El Gran Telescopio Milimétrico/ The Large
Millimeter Telescope
E. Carrasco1 and the GTM/LMT collaboration2
1
2
Instituto Nacional de Astrofı́sica, Óptica y Electrónica, Luis Enrique Erro 1,
Tonantzintla, Puebla, Mexico bec@inaoep.mx
INAOE, Luis Enrique Erro 1, Tonantzintla, Puebla, Mexico and the University
of Massachusetts Amherst, 710 North Pleasant Street, Amherst Massachusetts
Summary. The Large Millimeter Telescope is a single, high-precision mm-wavelength
telescope, 50 meters in diameter located at an elevation of 4580 m on Tlilteptl, in
central Mexico. It will operate with good efficiency at wavelengths as short as 1 mm
and it will be capable of observations at 0.8 mm. The telescope will be the largest in
the world operating at these wavelengths, equipped with innovative and extremely
sensitive receivers which will function as cameras for both heterodyne and continuum observations. The main characteristics of the telescope and its instrumentation
are presented.
1 Introduction
The Large Millimeter Telescope (LMT) is a major new astronomical facility
developed as a collaboration between the Instituto Nacional de Astrofı́sica,
Óptica y Electrónica and the University of Massachusetts Amherst. Located
at 18.9◦ N latitute, at 7 km from Citlaltepetl -the highest peak in Mexico- it
has full access to the Galactic Center. LMT is due to enter comissioning and
first light science phase in 2007 and will be the largest millimeter telescope
worldwide. With an area of almost 2000 m2 and its innovative instrumentation, it will allow sensitive measurements of all cosmic structures from dust
grains to the Universe as a whole [4].
LMT will be able to penetrate the dust that obscures the process of star
formation in distant galaxies to elucidate the history of star formation over
time. It will allow to analize the environment of active galactic nuclei to probe
the relation of supermassive black holes to their host galaxies and will quickly
follow up detections of gamma-ray burst to increase our understanding of
the death of massive stars and the origin of the heaviest chemical elements.
Furthermore, it will complement GLAST observations [1] and contribute to
disentangling the nature of the new and old unidentified gamma-ray sources.
For the study of our galaxy and other galaxies in the local Universe the
LMT will provide new insights into the nature and distribution of the inter-
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E. Carrasco and the GTM/LMT collaboration
Fig. 1. The Large Millimeter Telescope on November 22, 2006. [Photo G. Cerón].
stellar gas and dust from which starts forms. It will contribute to elucidate
the process of star formation itself and to establish the existence and nature
of the massive black hole at the center of the Milky Way by providing critical north-south coverage and unparalleled sensitivity to Very Long Baseline
Interferometry observations.
In addition, the LMT will be a powerful tool for astrobiology and planetary science by providing the sensitivity to allow searches for complex organic
molecules in space. LMT will be able to detect and provide an initial characterization of the disks of gas and dust around starts from which planets form.
It will analize with unprecedented sensitivity the chemistry and physics of
comets. The telescope will carry out the first comprehensive survey of small
bodies in the solar system, including near Earth objects, main belt asteroids, centaurs and Kuiper belt objects. More information about the project is
available at the official LMT/GTM webpage: www.lmtgt.org.
2 The telescope
The LMT is a 50 m diameter filled aperture mm-wave telescope. For the
effective surface area figure of 75 µm rms, the primary reflector accounts for
55 µm rms. The innovative and unique telescope design includes an active
surface formed by 180 segments. Each segment is supported by a reaction
structure which is attached to the reflector backstructure by a subframe. Four
El Gran Telescopio Milimétrico/ The Large Millimeter Telescope
3
actuators can adjust each subframe in relation to the backstructure to correct
the deformations due to gravity and thermal gradients. Temperature sensors
on all relevant parts will report to the control system and the surface will
be periodically measured by holographic techniques. The sensitive surface is
composed by electroformed nickel panels. Simulations indicate that the LMT
will be able to maintain surface accuracy in the presence of winds up to 10
m/s. The main telescope specifications are shown in table 1.
