines in the entire Milky Way Galaxy.  When combined
with the HEAT telescope, the receiver system represents
a uniquely powerful instrument for reconstructing the history of 
star formation in our Galaxy, with application to the distant Universe.  
The receiver system itself serves as a valuable 
testbed for heterodyne Terahertz components, using leading-edge 
mixer, local oscillator, low-noise amplifier, cryogenic, and digital 
signal processing technologies that will play essential roles in
future Terahertz observatories. 
The proposed study will pave the way for future astronomical
investigations from Dome~A.  
\end{abstract}

%>>>> Include a list of keywords after the abstract 

\keywords{Interstellar medium, heterodyne instrumentation, terahertz, star formation, molecular spectroscopy}


\begin{figure}[!htb]
\centering
\includegraphics[scale=0.85,angle=0]{../figures/2mass.eps}
\caption{\label{2mass} {\it The power of HEAT:  a definitive
    survey of the interstellar medium and star-forming clouds over 240
    square degrees of the sky can be performed in the proposal period in the
    spectral lines of atomic carbon and N$^+$.  Over 30 square degrees
    of sky can be mapped in two months of the best winter weather in
    the pivotal 158~$\mu$m C$^+$ line.  No other ground-based
    site is capable of routine observations at these highest
    frequencies. }}
\end{figure}

\section{Scientific Justification}


Although great progress in understanding aspects of star and planet
formation have resulted from the technological 
advent of sensitive (sub)millimeter
wave detectors and the fabrication of large-format
near-infrared detector arrays, many fundamental aspects of the
formation and evolution
of galaxies, stars, planets, and life remain shrouded in
uncertainty.  The common element of these mysteries is the evolution
of interstellar clouds of dust and gas, from which stars and planets
are born, and to which stars return enriched material at the end of
their lives.  These clouds sculpt the evolution of entire galaxies.  
The far-infrared (FIR) contains the brightest and most 
diagnostic spectral lines of the entire electromagnetic spectrum; in
particular the pivotal fine structure lines of C, N$^+$ and C$^+$ at
0.8, 1.4 and 1.9~THz (370, 205 and 158 $\mu$m), respectively.  Only on
the high Antarctic plateau is the atmosphere dry, cold and stable
enough to permit survey observations at all three wavelengths.  In
performing a Galactic Plane survey of these spectral lines,
fundamental new insights into Galactic evolution, and star formation
will be pioneered (Figure~\ref{2mass}). In particular, HEAT will: 

\begin{enumerate}
\item {\bf Directly witness the formation of interstellar molecular clouds
  for the first time}, and answer where and how cloud formation takes
  place in the context of the Galaxy as a whole, with direct impact on
  star formation and Galaxy evolution.  Natal molecular
  clouds are mostly comprised of spectroscopically elusive H$_2$, but the
  principal forms of carbon in forming will be C$^+$ and C -- not CO,
  as found in developed clouds.  2MASS J-H/H-K extinction maps, in
  comparison with existing CO surveys, will guide
  the selection of fields for mapping. 

\item {\bf Derive a definitive star formation rate
  as a function of radius in the plane of the Milky Way}, providing an
  optimum set of data to calculate the Schmidt Law in the Galaxy.
  N$^+$ emission at 1.4~THz provides a measure of the ionizing
  luminosity with unmatched sensitivity, angular and spectral
  resolution, and is unaffected by extinction.  

\item {\bf Provide the first map of warm dense molecular gas} via CO emission in the
  CO J=7$\rightarrow$6 line at 0.8~THz, which can be observed
  simultaneously with the J=2$\rightarrow$1 fine structure line of neutral
  carbon.  Measurement of this energetic gas probes the pivotal 
  feedback of stars with their parent interstellar clouds.  It is
  energetic molecular gas that
  participates in molecular outflows, is the best probe of radiation
  fields at the surfaces of clouds, and is warmed by star formation in
  cloud cores. 
\end{enumerate}

