Minimize APEX

APEX (Atacama Pathfinder Experiment)

APEX Instrumentation   SEPIA    Mission Status   References

ESO operates the Atacama Pathfinder Experiment, APEX, at one of the highest observatory sites on Earth, at an elevation of 5100 m, high on the Chajnantor plateau in Chile's Atacama region. APEX is a collaboration between the MPIfR (Max Planck Institute for Radioastronomy) Bonn Germany, the OSO (Onsala Space Observatory) of Chalmers University of Technology, located on Onsala peninsula (57°23'35"N 11°55'04"E), Kungsbacka Municipality, Sweden, and ESO (European Southern Observatory) with HQs in Garching, Germany. The telescope is operated by ESO and started operations in 2005. 1) 2)

APEX is a 12 m diameter telescope, operating at millimeter and submillimeter wavelengths — between infrared light and radio waves. Submillimeter astronomy opens a window into the cold, dusty and distant Universe, but the faint signals from space are heavily absorbed by water vapor in the Earth's atmosphere. Chajnantor is an ideal location for such a telescope, as the region is one of the driest on the planet and is more than 750 m higher than the observatories on Mauna Kea, and 2400 m higher than the VLT (Very Large Telescope) on Cerro Paranal.

APEX is the largest submillimeter-wavelength telescope operating in the southern hemisphere. It has a suite of different instruments for astronomers to use in their observations, a major one being LABOCA (Large APEX Bolometer Camera). LABOCA uses an array of extremely sensitive thermometers — known as bolometers — to detect submillimeter light. With almost 300 pixels, it is the largest such camera in the world. In order to be able to detect the tiny temperature changes caused by the faint submillimeter radiation, each of these thermometers is cooled to less than 0.3 degrees above absolute zero — a frigid minus 272.85 degrees Celsius. LABOCA's high sensitivity, together with its wide field of view (one third of the diameter of the full Moon), make it an invaluable tool for imaging the submillimeter Universe.

APEX was a pathfinder for ALMA (Atacama Large Millimeter/submillimeter Array), a revolutionary new telescope that ESO, together with its international partners, operates on the Chajnantor plateau. APEX is based on a prototype antenna constructed for the ALMA project, and it will find many targets that ALMA will be able to study in great detail.

APEX_Auto13

Figure 1: A view of APEX (Atacama Pathfinder Experiment) antenna, located at the highest observatory site on Earth, at an elevation of 5100 m on the Chajnantor plateau in Chile's Atacama region (image credit: ESO)

Location

Llano de Chajnantor, 50 km east of San Pedro de Atacama, Northern Chile

Coordinates

Latitude : 23º 00'20.8" South; Longitude :67º 45' 33.0" West; Altitude : 5105 m

Diameter

12 m

Mass

125,000 kg

Main reflector

264 aluminum panels, average panel surface r.m.s. 5 µm

Secondary reflector

Hyperboloidal aluminum, Diameter 0.75 m

Surface accuracy (r.m.s.)

17 µm

Pointing accuracy (r.m.s.)

2" rms over sky, Pointing accuracy on track 0.6"

Manufacturer

Vertex Antennentechnik, Duisburg, Germany

f/D

8

Beam width (FWHM)

7.8" x (800 / f [GHz])

Receiver cabins

2 Nasmyth (A,B) + 1 Cassegrain (C)

Table 1: The APEX Telescope 3)

 


 

APEX instrumentation

Instrument

Type

Mode

Frequency (GHz)

HPBW
(arcsec)

IF range
(GHz)

No of
beams

Location

Status

Comment

APEX-1
(SHeFI)

Heterodyne
SIS

SSB

213-275

30-25

4-8

1

Nasmyth-A

Decommissioned from Oct 2017

APEX-2

Heterodyne
SIS

SSB

267-378

23-17

4-8

1

Nasmyth-A

Decommissioned from Oct 2017

APEX-3

Heterodyne
SIS

DSB

385-506

17-13

4-8

1

Nasmyth-A

Decommissioned from Oct 2017

APEX-T2

Heterodyne
HEB

DSB

1250-1390

5

2-4

1

Nasmyth-A

x

Science Verification pending

LABOCA

Bolometer
array

 

345

19

 

295

Cassegrain

 

SABOCA

Bolometer
array

 

850

8

 

39

Cassegrain

x

warmed up

 

APEX-2A

Heterodyne
SIS

DSB (Double
SideBand)

279-381

18

4-8

1

Nasmyth-A

x

Decommissioned in
Feb 2008 and replaced
by APEX-2

Table 2: Facility instruments 4)

SHeFI receivers: This is a set of SIS heterodyne receivers covering the bands at 213-275 GHz, 267-378 GHz, and 385-506 GHz, and a heterodyne HEB receiver operating at 1.25-1.39 THz. All receivers are mounted in a single dewar in the Nasmyth A cabin. Band 2 has replaced the APEX-2A receiver.

For more information consult:

- A Swedish heterodyne facility instrument for the APEX telescope. A&A 490, 1157-1163 (2008).

- Facility heterodyne receiver for the Atacama Pathfinder Experiment Telescope.

- Group for Advanced Receiver Development (GARD).

