In a radio interferometer, the geometrical antenna positions are determined from measurements of the observed delay to each antenna from observations across the sky of many point sources whose positions are known to high accuracy. The determination of accurate antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), with baseline lengths up to 15 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of October through November 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-to-day variations in the wet delay. We describe possible new avenues of improvement, which include recalibrating the baseline measurement datasets using the contemporaneous measurements of the water vapor scale height and temperature lapse rate from the oxygen sounder, and applying more accurate measurements of the sky coupling of the WVRs.
The Atacama Large Millimeter/submillimeter Array finds itself in the transition into full operations. Previous construction activities are being wrapped up, and regular, repetitive maintenance and upkeep will dominate the daily life, which asks for a consolidation and streamlining of the activities at the observatory. Especially the shifting focus to the high site of the observatory deserves more attention, since assembly, integration and verification activities at the base camp have ceased by now. In parallel, adjustments in the host country's labor legislation for operations at high geographic altitudes demand a review of the way things are done. This talk outlines the underlying operational concepts, lists the limiting constraints, describes the implementation of our reactions to those, and outlines our future intentions, which will be one in a number of steps towards optimization of the productivity of the observatory. The latter is the top level goal, which the Joint ALMA Observatory (JAO) has signed up for.
The ALMA Test Interferometer appeared as an infrastructure solution to increase both ALMA time availability for science activities and time availability for Software testing and Engineering activities at a reduced cost (<30000K USD) and a low setup time of less than 1 hour. The Test Interferometer could include up to 16 Antennas when used with only AOS resources and a possible maximum of 4 Antennas when configured using Correlator resources at OSF. A joined effort between ADC and ADE-IG took the challenge of generate the Test Interferometer from an already defined design for operations which imposed a lot of complex restrictions on how to implement it. Through and intensive design and evaluation work it was determined that is possible to make an initial implementation using the ACA Correlator and now it is also being tested the feasibility to implement the Testing Interferometer connecting the Test Array at AOS with Correlator equipment installed at the OSF, separated by 30 km. app. Lastly, efforts will be done to get interferometry between AOS and OSF Antennas with a baseline of approximately 24 km.
Current large observatories, both in operation and projects in development or construction, face the challenge
to find skilled personnel for integration and operation. Typical locations of these observatories are found to
be remote, mainly due to electromagnetic pollution prevention, which in many if not all cases reduces the
attractiveness of the work posts. Additional budgetary limitations restrict the recruitment radius for certain
positions to the local labor market. This paper outlines these staffing constraints in more detail and elaborates
on the need for training programs on various levels, which can be costly. This, in turn, drives the need for
creative retention efforts. Therefore, financial modeling, contingency, risk and quality management, and the
reliability, availability, and maintainability of an observatory are directly coupled to the local embedding in the
labor market of the host country.
The content of precipitable water vapor (PWV) in the atmosphere is very important for astronomy in the infrared and
radio (sub-millimeter) spectral regions. Therefore, the astrometeorology group has developed different methods to derive
this value from measurements and making forecasts using a meteorological model. The goal is use that model to predict
the atmospheric conditions and support the scheduling of astronomical observations. At ESO, several means to
determine PWV over the observatories have been used, such as IR-radiometers (IRMA), optical and infrared
spectrographs as well as estimates using data from GOES-12 satellite. Using all of these remote sensing methods a study
undertaken to compare the accuracy of these PWV measurements to the simultaneous in-situ measurements provided by
radiosondes. Four dedicated campaigns were conducted during the months of May, July, August and November of 2009
at the La Silla, APEX and Paranal observatory sites. In addition, the astrometeorological group employs the WRF
meteorological model with the goal of simulating the state of the atmosphere (every 6 hours) and forecasting the PWV.
With these simulations, plus satellite images, radiosonde campaign data can be classified synoptically and at the same
time the model can be validated with respect to PWV.