%\vspace{-0.3cm}
\begin{figure}[!hbt]
\centering
\begin{minipage}{6.5in}
{\parbox{3in}{\includegraphics[scale=0.35,angle=0]{../figures/footprint.eps}}}
{\parbox{3in}{\caption{\label{beams} {\it The 3 beams of HEAT overlaid upon
    a 2MASS image of NGC~6334.  The size of the cospatial 0.8, 1.4, and 1.9~THz
    beams is 3, 1.7, and 1.3 arcminutes, respectively.}}}}
\end{minipage}
\end{figure}

The adoption of heterodyne receivers for HEAT naturally provides the
high spectral resolution ($<$0.4 km~s$^{-1}$) needed to disentangle
kinematically the many cloud components along any particular line of
sight through the Galactic Plane.  The angular resolution provided by
the 0.5-meter clear aperture (Figure~\ref{beams}) 
is optimized for resolving individual
cloud components throughout the Galaxy while providing adequate
mapping speed to perform a substantial survey of the Galaxy in a
single observing season.  The entire
Galactic Plane visible from Dome~A ($+10^o<l<-120^o$) will be mapped
in 6 months to 1.5$'$ resolution and 
RMS sensitivity of $\sim$0.1~K at CO and C, and $<$1~K at N$^+$, 
assuming that observations are being performed 75\% of the time. 
A total of $\sim$30 square degrees can be mapped in
C$^+$ during the coldest two months (typically August--September) to a
resolution of 0.7$'$ and a sensitivity of $\sim$1~K RMS (Figure~\ref{2mass}). 




\section{Instrument Description}

HEAT will be a fully automated, state-of-the-art THz observatory
designed to operate autonomously from Dome A in Antarctica. The
combination of high altitude (4,200~m), low precipitation, and extreme
cold make the atmospheric transmission in the FIR exceptionally good
from this site. In Figure~\ref{trans} we present a plot of the
expected atmospheric transmission above Dome A as a function of
wavelength (Lawrence 2004), indicating that winter weather at Dome~A
approaches (within an order of magnitude) the quality of that achieved
by SOFIA.  The wavelengths of several important
astrophysical lines are indicated with arrows. HEAT is designed to
take advantage of these unique atmospheric conditions and observe
simultaneously in [C~II](158 $\mu$m), [N~II](205 $\mu$m), and CO J =
7$\rightarrow 6$/[C~I] $2\rightarrow 1$ (370 $\mu$m).  

\begin{figure}[!hbt]
\centering
\begin{minipage}{6.75in}
{\parbox{4.4in}{\includegraphics[scale=0.44,angle=-90]{../figures/transplot.eps}}}
{\parbox{2in}{\caption{\label{trans} {\it Atmospheric transmission in the far
    infrared/submillimeter, derived from the PWV estimates for Dome~A of Lawrence
    (2004).  The pwv content 
    for each model atmosphere is 120 and 30 microns respectively,
    corresponding to $\sim$75 percentile and $\sim$10 percentile
    winter weather at Dome~A.  Arrows indicate the
    wavelengths of the [N~II], [C~II], and CO/[C~I] lines.}}}}
\end{minipage}
\end{figure}


A conceptual drawing of HEAT is shown in Figure~\ref{concept}. For
robustness and efficiency, the telescope and instrument are integrated
into a common optical support structure (OSS). 
HEAT will be mounted on top of a University of New South Wales AASTINO
(Automated Astrophysical Site-Testing InterNational Observatory). The
AASTINO provides power and communications for the HEAT telescope and
instrument. The HEAT/AASTINO facility is functionally equivalent to a
space-based observatory. 
Indeed, many of the key components used in the instrument were
originally developed for space applications.   

