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\centerline {\large \bf Continued Development of the HEAT telescope at Ridge A}

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Based on the results of an NSF-funded design study in 2006 and
successful deployment in 2012, we propose the next
development stage for the High Elevation Antarctic Terahertz (HEAT)
telescope, a robotic, 0.6-meter THz observatory at the summit of
the Antarctic 
plateau.  HEAT operates from 158 to 609 microns, and
observes the brightest and most diagnostic spectral lines from the
Galaxy.  It is tasked with the dual purpose of performing both site testing and
leading-edge terahertz astronomy. The (first) telescope was deployed in
2012 with the University
of New South Wales' PLATeau Observatory (PLATO-R) to Ridge A, the driest,
calmest and clearest point on the summit. The facility operates 
with no direct human contact for a year at a time between servicing
missions, with commands and data being transfered to and from the
experiment via satellite daily.  The site is truly 
exceptional, and HEAT has already made the most sensitive large-scale maps in
the 370 micron line of neutral carbon and performed landmark
observations at 200 microns, unveiling a population of
``CO dark'' molecular clouds and new regions where molecular clouds
may be forming.  With an established facility, we
propose here the next level of instrument development, namely the
augmentation of a hot
electron bolometer (HEB) receiver operating at 4K, 
whose increased sensitivity will allow the
facility to map the Galaxy more than 10 times faster.  
This effort is the second part of a longer term plan for the facility.  
This proposal requires fieldwork in the Antarctic.

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{\bf What is the intellectual merit of the proposed activity?}
The HEAT telescope forges entirely new capabilities for ground based
infrared and submillimeter astronomy which otherwise would be
unachievable except via expensive airborne or space-based
platforms. HEAT and PLATO-R represent a new generation of polar
instrumentation that permits the excellent conditions available from
remote sites like Ridge A to be harnessed without
the costs and hazards associated with manned operations. The
unparalleled stability, exceptional dryness, low wind and extreme cold
make Ridge A a site without equal for astronomy at infrared and
submillimeter wavelengths. HEAT operates in the atmospheric windows
between 158 and 609 microns, in which the most crucial astrophysical
spectral diagnostics of the formation of
galaxies, stars, planets, and life are found. HEAT is 
addressing timely and fundamental questions about the evolution of
the interstellar medium and star formation.
In particular, through large-scale Galactic surveys, the measurement
and impact of the Galactic environment on the life cycles of
interstellar clouds and their relation to star formation
are gradually being realized. The proposed upgrades of mixer, local
oscillator, low-noise amplifier, cryogenic, and DSP technologies 
will play essential roles in future Terahertz
observatories. This pioneering mission paves the way for future
astronomical investigations from the high plateau and beyond.

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{\bf What are the broader impacts of the proposed activity?}
HEAT's key project is to map, with great sensitivity and precision,
portions of the Southern Galactic Plane in the spectral light of the
dominant coolants of the interstellar medium.
Already, comprehensive science products from the survey and its
collaborations are being freely made available to the astronomy &
aeronomy community with no proprietary period.
These survey products enhance the value of numerous contemporary
surveys. Beneficiaries include Legacy programs from the Spitzer Space
Telescope, Key Projects from Herschel, the most recent
HI and CO surveys of the Galactic Plane, and the 2MASS \& UKIDSS
infrared sky surveys. The wide-field terahertz surveys provided by
HEAT place Herschel, ALMA, SOFIA and balloon-borne observations in a
broader, richer context. Thus, HEAT will serve both as a scientific
and technological pathfinder for contemporary and future suborbital
and space-based missions. 
As a portable, accessible terahertz observatory, the local copy of the
HEAT telescope transforms into
an outstanding educational and outreach tool. Furthermore, the HEAT project
uniquely captures the kind of high adventure spirit that attracts many
to science in the first place, and we aim to provide video and
photographic documentation of our experience for everyone via PBS's NOVA
program. Finally, the design
and fabrication of HEAT has been an interdisciplinary team effort
involving students from astronomy, optical sciences, and electrical
engineering. Astronomical instrumentation is becoming ever more
complex, requiring the talents of many individuals to bring them to
fruition. Providing students with both technical training and
team-work experience increases their probability
of success, both in science and in society.

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