In support of characterization of potential sites for the European Extremely Large Telescope (E-ELT) the European
Southern Observatory (ESO), the Institute for Space Imaging Science (ISIS) and the astrometeorology group of the
Universidad Valparaiso have jointly established an improved understanding of atmospheric precipitable water vapour
(PWV) above ESO's La Silla Paranal Observatory. In a first step, 8 years worth of high resolution near-IR spectra taken
with VLT-UVES have been statistically analysed to reconstruct the PWV history above Paranal. To this end a radiative
transfer model of Earth's atmosphere (BTRAM) developed by ISIS has been used. A median PWV of 2.1 mm is found
for Paranal based on UVES data covering the period 2001-2008. Furthermore we conclude that Paranal can serve as a
reference site for Northern Chile due to the stable atmospheric conditions in the region. The median offset between
Paranal and Armazones is derived to be 0.3 mm, but local arbitrary variations of a few tenths of a mm between the sites
have been found by measurement. In order to better understand the systematics involved two dedicated campaigns were
conducted in August and November 2009. Several methods for determining the water column were employed, including
radiosonde launches, continuous measurements by infrared radiometer, and VLT instruments operating at various
wavelengths: CRIRES, UVES, VISIR and X-shooter. In a first for astronomical instruments all methods have been
evaluated with respect to the radiosondes, the established standard in atmospheric research. Agreement between the
radiosondes and the IR radiometer (IRMA) is excellent while all other astronomical methods covering a wavelength
range from 700 - 20000 nm have also been successfully validated in a quantitative manner. All available observations
were compared to satellite estimates of water vapour above the observatory in an attempt to ground-truth the satellite
data. GOES can successfully be used for site evaluation in a purely statistical approach since agreement with the
radiosondes is very good on average. For use as an operational tool at an observatory GOES data are much less suited
because of significant deviations depending on atmospheric conditions. We propose to routinely monitor PWV at the
VLT and to use it as an operational constraint to guide scheduling of IR observations at Paranal. For the E-ELT we find
that a stand-alone high time resolution PWV monitor will be essential for optimizing the scientific output.
APEX, the Atacama Pathfinder EXperiment, is being operated successfully, now for five years, on Llano de Chajnantor
at 5107m altitude in the Chilean High Andes. This location is considered one of the worlds outstanding
sites for submillimeter astronomy, which the results described in this contribution are underlining. The primary
reflector with 12 m diameter is cautiously being maintained at about 15 μm by means of holography. This
allows to access all atmospheric submillimeter windows accessible from the ground, up to 200 μm. Telescope and
instrument performance, operational experiences and a selection of scientific results are given in this publication.
The ArTeMiS submillimetric camera will observe simultaneously the sky at 450, 350 and 200 μm using 3 different focal
planes made of 2304, 2304 and 1152 bolometric pixels respectively. This camera will be mounted in the Cassegrain
cabin of APEX, a 12 m antenna located on the Chajnantor plateau, Chile.
To realize the bolometric arrays, we have adapted the Silicon processing technology used for the Herschel-PACS
photometer to account for higher incident fluxes and longer wavelengths from the ground. In addition, an autonomous
cryogenic system has been designed to cool the 3 focal planes down to 300 mK. Preliminary performances obtained in
laboratory with the first of 3 focal planes are presented.
Latest results obtained in 2009 with the P-ArTeMiS prototype camera are also discussed, including massive protostellar
cores and several star forming regions that have been clearly identified and mapped.
We report on developments of submillimeter heterodyne arrays for high resolution spectroscopy with APEX. Shortly, we will operate
state-of-the-art instruments in all major atmospheric windows accessible from Llano de Chajnantor. CHAMP+, a dual-color 2×7 element heterodyne array for operation in the 450 μm and 350 μm atmospheric windows is in operation since late 2007. With its
state-of-the-art SIS detectors and wide tunable local oscillators, its cold optics with single sideband filters and with 3 GHz of processed IF bandwidth per pixel, CHAMP+ does provide outstanding observing capabilities. The Large APEX sub-Millimeter Array (LAsMA) is in the final design phase, with an installation goal in 2009. The receiver will operate 7 and 19 pixels in the lower submillimeter windows, 285-375 GHz and 385-520 GHz, respectively. The front-ends are served by an array of digital wideband Fast Fourier Transform spectrometers currently processing up to 32×1.5 (optionally 1.8) GHz of bandwidth. For CHAMP+, we process 2.8 GHz of instantaneous bandwidth (in 16.4 k channels) for each of the 14 pixels.
ArTeMiS is a camera designed to operate on large ground based submillimetric telescopes in the 3 atmospheric windows
200, 350 and 450 µm. The focal plane of this camera will be equipped with 5760 bolometric pixels cooled down at 300
mK with an autonomous cryogenic system. The pixels have been manufactured, based on the same technology processes
as used for the Herschel-PACS space photometer. We review in this paper the present status and the future plans of this
project.