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.1,angle=0]{../figures/concept.eps}
\caption{\label{concept} {\it HEAT concept: The telescope has an effective
collecting area of 0.5m. Elevation tracking is accomplished by
rotating the $45^\circ$ flat reflector. The entire telescope structure
is warmed by waste heat from the AASTINO module
below and rotates for azimuth tracking. The HEB and SIS mixers used in
the instrument package are cooled to 
$\sim 4$~K using closed-cycled cryo-cooler technology developed for
space-based applications.}} 
\end{figure}


\subsection{Telescope}

The telescope is designed to have maximum efficiency and the minimum
number of optical components. Its design is similar to that of Kraus
(1966). Incoming light is reflected horizontally off a 45$^\circ$, $0.5
\times 0.7$~m flat reflector to an f/5 off-axis parabolic mirror. The
converging beam is intercepted by 
a flat tertiary mirror that directs it into the receiver. The tertiary
mirror can chop the incoming beam between source and reference
positions ($\Delta {\rm az} \sim 10^\prime$) at a rate of 0 to 4~Hz. All
three mirrors are fabricated from aluminum on a numerical milling
machine and have a surface roughness $\le 3\,\mu$m rms.  Elevation
tracking is achieved by rotating the first flat reflector. Azimuth
tracking is achieved by rotating the OSS on a bearing attached to the
roof of the AASTINO. The absolute pointing accuracy will be
15$^{\prime\prime}$, 1/5 of the smallest diffraction-limited beam. The
slew speed will be 1$^\circ$
per sec. Vertex-RSI has reviewed the proposed design and provided a
cost estimate for the delivery of a telescope meeting these
specifications.  
The cost estimate, which includes detailed design, fabrication, and
testing of the telescope and drive system, is provided in the budget. 

To prevent ice accumulation, the telescope is
enclosed and warmed by waste heat forced up through the
azimuth bearing from the AASTINO. A small radome made of a low-loss
dielectric (e.g. Goretex or polyethylene) encircles the first flat
reflector. This optical configuration provides an unobstructed view of
the sky. A CCD camera (provided by UNSW) 
and 1--5.3~\micron\ IR camera are mounted just
outside the radome on an extension of the elevation axis. The cameras
provide optical pointing and crucial site testing data. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.5,angle=0]{../figures/AASTINO.eps}
\caption{\label{AASTINOHEAT} {\it Rendering of HEAT atop a UNSW AASTINO}}
\end{figure}

\subsection{Receiver}

\subsubsection{Design Approach}

Heterodyne receivers are needed to achieve the sensitive, high
spectral resolution ($R=\lambda/\Delta\lambda > 10^6$) observations of
[N~II], [C~II], and CO/[C~I]
required for the proposed Galactic plane survey. The key components of a
submillimeter-wave heterodyne receiver are the mixer and local
oscillator (LO).  There are 3 types of mixers in common
use; the Schottky diode mixer, the SIS mixer, and, more recently, the
Hot Electron Bolometer (HEB) mixer. The Schottky diode mixer is
less sensitive than either SIS or HEB mixers and requires orders of
magnitude more LO power. Their principal advantages are robustness and
ability to operate at room temperature. 
Below $\sim$1~THz SIS mixers provide the lowest
receiver noise temperatures ($T_{rec} \le 400$~K). Above 1~THz HEB
mixers have the best performance ($T_{rec} \le 900$~K). The $\sim1$~THz
upper limit to SIS mixer performance is currently set by the
availability of suitable superconducting materials. Therefore, HEAT
will use an SIS mixer for the CO/[C~I] channel and HEB mixers for the
[N~II] and [C~II] channels.  The specific 
mixers and LO's that will be used in HEAT were
originally developed for use in the HIFI instrument for the {\it
Hershel Space Observatory}.