Table 1. LMT technical specifications
Property
Specification Goal
Effective surface area
Pointing accuracy
Aperture efficiency (3mm)
Aperture efficiency (1.2mm)
Sensitivity (3mm)
Sensitivity (1.2mm)
FHWM beam size (3mm)
FWHM beam size (1.2mm)
75 µm rms
1 arcsec
0.65
0.40
2.2 Jy/K
3.5 Jy/K
15 arcsec
6 arcsec
70 µm rms
0.6 arcsec
0.70
0.45
2.0 Jy/K
3.1 Jy/K
3 Instrumentation
AzTEC, the astronomical thermal emission camera, is a large format array
made of 144 silicon nitride micromesh bolometers. The camera operates at
1.1, 1.4 and 2.1 mm although only one bandpass will be available per observing run. To optimize the instrument performance the warm readout electronics have been designed to minimize the analog input path, simplifying
the electrical connections between the front-end and the back-end electronics
and to eliminate signal ground connections between all computers and the
radiometer. The system architecture utilizes fiber optic connections carrying
the AES/EBU for commanding, clock distribution and signal transmission.
AzTEC field of view on LMT will be about 2.4 arcmin2 . The instrument has
been used in the James Clerk Maxwell Telescope (JCMT) already achieving
sub-mJy flux limits [6] acomplish its expected performance. On LMT, with
10 times larger collecting area than the JCMT, AzTEC 1σ per pixel sentivity
will reach <3mJyHz−1/2 and a mapping speed of 0.36 deg2 /hr/mJy2 .
SPEED, the spectral energy distribution camera, uses the new technology
of frequency selective bolometers to perform multifrequency observations [5].
The camera is configured as a 2 x 2 array with each pixel housing a 2.1,
1.3, 1.1 and 0.85 mm detector. By sampling these wavebands it measures the
millimetric spectral energy of a source in a single pointing eliminating the
need for repeated observations of the same target. It is the ideal instrument
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E. Carrasco and the GTM/LMT collaboration
to follow up AzTEC sources. The sensitivies of these instruments on LMT are
shown in table 2.
Table 2. Estimated sensitivities for LMT continuum instruments
Channel
Center frequency
Beam size [arcsec]
√
NEP [aW/ Hz]
√
NETRJ [mK/√ Hz]
NEFD [mJy/ Hz]
Mapping speed [deg2 /hr/mJy2 ]
AzTEC
...
214
7
93
1400
2.95
0.36
1
145
11
139
593
0.93
...
SPEED
2
3
214 273
11 11
194 266
449 382
1.50 1.83
... ...
4
375
11
324
505
3.26
...
Fig. 2. AzTEC cryostat and readout electronics in the UMass Amherst Cryogenic
Device Laboratory [4]. [Photo: J. Austermann].
SEQUOIA, the second Quabbin optical imaging array [2], is a 3mm heterodyne focal plane array tunable in the 85-115.6 GHz range. It has 32 pixels
arranged in dual polarized 4 x 4 arrays. Two dewars contain 16 pixels each,
the beams from which are combined using a wire grid. The array uses indium
phosphide monolitic microwave integrated circuit (MMIC) preamplifiers followed by subharmonic mixers with an intermediate frequency (IF) band of
El Gran Telescopio Milimétrico/ The Large Millimeter Telescope
5
5-20 GHz. SEQUOIA was in operation at the Five College Radio Astronomy
Observatory (FCRAO) 14 m telescope, mostly used for extended mapping
of molecular clouds regions [3]. When mounted on LMT, SEQUOIA will be
able to cover similar extensions to those achieve on the 14 m with improved
signal to noise ratio and angular and velocity resolutions. The instruments
characteritcs are shown in table 3.