A prototype camera, named P-ArTeMiS, has been developed and successfully tested on the KOSMA telescope in 2006 at
Gornergrat 3100m, Switzerland. Preliminary results were presented at the previous SPIE conference in Orlando (Talvard
et al, 2006). Since then, the prototype camera has been proposed and successfully installed on APEX, a 12 m antenna
operated by the Max Planck Institute für Radioastronomie, the European Southern Observatory and the Onsala Space
Observatory on the Chajnantor site at 5100 m altitude in Chile. Two runs have been achieved in 2007, first in March and
the latter in November. We present in the second part of this paper the first processed images obtained on star forming
regions and on circumstellar and debris disks. Calculated sensitivities are compared with expectations. These illustrate
the improvements achieved on P-ArTeMiS during the 3 experimental campaigns.
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel
heterodyne PI−instrument for high resolution spectroscopy. The system is developed by a consortium of German
research institutes. The receiver will allow simultaneous observations in two out of the following three far−infrared
frequency bands:
* a low−frequency (1.4−1.9 THz) channel for e.g. the fine-structure lines of ionized nitrogen [NII] at 205μm
and ionized carbon [CII] at 158μm;
* a mid−frequency (2.4−2.7 THz) channel for e.g. the 112μm transition of HD; and
* a high−frequency (4.7 THz channel) for the 63 μm fine−structure line of neutral atomic oxygen.
Hot electron bolometers (HEB) mixers provide state of the art sensitivity. A spectral resolving power of up to
108 is achieved with chirp transform spectrometers, and a total bandwidth of 4 GHz at 1 MHz resolution is
reached with wide band acousto-optical spectrometers. The modular concept of GREAT allows to observe with
any combination of two out of the three channels aboard SOFIA. A more complete frequency coverage of the
THz regime by adding additional GREAT channels is possible in the future. The adaptation of new LO−, mixer−
or backend−techniques is easily possible.
We describe details of the receiver and the results of first performance tests of the system at 1.9 THz. As an
outlook to future developments we show first results obtained with phase locking a quantum cascade laser, the
most promising option for future high power local oscillators in the Terahertz regime.
GREAT, the German REceiver for Astronomy at Terahertz frequencies, is a first generation SOFIA dual channel
heterodyne PI-instrument for high resolution spectroscopy. The system is developed by a consortium of German
research institutes. The receiver will allow simultaneous observations in two out of the following three far-infrared
frequency bands:
a 1.4-1.9 THz channel for e.g. the fine-structure line of ionized carbon [CII] at 158μm;
a 2.4-2.7 THz channel for e.g. the 112μm transition of HD; and
a 4.7 THz channel for the 63 μm fine-structure line of neutral atomic oxygen.
Hot electron bolometers (HEB) mixers provide state of the art sensitivity. A spectral resolving power of up to
108 is achieved with chirp transform spectrometers, and a total bandwidth of 4 GHz at 1 MHz resolution is
reached with wide band acousto-optical spectrometers. The modular concept of GREAT allows to observe with
any combination of two out of the three channels aboard SOFIA. A more complete frequency coverage of the
THz regime by adding additional GREAT channels is possible in the future. The adaptation of new LO-, mixer-
or backend-techniques is easily possible. We describe details of the receiver and the results of first performance
tests of the system at 1.9 THz.
We present the opto-mechanical layout of KOSMA's (Kolner Observatorium fur SubMillimeter Astronomie) submillimeter and terahertz heterodyne array receiver STAR (SOFIA Terahertz Array Receiver) which is derived from SMART (Sub-Millimeter Array Receiver for Two frequencies). To reduce the alignment effort, SMART, for the first time, uses an integrated optics concept with no adjustable optical components inside the receiver dewar. For STAR this successful design concept will be extended and adapted for 1.9 THz and for use aboard SOFIA (Stratospheric Observatory For Infrared Astronomy).
The design of STAR's cryostat and cryogenic optics is described. Emphasis is laid on the required accuracy for 4x4 spatially multiplexed Terahertz heterodyne receivers. The proposed design of the local oscillator, a frequency-tripled BWO (Backward Wave
Oscillator), is outlined. The presentation comprises the scheme
for multiplexing the local oscillator, dense arrangement of mixer elements in a cryogenic focal plane and manufacturing techniques of integrated optics units for reduction of optical adjustment efforts in astronomical submillimeter and terahertz receivers.
The German REceiver for Astronomy at Terahertz frequencies (GREAT) is a first generation PI instrument for the SOFIA telescope, developed by a collaboration between the MPIfR, KOSMA, DLR, and the MPAe. The first three institutes each contribute one heterodyne receiver channel to operate at 1.9, 2.7 and 4.7 THz, respectively. A later addition of a e.g. 1.4 THz channel is planned.