\subsubsection{Receiver Optics}


A close-up of the receiver optics is shown in Figure~\ref{optics}. The
incoming beam from the folding tertiary encounters two bandpass filters,
the first centered on the [N~II] (1.459~THz) line and the second on the
[C~II] (1.9~THz) line.  
Outside of their nominal passband, the filters are highly
reflective. Therefore, when the incoming beam encounters the first
filter, all but a narrow range of frequencies around the [C~II] line
are reflected to the [N~II] filter. The light reflected off the [N~II]
filter contains the CO J=7-6 and [$^{12}$C~I] and
[$^{13}$C~I] J=2-1 lines. The three emerging beams are
collimated and directed into the instrument cryostat. Before entering
the cryostat, the corresponding LO signals are injected into the
collimated beams using Mylar (for the 810~GHz mixer) and silicon (for
the 1.45 and 1.9~THz mixers) beam splitters. The Mylar is used to
inject the 810~GHz LO because it is both low-loss and wideband,
permiting DSB operation at the low IF frequency (1.5~GHz) required for
simultaneous observations of CO and [CI] lines. Silicon etalons can
serve as efficient LO diplexers, passing $>$95\% of the signal while
reflecting $>$50\% of the LO power (Mueller and Waldman 1994). These
properties make silicon etalons attractive beam splitters for THz
mixers where LO power is precious and the size and
complexity of Martin-Puplett diplexers can be prohibitive. The IF
frequency of the [C~II] and [N~II] mixers is 3~GHz. 
Upon entering the cryostat, the beams pass through infrared blocking
filters. For the HEB mixers, the same type of bandpass filters used
for frequency diplexing will be used at the entrance window of the 4~K
radiation stage. Here the filters not only prevent infrared loading on the
4~K stage, they also help to prevent the HEB mixers from being
saturated by the incoming radiation field.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.15,angle=0]{../figures/optics.eps}
\caption{\label{optics} {\it Optical subsystem and LO injection for
    HEAT's HEB and SIS mixers.}}
\end{figure}



\subsubsection{Mixer Performance}

The HEB mixers for HEAT will be provided by SRON/Delft 
and follow the successful quasi-optically coupled, NbN
phonon-cooled design of Gao {\it et. al.} (2004). {\it These mixers
have the lowest noise 
temperatures and LO power requirements of any mixers measured to
date ($T_{rec}= 940$~K and $P_{LO} = 0.17\,\mu$W at 1.89~THz).} A
plot summarizing  the noise performance of HEB mixers world-wide is
shown in Figure~\ref{mixers}. Recent Allan variance measurements (Kooi
2004) of HEB stability (Figure~\ref{AllanResultJB}) show excellent
performance can be achieved if the incoming beam is chopped at $\ge
1$~Hz. The HEAT telescope is designed to accommodate sky chopping at
frequencies up to $\sim 4$~Hz. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.5,angle=0]{../figures/mixers.eps}
\caption{\label{mixers} {\it Comparison of measured HEB mixer noise
    temperatures worldwide (from Gao et al. 2004)}} 
\end{figure}

The SIS mixer will be provided by the University of Cologne. The
design and fabrication of the SIS mixer is described by Puetz {\it
et al.} (2004, this volume) and has been successfully used on the
AST/RO telescope at the South Pole for several years. The noise
temperature is expected to be $\sim$200K. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1,angle=0]{../figures/AllanresultJB.eps}
\caption{\label{AllanResultJB} {\it Allan variance stability tests for
    Delft HEB mixers are in accordance with the radiometer equation if
    a chopping secondary is used. }}
\end{figure}


\subsubsection{Local Oscillators}

HEAT will employ 3 solid-state local oscillator chains built by the
Jet Propulsion Laboratory. The
809~GHz chain will consist of three doublers. The 1.45 Thz chain will
have four doublers, and the 1.9~THz chain will be comprised of  
one doubler and two triplers. {\it These chains leverage heavily
from the technology developed for {\it Herschel} and have had their
performance validated in lab measurements.} All three chains easily
meet our LO power requirements of $> 100$~$\mu$W at 809~GHz and $>
1$~$\mu$W at 1.46 and 1.9~THz. The JPL LO chains will be pumped by
CTI/Millitech DRO (dielectric resonant oscillator)/active multiplier
chains that can source $\sim 3$~dBm of power between 90-106~GHz. Each
of the phase-locked DRO's is fixed tuned to a single frequency, that
when up-converted, corresponds to one of the three target
astrophysical lines. A CAD drawing illustrating the integration of
the LO's is provided in Figure~\ref{optics}. Since each of the three
receivers acts independently, any increase in phase-noise that might
occur by using an active multiplier chain has minimal, if any, impact
on the quality of the collected data. The frequency stability of the
LO chains will be better than $\pm 0.5\,$MHz at 809~GHz and $\pm
1.0\,$MHz at 1.46 and 1.9~THz.  