Table 3. Specifications of SEQUOIA
Num of pixels
32 (2 de 4 x4)
Beam size [arcsec]
15
Space between beams [arcsec]
30
Polarizations
2
Instantaneous RF bandwidth [GHz]
85 - 115
Instantaneous FI bandwidth [GHz]
5 - 20
Tsys (one pixel)
< 60 K, 85 - 115 GHz
< 90 K, 105 - 115 GHz
Tsys (sky)
100 - 250 K
A 1 mm commisssioning receiver is being built to operate in the 210-275 GHz
atmospheric window providing the state of the art sensitivity using a side band
separation squeme to separate the upper and lower sidebands. This receiver
will employ detectors based on the superconductor-insulator-superconductor
technology.
The redshift receiver (RR) is a ultra wide band spectrograph to cover the
spectral range of 75 - 111 GHz. It uses the new MMIC amplifiers technology
that achieve noise temperature below 50 K over the same frequency range.
It has a very low loss waveguide polarization combiner that covers the full
band and a new fast electrical beam switching based on a Faraday rotation
polarization switch. The receiver will have two dual polarized interchangeable beams so that one beam remains on the source at all times. The four
receivers, making up the autocorrelation spectrometer, have a combined IF
bandwidth of 144 GHz and the entire band needs to be spectrally processed
simultaneously. Recently the RR saw first light with the acquisition of a 6
GHz wide spectrum of M82 on the 14 m FRCAO telescope, that used only
1/4 of the full coverage. Designed to determine the redshift of dusty galaxies
via the frequency measurement of contiguous CO lines, the RR can also be
used as a low dispersion spectrograph for identifying Galactic sources.
The wide band spectrometer is the spectroscopic capability for the heterodyne receivers as SEQUOIA, the 1mm commissing receiver and future focal
plane arrays. It computes the autocorrelation function (ACF) of the input
signal and the Fourier transform of the ACF then gives the spectrum. The
correlators will support all the envisioned data collection modes including positioning (< 1 Hz), beam (∼ 1 Hz) and frequency (> 1 Hz) switching and
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E. Carrasco and the GTM/LMT collaboration
on the fly mapping. The bandwidth (BW) and resolution requirements are
imposed by the scientific goals as indicated in table 4. The spectrometer will
be optimally configured for broadband coverage or high spectral resolution or
some combination of both extremes.
Table 4. Wideband spectrometer bandwidth (BW) and resolution (δν) requirements, in km/s, for some of the main scientific projects.
Scientific goals
BW
δν
Identification of unknown redshifts of primeval galaxies > 1000
Extragalactic imaging
∼ 1000
Galactic surveys and high velocity sources
∼ 300
Giant molecular clouds
∼ 50
Dark clouds
∼ 20
Spectral line searches
∼ 1 GHz
∼100
∼10
1
∼0.1
∼0.01
∼-0.1
4 Conclusions
The Large Millimeter Telescope with a 50 m diameter dish will be the largest
single, high precision antena. The telescope commissioning will start en 2007
with initial science late the same year. As most of the first generation instruments have been commissioned -and the rest will be- on other telescopes, it is
foreseen that when LMT enters full operation in 2008, it will produce scientific
results inmediatly.
References
1. Carramiñana, A., Astrop. Spa. Sci. proc. of “The multimessenger approach to
unidentified gamma-ray sources” eds. J. Paredes, O. Reimer, D. Torres (2006)
2. N.R. Erickson, R.M. Grosslein, R.B. Erickson et al. IEEE Trans. Microwave
Theory Techniques 47, 2212 (1999)
3. J. Jackson, et al: ApJS 163, 145 (2006)
4. W.M. Irvine, E. Carrasco & I. Aretxaga: The Large Millimeter Telescop - Neighbors explore the cosmos. ed by W.M. Irvine, E. Carrasco & I. Aretxaga (Creative Services, University of Massachusetts Amherst, 2005)
5. G.W. Wilson, J. Austermann, D.W. Logan, M. Yun: SPIE 5498, 246 (2004)
6. G.W. Wilson, et al: AAS 208, 6606 (2006)