The GREAT instrument is developed to carry two cryostats at once. That means that any two of the three frequencies can be observed simultaneously. Therefore, we need to be able to quickly exchange the optics benches, the local oscillator (LO) subsystems, and the cryostats containing the mixer devices. This demands a high modularity and flexibility of our receiver concept. Our aim is to avoid the need for realignment when swapping receiver channels.
After an overview of the common GREAT optics, a detailed description of several parts (optics benches, calibration units, diplexer, focal plane imager) is given. Special emphasis is given to the LO optics of the KOSMA 1.9 THz channel, because its backward wave oscillator has an astigmatic output beam profile, which has to be corrected for. We developed astigmatic off-axis mirrors to compensate this astigmatism. The mirrors are manufactured in-house on a 5 axis CNC milling machine. We use this milling machine to obtain optical components with highest surface accuracy (about 5 microns) appropriate for these wavelengths. Based on the CNC machining capabilities we present our concept of integrated optics, which means to manufacture optical subsystems monolithically. The optics benches are located on three point mounts, which in conjunction with the integrated optics concept ensure the required adjustment free optics setup.
We present the concept for KOSMA's 16 element 1.9 THz heterodyne array
STAR (SOFIA Terahertz Array Receiver) which is being developed for
SOFIA. The instrument will consist of two interleaved sub-arrays of 8
pixels each. Together we will have a 4 × 4 pixel array with a beam spacing on the sky of approximately 1.5 times the beam size of 15 arcsec (FWHM). The receiver is mainly targeted at measuring the fine structure transition of ionized atomic carbon at 1.9 THz (158 microns). STAR's optics setup is modeled after the successful design used in KOSMA's SMART receiver. It will contain a K-mirror type beam rotator, a Martin-Puplett diplexer for LO coupling and an LO multiplexer using imaging Fourier gratings. Complete optical sub-assemblies will be machined monolithically as integrated optics units, to reduce the need for optical alignment. STAR will probably use waveguide mixers with diffusion cooled hot electron bolometers, which are being developed at KOSMA. The receiver backends will be KOSMA Array-AOSs. Local oscillator power will be provided by a backward wave oscillator (BWO), followed by a frequency tripler.
We have designed and prototyped an array of Ge:Sb photoconductors for use in AIRES, the Airborne InfraRed Echelle Spectrometer, on SOFIA. The 16 X 24 flight array will operate between 33 micrometers and 120 micrometers . In this paper we discuss the testing of a 3 X 3 prototype array and the resulting design of the flight array.
In this paper we present the considerations for design and assembly of a stressed gallium doped germanium photoconductor array for the Airborne InfraRed Echelle Spectrometer on SOFIA. This 8 X 12 element array will cover the wavelength range from 125 to 210 micrometers . The considerations cover the aspects of the mechanical design for stressing the detectors in a uniform way, assembly of the components, contacting them electrically with minimized stray capacitance, and the layout of the light collecting cone assembly.
The spectral-photometric IR camera SPICA is proposed as one of the German science instruments of the Stratospheric Observatory for IR Astronomy (SOFIA). It will cover a wavelength range of 20-220 micrometers with three large area detector arrays. With the 2.5 m SOFIA telescope, SPICA will provide unprecedented diffraction limited spatial resolution in the far-IR. In addition, low resolution 3D-imaging spectroscopy is planned. While the silicon array will be commercially available, the germanium arrays are being developed, including their cryogenic multiplexers. The overall instrument concept, its camera optics and the status of the detector development will be presented. The instrument is being developed by the DLR Institute of Space Sensor Technology in Berlin with support of several German and US partners.
The Stratospheric Observatory for Infrared Astronomy, SOFIA, is a joint US and German project and will start observations from altitudes up to 45,000 ft in late 2001. The 2.5 m telescope is being developed in Germany while the 747- aircraft modifications and preparation of the observatory's operations center is done by a US consortium. Several research institutions and universities of both countries have started to develop science instruments. The DLR Institute of Space Sensor Technology in Berlin plans on a spectral-photometric camera working in the 20 to 220 micrometers wavelength range, using doped silicon and germanium extrinsic photoconductors in large, 2D arrays: silicon blocked-impurity band detectors, Ge:Ga and stressed Ge:Ga. While the silicon array will be commercially available, the germanium arrays have to be developed, including their cryogenic multiplexers. Partner institutions in Germany and the US will support the development of the instrument and its observations.
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