\subsubsection{IF Processing \& Spectrometers}

The IF output of the HEB mixers is centered at 5~GHz. In order to
simultaneously detect the CO $J=7\rightarrow 6$ line in the lower
sideband and the [C~I] line in the upper sideband, the SIS mixer will
have an IF center frequency of 1.5~GHz.  
The IF output of each mixer is amplified by a low-noise MMIC amplifier
and downconverted to baseband for processing by three, 1024 lag
autocorrelators. The autocorrelators for the [N~II] and [C~II] channels
will each have 2~GHz of instantaneous bandwidth and $\sim 2$~MHz
resolution. The autocorrelator for the lower frequency CO/[C~I] channel
will run at half the clockspeed of the other two channels, yielding an
instantaneous bandwidth of 1~GHz and frequency resolution of $\sim
1$~MHz resolution. These correlators provide velocity coverage in
access of 316~km/s at a resolution $\le 0.4$~km/s, enabling all three
lines to be resolved and observed throughout the Milky Way without
needing to retune. Spaceborne Inc. of La Canada, CA has demonstrated a
2~GHz wide, 1024 lag correlator that meets our design specifications 
(see Figure~\ref{correlator}) and provided a quote for the complete
HEAT correlator system.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.6,angle=0]{../figures/correlator.eps}
\caption{\label{correlator} {\it The Spaceborne digital
  autocorrelator provides an ideal ``off-the-shelf'' multichannel
  spectrometer for HEAT. a) 2~GHz Spaceborne correlator chip.  b)
  Laboratory measurement of spectral response.}} 
\end{figure}


\subsubsection{Cryogenics}

A reliable, efficient, low-vibration 4~K cryogenic system is essential
to the success of HEAT. Experience has shown that care must be taken
to insure the 
uninterrupted operation of cryogenic systems over extended
periods. This philosophy is even more true for the proposed remote,
automated facility at DOME~A. Indeed, the system requirements for
HEAT are more like those of a 
spaceraft than a ground-based observatory. It is for this reason that
we have a teamed with Ball Aeropace to adapt one of their successful
spacebased cryogenic systems for work on HEAT. 

\medskip

\noindent{\it HEAT Cryogenic Subsystem Trades and Approach}

The cryogenic subsystem for HEAT must maintain the detectors
at 4~K. A number of approaches, including off-the-shelf
commercial cryocoolers (or mechanical refrigerators), were
evaluated against the system requirements. The need for
unattended operation for at least 1 year eliminates the
possibility of using liquid expendable cryogens. The remote
and harsh Antarctic environment prevents the use of
off-the-shelf commercial cryocoolers primarily due to limited
power for HEAT ($<$600~W total) provided by the AASTINO. High
cost prevents the use of fully space qualified cryocoolers for
HEAT. The resulting optimum cryogenic system that meets both
the technical and cost requirements is a combination of
commercial and aerospace cryocooler hardware. 
The HEAT cryogenic system requirements are shown in
Table~\ref{cryoreq}. 

\begin{deluxetable}{cc} 
\tablecaption{\label{cryoreq} Cryogenic system requirements for HEAT}
\tablewidth{0pt}
\tablecolumns{2}
\tablehead{ 
\colhead{Requirement } & 
\colhead{ Specification} 
}
\startdata
4K cooling capacity  &  4 mW \\ 
14 K cooling capacity  &  220 mW \\ 
70 K cooling capacity  &  3.6 W \\ 
4 K temperature stability  &  $\pm$0.01 K \\ 
Input power  &  $<$500 W \\ 
Lifetime  &  2 years total with 1 year maintenance interval \\ 
Vibration  &  Low \\ 
Vacuum level  &  $<1\times10^{-5}$ torr at 1 year \\ 
Cost  &  Low to Moderate \\ 
\enddata
\end{deluxetable}


After lifetime, the most demanding requirement is the input
power. To meet the power requirement, we have combined
improvements in a number of areas: 

\begin{enumerate}
 
\item The cooler is part of a custom cryostat that is designed to take
advantage of the low environmental temperatures to minimize heat load
parasitics and save power. 