To estimate the necessary integration time for ON/OFF observations, please use the Integration Time Estimator Facility Heterodyne Receivers. For OTF observations, please use the On The Fly Integration Time Estimator. To design a OTF map use, OTFSimulator

LABOCA: The Large Apex BOlometer CAmera is a 295-pixel, 345 GHz (870 µm) bolometer array built by the bolometer development group at MPIfR Bonn. The field of view is 11 x 11 arcmin2. It was successfully commissioned in May 2007. More information:LABOCA A&A paper

SABOCA: The Submillimeter APEX Bolometer Camera (SABOCA), a 39-channel bolometer array operating at 850 GHz (350 µm), has been commissioned in March 2009.
More information here: ESO Messenger 139

APEX-2A: APEX-2A is a heterodyne receiver remotely tunable in the range 279-381GHz. It is a double-sideband (DSB) receiver with typical receiver temperatures of Trec=60-80 K. Under good weather conditions, this leads to DSB system temperatures of ~150 K over most of the tuning range, but up to 250 K towards 370 GHz.
APEX-2A was decommissioned in February 2008.
More information: A 0.8 mm heterodyne facility receiver for the APEX telescope, A&A 454, L17-L20 (2006).

 

PI instruments

Apart from the facility receivers, PI instruments will be installed at APEX during longer or shorter time periods. Some of them can be used by the community in collaboration with people from the corresponding PI group. In case of the MPIfR instruments, contact Rolf Güsten (rguesten@mpifr-bonn.mpg.de) before submitting any proposals.

Instrument

Type

Usage

Frequency
(GHz)

HPBW
(arcsec)

No of
beams

Location

Status

Comment

FLASH+

Heterodyne
SIS

PI (MPIfR)

268-374
374-516

17-22
12-14

1
1

Nasmyth-A
Nasmyth-A


 

CHAMP+

Heterodyne
SIS

PI (MPIfR)

620-720
780-950

9-7
7-6

7
7

Nasmyth-B
Nasmyth-B


 

ASZCA

Bolometer
array

PI (MPIfR)

150

42

330

Cassegrain

x

Decommissioned in
December 2010

ARTEMIS+

Bolometer
array

PI (ESO)

1499 (200 µm)
856 (350 µm)
666 (450 µm)

4
7
9


288 x 8


Cassegrain

x

not available yet
Commissioning June 2013

PolKa

Polarimeter

PI (MPIfR)

345

19

295

Cassegrain

x

Commissioning pending

SEPIA

Heterodyne
SIS

PI (ESO/
Swedish)

159-211 (Band 5)
272-376 (Band 7)
600-722 (Band 9)

30-39
17-23
10-9

1
1
1

Nasmyth-A
Nasmyth-A
Nasmyth-A


x

Commissioning Feb/Mar 2015
Installation in 2018
DSB version available

PI230

Heterodyne
SIS

PI (MPIfR)

200-270

31-23

1

Nasmyth-B

Offered by MPIfR to all
partners till new 230 GHz
facility will be in operation

Table 3: PI instrument parameters

FLASH+: FLASH is a dual-frequency MPIfR principal investigator (PI) receiver, operating simultaneously - on orthogonal polarizations - in the 345 GHz and the 460 GHz atmospheric windows. This PI instrument is available to the APEX user community on a collaborative basis with the MPIfR. To estimate the integration time in ON/OFF observation use Integration Time Estimator for Heterodyne Receivers. To estimate the integration time in OTF observation use On The Fly Integration Time Estimator. To design a OTF map use, OTFSimulator.

CHAMP+: CHAMP+ is dual channel heterodyne receiver array of 7 pixels operating in the 602-720 GHz and 790-950 GHz atmospheric windows.

ASZCA: ASZCA is a close-packed bolometer array operating at 2 mm. The field of view is 0.4 x 0.4 degrees. It was decommissioned in December 2010.

PolKa: PolKa - A new concept of polarimeter, named PolKa after the German "Polarimeter fur Bolometer-Kameras", was developed by the Bolometer Group at the MPIfR in Bonn and installed on APEX in October 2009. PolKa works in combination with LABOCA to produce high-resolution maps of linear (and possibly also circular) polarization. The new polarimeter has the unique characteristic of being tunable and of producing a negligible absorption.

ARTEMIS : ARTEMIS is a bolometer array working at 200, 350, and 450 µm. It is developed by CEA Saclay (France), and its final version is expected at APEX at June 2015. To estimate the integration time and RMS in observation use Observing Time Estimator.

SEPIA : SEPIA (Swedish ESO PI receiver for APEX) contains currently 2 bands: Band 5 (sideband separating dual polarization receiver, operating in the frequency range 159-211 GHz) and Band 9 (DSB, dual polarization, operating in the frequency range 600-722 GHz). Band 7 is expected to be installed during 2018. To estimate the integration time in ON/OFF observation use Integration Time Estimator for Heterodyne Receivers. To estimate the integration time in OTF observation use On The Fly Integration Time Estimator. To design a OTF map use, OTFSimulator.

 

SEPIA (Swedish-ESO PI Instrument for APEX) Receiver

The new Swedish-ESO PI Instrument for the APEX telescope (SEPIA) receiver was brought to APEX in the beginning of 2015 via a joint effort from the GARD (Group for Advanced Receiver Development at OSO, and ESO. GARD performed the optics and cryostat design and construction and the refurbishing of the Band 5 pre-production cartridge (owned by ESO, earlier built by GARD under the European Community FP6 funded project) with the full-production LO system and WCA (Warm Cartridge Assembly), both provided by NRAO (National Radio Astronomy Observatory , USA) via ESO. GARD has built the control system and software, installed the SEPIA receiver in the APEX Cabin A and performed technical commissioning. 5)

Initially, the SEPIA receiver contained only an updated ALMA Band 5 pre-production receiver cartridge to address the growing interest in future observations with Band 5 at ALMA (first available during cycle 5). The ALMA Band 5 center frequency nearly coincides with the 183.3 GHz water absorption line. The key applications of the Band 5 receivers for ALMA range from observations of the 313–220 H2O line at 183.310 GHz in both Galactic objects and nearby galaxies to the 158 µm emission line of C+ from high redshift galaxies.

In the beginning of 2016, SEPIA was equipped with an additional ALMA Band 9 cartridge, providing observers with two state-of-the-art, single-pixel, dual polarization receivers delivering each a total 16 GHz IF band. The realization of the SEPIA receiver project took about one year from the moment the decision was taken, February–March 2014, to the time SEPIA was installed at APEX in February 2015. The success of the SEPIA project was guaranteed by ESO, providing optical windows and filters, access to hardware from ALMA, support from NRAO by providing local oscillator (LO), Band 5 WCA and FE bias electronics – all purchased by ESO. GARD OSO expertise in designing and constructing mm-wave receivers, in particular the ALMA Band 5 receiver and a long time heritage of APEX instrumentation, allowed the successful completion of the entire project over this short time. In this manuscript, the SEPIA receiver design, the receiver channel performance and the commissioning results are described.

SEPIA receiver design:

Optics: One of the major challenges of bringing the SEPIA receiver with installed ALMA cartridges to the APEX telescope is the necessity to implement tertiary optics, which should not only provide the required and frequency independent illumination of the secondary but also be compatible with the existing optical layout of the APEX Cabin A where all single-pixel heterotion offset from the FE center, the beam tilt offset compensating is different for, for example, ALMA Band 5 – 2.38º and for ALMA Band 9 cartridges – 0.93º. SEPIA tertiary optics shall accommodate all these constrains and differences with minimum number of reflecting surfaces (thus minimizing the reflecting loss) and fit a very confined volume within the APEX Cabin A. Specific for SEPIA and in contrast to ALMA FE, we use a rotating cartridge-selection mirror. Such an optical switch addresses limitations of the Nasmyth layout when one receiver channel has access to the sky at a time (Figure 2).

The cartridge-selection mirror (NMF3; Figure 2) with its precision computer-controlled rotating mechanism facilitates the accommodation of different ALMA cartridges having sdyne receivers are installed. Another serious constraint is a clearance of the APEX Cabin A Nasmyth tube, whose rim is limited by the elevation encoder down to 150 mm in diameter requiring precision alignment possibilities to avoid Band 5 receiver beam truncation. ALMA receiver cartridges have built-in cold optics optimized for their respective position inside the ALMA Front-End (FE) receiver cryostat place at the antenna focal plane. In particular, depending on the cartridge posipecific differences in the incoming beam positioning as outlined above.

APEX_Auto12

Figure 2: SEPIA optics (in scale). Mirrors CMF1, CMA1, CMF2, CMA2 and Cabin A Instrument switch, NMF1, are part of earlier existing tertiary optics for the APEX multi-channel facility instrument SHeFI and PI instruments of the APEX Cabin A. Mirrors NMA1, NMF2, NMF3 (SEPIA channel switch), NMA2 are parts of the SEPIA dedicated relay optics. Letters A and F in the mirror name indicate active and flat mirrors, respectively. Gaussian beam visualization is done for the lowest ALMA Band 5 RF frequency 158 GHz. The entire relay optics (APEX Cabin A+C+SEPIA) provides frequency independent illumination of the secondary with -12:5 dB edge taper and was designed to provide the beam clearance and optical element rims at 5ω (image credit: SEPIA Team)

The SEPIA optics have been designed and manufactured with its supporting parts by GARD. The optics is mounted on a separate supporting frame and employs precision mechanical reference devices; these devices were used for initial alignment of the optics with help of a reference laser. Once the optics as a whole is aligned with the tertiary optics of the APEX Cabin A, the reference devices have been locked to allow easier re-installation of the receiver optics and the cryostat without further alignment. The SEPIA tertiary optics unit is normally attached to the APEX Cabin A receiver supporting frame whereas the SEPIA cryostat is attached to the optics-supporting frame. The cryostat-optics frame mechanical interface allows accurate remating of these two parts of the SEPIA receiver without disturbing the optical alignment.

SEPIA cryostat: In order to accommodate the ALMA receiver cartridges at APEX, a compatible cryostat should be used which provides the required mechanical, cryogenic, vacuum, optical and electrical interfaces to the receiver cartridges. The space available in APEX Cabin A and the cabin door opening completely excluded the use of the ALMA FE cryostat. The Nasmyth layout of APEX allows using the receiver cryostat in a steady position with the cryocooler and the receiver cartridges placed vertically, thus achieving a more compact cryostat design. A similar approach though with differently designed receiver cartridges was used for the ASTE telescope.