\item The vacuum-sustaining pumping system ($\sim$150~W input) has been
eliminated by using the cryocooler as a cryopump. 

\item To circumvent the problem that makes traditional mechanical coolers
at 4~K so power inefficient, our design uses two cooling cycles, each
to best advantage. The G-M cools down to 14~K, where its regenerator
is still efficient. Then, a J-T cold head is used for the remaining
step down to 4~K. To minimize complexity, both systems run off the
same compressor. The G-M is a modified commercial model. The J-T
system only consists of a few passive components and is highly
leveraged off our space cooler hardware (demonstrated 3.4~K cooling). 

\item The off-the-shelf G-M cooler is modified to take advantage of the
  low ambient temperatures. The standard CTI Model 350 cold head is
  used, but the compressor is replaced with a commercial oil-free
  linear compressor, which will operate in the colder environment and
  provide additional temperature control. 
	
\end{enumerate}

Table~\ref{cryoopts} shows a compilation of cryocooler approaches and their
power consumption. Only the hybrid system can consume less than the
500 W requirement. The 4~K refrigeration requirement for the
commercial 2-stage coolers must be $>$0.25~W because they don't have a
14~K stage available to help reduce parasitic loads. The Pulse Tube
G-M has a number of attractive features ({\it e.g.} low maintenance
and vibration), however low efficient makes its power consumption
greater than a standard G-M cooler and, therefore, not suitable for
HEAT. The extra cost of the semi-custom cryosystem enables the great
reduction in input power needed for operation from a remote site. 

\begin{deluxetable}{cccc} 
\tablecaption{\label{cryoopts} Cryocooler approaches and applicability
to HEAT}
\tablewidth{0pt}
\tablecolumns{4}
\tablehead{ 
\colhead{Capacity at 4K } & 
\colhead{ Manufacturer} & 
\colhead{ Input power (Watts) } & 
\colhead{Meets Requirement?} 
}
\startdata
0.1 W (G-M)  & Sumitomo SRDK-101D  & 1300  & No \\ 
0.5 W (G-M)  &  Sumitomo SRDK-205D  & 3500  & No \\ 
0.5 W (Pulse Tube G-M)  & Sumitomo SRP-052A  & 7500  & No \\ 
20 mW (Hybrid G-M/J-T)  & Ball Aero - HEAT  & 400 &  Yes \\ 
\enddata
\end{deluxetable}


The HEAT system requires low cryocooler induced vibration levels to
minimize microphonic effects. The HEAT J-T cold head inherently
results in very low cryocooler induced vibration as it employs long
flexible tubes coming from the compressor. The tubing allows for the
isolation of the compressor vibration from the detectors, leaving only
the completely passive J-T cold head components (essentially orifices
and tubing) in contact with the detectors.  Commercial G-M cold heads
produce significant vibration and even Pulse Tube versions can cause
issues due to the oil-canning (from the pressure waves) of the
cold-finger. Thus, the HEAT approach will produce significantly lower
vibration levels than the commercial alternatives. 

\medskip

\noindent {\it Cryo System Implementation}

The proposed cooler has a high degree of technology maturity. {\it All
of the cryocooler components and their integration have been proven
in test.} The foundation of the cryocooler design is component and
system development proven in test verification from Ball programs such
as the Active Cooler Test and Development Program (ACTDP) and JPL
Planck programs.  
The Ball ACTDP Study Phase program fabricated, integrated, and tested
the cryocooler components. Cooling capacities in excess of 25~mW were
measured for temperatures down to 5~K. Tests with $^3$He as the J-T
working fluid extended the temperatures down to 3.5~K, and
demonstrated 12~mW of cooling at 3.8~K, more than adequate for cooling
HEAT's complement of HEB mixers.  


Leveraging off Ball Aerospace's system and instrument expertise, the
cryogenic system has been designed to meet the HEAT's unique
requirements. A block diagram of the HEAT cryogenic system is shown in
Figure~\ref{cryoblock}. 