The SEPIA cryostat was built around a three-stage Sumitomo RDK-3ST cryocooler generation 6, with 1W at 4K lift off power. In order to provide mechanical and cryogenic compatibility with the ALMA receiver cartridges, we used the same distances and dimensions of the cartridge holders as in the ALMA FE cryostat. Furthermore, the cartridge cooling thermal links at 110 K, 15 K and 4 K temperature stages have been produced by RAL (Rutherford Appleton Laboratory), UK to the ALMA FE specifications and used in the SEPIA cryostat. Figure 3, left shows the interior of the SEPIA cryostat with the mounted cartridge cooling thermal links and the Sumitomo cryocooler integrated into the cryostat. At the center of the cryostat, we used a stainless steel tube (protected by coaxial thermal shields) that connects the bottom and top bulkheads of the cryostat vacuum vessel and precludes the deflection of the bulkheads made off anodized aluminum.

The cryostat vacuum vessel is made of two aluminum bulkheads and a stainless steel tube interconnected using standard ISO-K claw clams. The top bulkhead has three ports to connect the vacuum pump, vacuum gauge, and the venting valve that are integrated with the SEPIA cryostat. All vacuum sealing is done with VitonTM O-rings. The SEPIA cryostat employs anti-vibration suspension of the cryo-cooler by using a bellow between the 300 K flange of the RDK-3ST cryocooler and the cryostat to keep the vacuum with the vibration-damping rubber elements installed at the brackets providing the mechanical attachment of the cryocooler to the cryostat bottom bulkhead (Figure 3, left). All thermal links between the cryocooler temperature stages and the elements of the receiver are made with flexible thermal contacts using braided 0.05mm diameter oxygen-free copper wires with each thermal link having a cross-sectional area of about 6mm2. This ensures that the vibration of the cryocooler is not transferred to the receiver, its supporting structure and the cryostat. The entire inner mechanical structure was simulated using AnsysTM FESS to verify that eigenfrequencies exceed 80 Hz.

The total mass of the SEPIA receiver with tertiary optics and its supporting frame, integrated turbo-pump and with three receiver cartridges loaded is expected to be approximately 315 kg. This also includes three cartridge loaders mounted at the receiver cartridge loading interface that could be flexibly arranged to fit any ALMA receiver cartridge. In order not to overload the APEX Cabin A receiver support structure, a mass-compensation scheme was implemented, comprising of the three supporting legs each equipped with the rubber foot and joint to the SEPIA cryostat via gas-spring. The gas springs each provide force at the level of 620 ±20 N giving a total of 1860 N of the mass compensation. Figure 4 shows a picture of the SEPIA receiver installed at its PI 2 position in the APEX Cabin A.

The cryostat temperature is monitored and controlled via a CryoConTM cryogenic temperature controller, Model 24C, which allows stabilizing the 4 K temperature deck to facilitate the SIS mixers gain stability and ensure that the sideband rejection performance of the SIS mixers is not affected by changing the physical temperature. Additionally, the CryoConTM signal from the cryogenic temperature sensors is used by the receiver control system to interlock the receiver shut down at the events of a power blackout, cold head or compressor failure when the raised temperature does not allow operating SIS mixers. A typical SEPIA cryostat cool-down time is around 8 hours reaching a physical temperature of about 3.9 K in the laboratory conditions and about 3.4 K at the telescope, respectively. The cryo-refrigerator capacity and SEPIA cryostat design allow operation of 3 cartridges without substantial changes in the physical temperature.

The SEPIA cryostat RF vacuum windows have the same design as in the ALMA FE cryostat, they are made of crystal quartz with Teflon coating and were produced by QMC Inc. (for SEPIA cartridge Bands 5 and 9). The SEPIA cryostat uses the same infrared (IR) filters as the corresponding bands of ALMA FE and those were delivered by ESO. The design of the SEPIA RF vacuum window holders and the IR filter brackets is such that it allows their exchange using a specialized tool without disassembling the cryostat, thus providing full flexibility of installing different frequency ALMA cartridges, that is, any of the ALMA Band 5–10 cartridges, on three positions of the SEPIA cryostat. The first channel pick-up mirror with its corresponding support bracket should be individually introduced for each of the (new) ALMA cartridges in order to accommodate them to the SEPIA tertiary optics.

APEX_Auto11

Figure 3: SEPIA cryostat interior. Left: the cartridge cooling thermal links at 110 K, 15K and 4K temperature stages (purchased from RAL, UK) and the Sumitomo cryocooler integrated into the cryostat. All thermal connections are made via oxygen-free copper flexible braids to avoid vibration transfer. Below the bulkhead, the elements of the anti-vibration cold-head suspension system are shown. Right: SEPIA cryostat interior with ALMA Band 5 pre-production cartridge installed and the idle cartridge places blanked. Fiberglass support plates that also limit thermal conductivity flux sufficiently well providing the mechanical stability of the thermal decks (image credit: SEPIA Team)

APEX_Auto10

Figure 4: SEPIA receiver. Left: CAD picture showing the entire assembly of the SEPIA cryostat and dedicated SEPIA tertiary optics attached to the APEX Cabin A instrument-supporting frame. The SEPIA support toward the Cabin A floor uses gas springs unloading the frame and providing stable mechanical contact with the cabin floor. Right: picture of the SEPIA receiver installed at its outer PI 2 position in the APEX Cabin A (image credit: SEPIA Team)

SEPIA receiver control and back end

The SEPIA receiver control system hardware uses standard ALMA FE bias modules (FEBM) for each receiver cartridge, which communicate with the ALMA Monitoring and Control (M&C) unit, all the hardware with its respective firmware was developed and built by NRAO. The M&C unit communicates with the PC computer that is running the SEPIA control software via CAN bus using a PCAN-USB interface from PEAK. All peripheral hardware communication with the control software is based on an Ethernet internal network and communication with the APEX control system is via a separate Ethernet connection. Figure 5 shows the block diagram of the SEPIA receiver control system, which consists of the receiver itself together with all peripheral hardware.

The SEPIA control system PC is a dual boot system. Engineering software running under MS WindowsTM 7 is based on IRTECON (Ermakov et al. 2001) and used for advanced tuning and diagnostic of the ALMA Band 5 receiver channel. For the ALMA Band 9 receiver channel, NOVA uses specialized scripting software to tune the receiver and check the hardware.

The observing control software is run under Linux and provided for all SEPIA channels by OSO. Tuning of the receivers is fully automatic and controlled by APECS (APEX Control System) via network communication based on a command language which is modeled after Standard Commands for Programmable Instruments (SCPI). In addition, the program offers a graphical user interface (GUI), allowing the observer to visually check the state of the frontend, inspect entries in the tuning table and perform manual tuning for testing purposes. The GUI also provides a plot of the typical atmospheric transmission at Chajnantor over the frequency band of the receiver, and marks current LO tuning and sideband locations. Furthermore, a log of all SCPI commands as well as a tabular view of raw CAN bus readings are provided for debugging purposes.

At present, the Band 5 and Band 9 receivers are controlled via two independent programs, which share most of their code base, but differ in parameters and hardware addressing.

The SEPIA receiver uses the XFFTS spectrometer, provided as part of the APEX collaboration by MPIfR. The spectrometer has 4–8 GHz x 4 bandwidth, which covers 100% of the SEPIA ALMA Band 5 receiver channel IF band and only 4–8 GHz x 2 of the IF band of the SEPIA ALMA Band 9 receiver channel whereas the IF band 8–12 GHz is not available in the current configuration. An upgrade of the IF system is planned for late 2018, which will cover 4–12 GHz IF bandwidth.

APEX_AutoF

Figure 5: Block diagram of the SEPIA receiver control system with all peripheral hardware (image credit: SEPIA Team)

SEPIA ALMA Band 5 receiver channel

ALMA Band 5 receivers have been developed, designed and six pre-production cartridges have been built under EC FP 6 "Infrastructure Enhancement Program". The details of the receiver cartridge design and performance can be found in Billade et al. (2012). The produced pre-production ALMA Band 5 cartridges have been delivered to ALMA via ESO. When ALMA Band 5 full production started, it was decided to spare the pre-production cartridges and one of those was made available by ESO for the APEX SEPIA receiver. Several important modifications have been made by GARD to the pre-production ALMA Band 5 cartridge to bring it up-to-date. The most serious upgrade was done to the LO and WCA, the hardware version produced by NRAO for full ALMA Band 5 production was used with appropriate changes in the cartridge design.

The cartridge 2SB SIS mixer assemblies for both polarizations have been replaced by the latest version with modifications of the 2SB SIS mixers and IF hybrid assembly used for the full ALMA Band 5 production project, for example, a slightly extended RF band down to 158 GHz that allows to avoid the "wings" of the 183 GHz water line when observing with the upper sideband. The summary of the specifications for SEPIA ALMA Band 5 receiver channel is presented in Table 4. In Figure 6, the picture of the modified ALMA Band 5 pre-production cartridge is shown along with the noise temperature and sideband rejection ratio performance for both polarizations performed in the lab keeping the SIS mixers at 4 K physical temperature. As mentioned above, the SEPIA cryostat is equipped with an active temperature stabilization system, which allows stabilizing of the SIS mixer physical temperature down to milli-Kelvin level. This ensures that the optimum 2SB mixer tuning setting for the best noise temperature and the sideband-rejection will remain stable with changes of the cold-head physical temperature which may occur, for example, between the cold head service periods.

Technical commissioning of the SEPIA receiver with the ALMA Band 5 receiver band installed was performed in February–March 2015. The commissioning included hardware installation, for example, reference LO PLL synthesizer, IF switches, vacuum control gauge, turbo-pump, computerized control system, etc., putting in place and alignment of the SEPIA tertiary optics and installing the SEPIA cryostat and the cold head compressor. The entire system was tested for integrity and the SEPIA Band 5 beam alignment was verified using small size nitrogen-cooled absorbers at all tertiary optics components and at the APEX Cabin C at the antenna focal plane.