\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.6,angle=0]{../figures/cryoblock.eps}
\caption{\label{cryoblock} {\it The cryocooler supporting the HEAT
    cryogenic system is a hybrid cooler consisting of a modified
    Gifford McMahon precooler and a Joule-Thomson cooler.}} 
\end{figure}


The 4~K temperature control as well as the cryocooler on/off and a
safety defrost heater will be handled by a commercial temperature
controller from Lake Shore Cryotronics. A trim heater, providing just
a few milliwatts of power, will maintain the 4~K temperature with
changing environmental conditions. The analog output will provide
remote operation of a safety defrost heater used in case the JT
orifice plugs. The relay will allow the cryocooler to be remotely
turned on or off.  

To comply with the tight vibration and thermal requirements, Ball will
provide a separate structural mount for the detector package, and use
the J-T to refrigerate the isolated system (see
Figure~\ref{balldewar}). The temperature of the cold plate will be
actively regulated to within 10~mK. To meet the tight power
constraints, Ball has designed a special dewar that uses the cold
Antarctic environment to minimize power consumption. This type of
optimization and design strategy is common for space missions where
power conservation and reliability are essential. To our knowledge
this is the first time this philosophy is being employed to such a
degree on a ground-based instrument.   

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.3,angle=0]{../figures/balldewar.eps}
\caption{\label{balldewar} {\it Cut-away views of low-vibration, long-life, 4~K closed-cycle cryostat designed by Ball.}} 
\end{figure}


\subsubsection{Calibration}

HEAT will be able to calibrate observations through several means. 1)~A 
vane with an ambient temperature absorbing load will be located at
the dewar entrance window, allowing standard chopper wheel calibration
to be performed. 2)~HEAT will routinely perform sky-dips to
compute the atmospheric optical depth in each of it's three wavelength
bands. 3)~HEAT will regularly observe a standard list of calibration
sources. 4)~The AASTINO will also host a Fourier Transform Spectrometer
(funded through the Australian program) that will measure
atmospheric transmission throughout the FIR. These measurements will be
coordinated with HEAT spectral line observations to provide cross
calibration. 

\subsubsection{Control System and Communication}
\label{control}

The HEAT control system will consist of a centralized control computer
that communicates with the telescope drive system, receiver frontend
({\it e.g.} mixers, LO's, and cryostat) computer, backend
(spectrometer) computer, and the AASTINO via Ethernet.
HEAT will be designed to work
autonomously for up to a week at a time, performing pre-programmed
observational programs and storing astronomical and housekeeping data
in non-volatile memory.  Preprocessed sample data will be uploaded to
control centers at the Universities of Arizona and New South Wales
via (1-3) dedicated Iridium satellite channels.  Raw data will be recovered
from HEAT each year during maintenance and processed into the released
data products.


\section{Conclusion}

With the implementation of HEAT, the astronomical community will 
have a new, powerful capability for exploring the origin of stars, 
galaxies, and planetary systems like our own. It will serve as
a model for future Antarctic observatories and the first step toward
realizing the research potential of Dome A. We look forward both to 
the exciting science that will come from HEAT and the challenge of 
making it a reality. 


\clearpage

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%% References %%%%%


\begin{thebibliography}{}

\bibitem{Gau04}Gao, J. R., Hajenius, M., Baselmans, J., Klawijk, P., de Korte, Voronov, B.,
and Gol'tsman, G., 2004, ``NbN Hot Electron Bolometer Mixers with Superior
Performance for Space Applications'', International Workshop on Low Temperature
Electronics, 23-24 June 2004, (invited paper).

\bibitem{Kooi04} Kooi, J., 2004, private communication.

\bibitem[Krauss(1966)]{krau66}  Krauss, J., ``Radio Astronomy'', 1966,
  McGraw-Hill, NY

\bibitem{Lawrence04} Lawrence, J. S., 2004, ``Infrared and submillimetre atmospheric characteristics of high Antarctic plateau sites'', PASP, in press. 
\end{thebibliography}

\end{document}