Item

Specifications

Comment

Technology

2SB SIS mixers, dual polarization

 

RF band

157.36–211.64 GHz

 

IF band

4-8 GHz

For each sideband

IF bandwidth

4 x 4 GHz

 

SSB noise temperature

<50 K

Averaged over RF band < 45K

Sideband rejection ratio

>10 dB

Averaged over IF > 16 dB

Cross-polarization

<-23 dB

At the cartridge RF window

Table 4: Summary of the SEPIA ALMA Band 5 specifications and performance

APEX_AutoE

Figure 6: Left: SEPIA ALMA Band 5 pre-production receiver cartridge. Right: measured noise temperatures and the sideband rejection ratio of the SEPIA Band 5 receiver cartridge (image credit: SEPIA Team)

SEPIA ALMA Band 9 receiver channel

The NOVA instrumentation group at the Kapteyn Astronomical Institute in Groningen, The Netherlands, already produced a full set of ALMA Band 9 cartridges. For SEPIA, the NOVA group built another Band 9 cartridge at ALMA specifications and delivered it to the SEPIA PIs, ESO and OSO. This SEPIA ALMA Band 9 receiver has DSB dual polarization SIS mixers operating in the RF band of 599.77–722.15 GHz. The summary of the specifications for SEPIA ALMA Band 9 receiver channel is presented in Table 5. The details on technology and performance of the ALMA Band 9 cartridge can be found in Baryshev et al. (2015). The ALMA Band 9 LO, WCA and FE bias module were produced by NRAO and jointly purchased by ESO and OSO for the SEPIA project. Figure 7 shows the picture of the ALMA Band 9 production cartridge and the noise temperature for both polarizations with the measurements performed in the lab keeping the SIS mixers at 4 K physical temperature for the ALMA Band 9 Cartridge #74 installed in SEPIA.

Item

Specifications

Comment

Technology

DSB SIS mixers, dual polarization

 

RF band

599.77–722.15 GHz

 

IF band

4-12 GHz

DSB operation

IF bandwidth

2 x 8 GHz

Currently limited by the FFTS to 2 x 4 GHz

DSB noise temperature

75-125 K

Averaged over RF band ~ 100 K

Cross-polarization

-15.5 dB

At the cartridge RF window

Table 5: Summary of the SEPIA ALMA Band 9 specifications and performance

Technical commissioning of the SEPIA ALMA Band 9 receiver channel was performed in February–March 2016 by a joint NOVA-GARD team. The commissioning included hardware installation, IF switches, computerized control system extension and software update. The entire system was tested for integrity and the SEPIA Band 9 beam alignment was verified using small size nitrogen-cooled absorbers at all relay mirrors and finally at the APEX Cabin C at the antenna focal plane.

The current plan is to replace the SEPIA ALMA Band 9 DSB receiver channel in February–March 2018 with a new receiver cartridge, which employs 2SB SIS mixers with four IF outputs, that is, 4–12 GHz USB and LSB for each polarization, and otherwise has the same hardware and layout as the ALMA Band 9 cartridge.

APEX_AutoD

Figure 7: Left: SEPIA ALMA Band 9 pre-production receiver cartridge. Top right: typical co-polar beam pattern in a Band 9 cartridge (polarization 0 in cartridge #63, picture from Baryshev et al. (2015), with the secondary telescope mirror indicated (white circle). Bottom right: noise temperatures of the SEPIA Band 9 cartridge (image credit: SEPIA Team)

Sky commissioning

The sky commissioning of the SEPIA Band 5 receiver was successfully performed between February and September 2015. The sky commissioning of Band 9 has been started in June 2016. The atmospheric transmission at Llano de Chajnantor in the frequency range of the SEPIA Band 5 and Band 9 receivers is shown in Figure 8. A number of on-sky tests have to be conducted before a new instrument can be offered for observing time. Tuning Band 5 every 0.5 GHz, we confirmed that the whole tuning range from 157.36–211.64 GHz is accessible. Observing strong water masers as part of the science verification, Humphreys et al. (2017) determined the sideband suppression level to 17.7 dB from comparing the water line in the upper sideband to its ghost line in the lower sideband.

To determine the beam coupling efficiencies for Band 5, we observed Mars on UT 2016 April 6 and 7 at 208 GHz, and Jupiter on UT 2016 August 11 at 170 GHz. The respective apparent sizes of these planets were 12.700 for Mars and 31.400 for Jupiter. From the same planet observations, we derive deconvolved beam sizes of about 28.600 at 208 GHz and 31.700 at 170 GHz. This means the apparent size of Jupiter matched the beam size, while Mars was an unresolved source (semi-extended in the Herschel/HIFI nomenclature). Assuming an antenna far-field forward efficiency ηf = 0:95 (from Güsten et al. 2006, with 10% uncertainties across the 160–211 GHz range), we find beam coupling efficiencies of 0.67 ± 0.05 for Jupiter and 0.72 ± 0.07 for Mars,corresponding to Jy K-1 factors of 33.8 ± 2.5 and 38.4 ± 2.8 for resolved and unresolved sources, respectively.

To determine the beam coupling efficiencies for Band 9, we observed Jupiter on UT 2017 April 22, and Uranus on UT 2017 May 21, both at 691.5 GHz. The apparent sizes for Uranus and Jupiter were 3.400 and 42.600, respectively. From observations toward Callisto, which has an apparent size of 1.500, we determine a deconvolved beam size of 8.8 ± 0.5" at 691.5 GHz. Assuming the same antenna far-field forward efficiency ηf = 0:95 as above, we find beam coupling efficiencies of 0.46 ± 0.02 for Jupiter and 0.37 ± 0.03 for Uranus, corresponding to Jy K-1 factors of 63 ±3 and 79 ± 6, for resolved and unresolved sources, respectively.

These values were obtained after careful optimization of the antenna performance (replacement of the secondary surface and dish setting following holography results). At these high frequencies, the effective performance depends critically on both the antenna and the instrument. While we expect the performance of the instrument to be relatively stable, the antenna efficiencies can vary on time scales of weeks or months, due to the external weather factors. Users are encouraged to perform efficiency measurements as part of their science projects, or to contact APEX staff for the most recent values to be used.

Pointing and focus observations at APEX are conducted toward compact sources with either strong continuum or line emission. For the other APEX heterodyne instruments, these observations are often done in the main transitions of the CO molecule. Since the Band 5 frequency range does not contain a CO transition, we had to find sources that strongly emit in other molecular transitions. We chose evolved stars that are bright in HCN(2–1) or SiO (4–3) ν[0–3] masers. After testing their quality as pointing sources, we constructed a pointing catalog, which covers the whole LST range (Figure 9, Table B.1). The pointing sources are being used to compile a good pointing model for Band 5, using the professional version of the Telescope Pointing Analysis System TPOINT2 (Wallace 1994). With this receiver, pointing accuracies better than 2.5" are achieved.

The receiver stability was tested using a series of Allan variance measurements, both in spectral and total power mode. Results are shown in Figure 10. For all measurements, a channel resolution of 1.2 MHz was used. For both Band 5 and Band 9 spectroscopic stability times are always well above a few times 100 s. For the quoted resolution, even the total power stability is typically above 100 s, or above 30 s when scaled to 10MHz resolution.

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Figure 8: Atmospheric transmission at Llano de Chajnantor (Paine 2017) for the frequency ranges of the Band 5 (upper panel) and Band 9 (lower panel) receivers (image credit: SEPIA Team)

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Figure 9: Sky distribution of SEPIA Band 5 pointing sources (image credit: SEPIA Team)

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Figure 10: Results from Allan variance measurements. The bold horizontal lines mark the median results of all measurements taken, the top and bottom of the boxes show the 25th and 75th percentile, i.e., outline the middle 50% of the data (image credit: SEPIA Team)

 


 

Status and some science observations with the APEX telescope

Submillimeter astronomy is a relatively unexplored frontier in astronomy and reveals a Universe that cannot be seen in the more familiar visible or infrared light. It is ideal for studying the "cold Universe": light at these wavelengths shines from vast cold clouds in interstellar space, at temperatures only a few tens of degrees above absolute zero. Astronomers use this light to study the chemical and physical conditions in these molecular clouds — the dense regions of gas and cosmic dust where new stars are being born. Seen in visible light, these regions of the Universe are often dark and obscured due to the dust, but they shine brightly in the millimeter and submillimeter part of the spectrum. This wavelength range is also ideal for studying some of the earliest and most distant galaxies in the Universe, whose light has been redshifted into these longer wavelengths.

 

• May 14, 2018: At first glance, this image may resemble red ink filtering through water or a crackling stream of electricity, but it is actually a unique view of our cosmic home. It reveals the central plane of the Milky Way as seen by ESA's Planck satellite and the APEX (Atacama Pathfinder Experiment), which is located at an altitude of around 5100 m in the Chilean Andes and operated by ESO (European Southern Observatory). 6)

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Figure 11: This image was released in 2016 as the final product of an APEX survey mapping the galactic plane visible from the southern hemisphere at submillimeter wavelengths (between infrared and radio on the electromagnetic spectrum). It complements previous data from ESA's Planck and Herschel space observatories (image credit: ESO/ATLASGAL consortium; ESA/Planck)

- Planck and APEX are an ideal pairing. APEX is best at viewing small patches of sky in great detail while Planck data is ideal for studying areas of sky at the largest scales. It covers the entire sky – no mean feat. The two work together well, and offer a unique perspective on the sky.

- This image reveals numerous objects within our galaxy. The bright pockets scattered along the Milky Way's plane in this view are compact sources of submillimeter radiation: very cold, clumpy, dusty regions that may shed light on myriad topics all the way from how individual stars form to how the entire Universe is structured.

- From right to left, notable sources include NGC 6334 (the rightmost bright patch), NGC 6357 (just to the left of NGC 6334), the galactic core itself (the central, most extended, and brightest patch in this image), M8 (the bright lane branching from the plane to the bottom left), and M20 (visible to the upper left of M8).

• April 25, 2018: Using ALMA (Atacama Large Millimeter/submillimeter Array) and APEX (Atacama Pathfinder Experiment), two international teams of scientists led by Tim Miller from Dalhousie University in Canada and Yale University in the US and Iván Oteo from the University of Edinburgh, United Kingdom, have uncovered startlingly dense concentrations of galaxies that are poised to merge, forming the cores of what will eventually become colossal galaxy clusters. 7)

- Peering 90% of the way across the observable Universe, the Miller team observed a galaxy protocluster named SPT2349-56. The light from this object began travelling to us when the Universe was about a tenth of its current age.

- The individual galaxies in this dense cosmic pileup are starburst galaxies and the concentration of vigorous star formation in such a compact region makes this by far the most active region ever observed in the young Universe. Thousands of stars are born there every year, compared to just one in our own Milky Way.

- The Oteo team discovered a similar megamerger formed by ten dusty star-forming galaxies, nicknamed a "dusty red core" because of its very red color, by combining observations from ALMA and the APEX.

- Iván Oteo explains why these objects are unexpected: "The lifetime of dusty starbursts is thought to be relatively short, because they consume their gas at an extraordinary rate. At any time, in any corner of the Universe, these galaxies are usually in the minority. So, finding numerous dusty starbursts shining at the same time like this is very puzzling, and something that we still need to understand."

- These forming galaxy clusters were first spotted as faint smudges of light, using the South Pole Telescope and the Herschel Space Observatory. Subsequent ALMA and APEX observations showed that they had unusual structure and confirmed that their light originated much earlier than expected — only 1.5 billion years after the Big Bang.

- The new high-resolution ALMA observations finally revealed that the two faint glows are not single objects, but are actually composed of fourteen and ten individual massive galaxies respectively, each within a radius comparable to the distance between the Milky Way and the neighboring Magellanic Clouds.

- "These discoveries by ALMA are only the tip of the iceberg. Additional observations with the APEX telescope show that the real number of star-forming galaxies is likely even three times higher. Ongoing observations with the MUSE instrument on ESO's VLT are also identifying additional galaxies," comments Carlos De Breuck, ESO astronomer.

- "How this assembly of galaxies got so big so fast is a mystery. It wasn't built up gradually over billions of years, as astronomers might expect. This discovery provides a great opportunity to study how massive galaxies came together to build enormous galaxy clusters," says Tim Miller, a PhD candidate at Yale University and lead author of one of the papers.

- This research was presented in two papers, "The Formation of a Massive Galaxy Cluster Core at z = 4.3", by T. Miller et al., to appear in the journal Nature, and "An Extreme Proto-cluster of Luminous Dusty Starbursts in the Early Universe", by I. Oteo et al., which appeared in the Astrophysical Journal.

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Figure 12: Artist's impression of ancient galaxy megamerger. The ALMA and APEX telescopes have peered deep into space — back to the time when the Universe was one tenth of its current age — and witnessed the beginnings of gargantuan cosmic pileups: the impending collisions of young, starburst galaxies. Astronomers thought that these events occurred around three billion years after the Big Bang, so they were surprised when the new observations revealed them happening when the Universe was only half that age! These ancient systems of galaxies are thought to be building the most massive structures in the known Universe: galaxy clusters (image credit: ESO)

• October 2017: Comet composition provides critical information on the chemical and physical processes that took place during the formation of the Solar system. Reported are millimeter spectroscopic observations of the long-period bright comet C/2014 Q2 (Lovejoy) using the APEX (Atacama Pathfinder Experiment) band 1 receiver between 2015 January UT 16.948 and 18.120, when the comet was at heliocentric distance of 1:30 au and geocentric distance of 0:53 au. Bright comets allow for sensitive observations of gaseous volatiles that sublimate in their coma. These observations allowed the study team to detect HCN, CH3OH (multiple transitions), H2CO and CO, and to measure precise molecular production rates. Additionally, sensitive upper limits were derived on the complex molecules acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the strongest lines in the targeted spectral range to improve the signal-to-noise ratio. 8)

• May 12, 2017: An extension of the agreement between the partners of the Atacama Pathfinder Experiment (APEX) has been signed, ensuring that this very productive collaboration will continue until the end of 2022. The 12 m APEX telescope saw first light in 2005 and has provided astronomers with detailed views of the coldest objects and processes in the Universe. 9)

- APEX is a collaborative effort between the Max-Planck-Institute for Radio Astronomy (MPIfR) in Bonn, Germany, ESO and the Onsala Space Observatory (OSO) in Onsala, Sweden, and the agreement was signed by ESO's Director General, Tim de Zeeuw, Karl Menten, Director at the Max-Planck-Institut für Radioastronomie and John Conway, Director of the Onsala Space Observatory. The ceremony took place at Chalmers University of Technology in Gothenburg, Sweden.

- Under the APEX extension agreement, the telescope will be upgraded to significantly improve the overall observing efficiency, and the suite of instruments will be upgraded to a new generation. These new instruments include several prototype receivers for ALMA, opening up new atmospheric windows (eso1543) and increasing the bandwidth of existing receivers. In order to better accommodate the high demand for APEX from the ESO community, ESO's share will increase from 27% to 32%. The MPIfR share will also increase from 50% to 55%, while the OSO share will decrease from 23% to 13%.

• February 24, 2016: A spectacular new image of the Milky Way has been released to mark the completion of the ATLASGAL (APEX Telescope Large Area Survey of the Galaxy). The APEX telescope in Chile has mapped the full area of the galactic plane visible from the Southern Hemisphere for the first time at submillimeter wavelengths — between infrared light and radio waves — and in finer detail than recent space-based surveys. The pioneering 12 m APEX telescope allows astronomers to study the cold universe: gas and dust only a few tens of degrees above absolute zero. 10)

- The new ATLASGAL maps cover an area of sky 140 degrees long and 3 degrees wide, more than four times larger than the first ATLASGAL release. The new maps are also of higher quality, as some areas were re-observed to obtain a more uniform data quality over the whole survey area.
Note: The first data release covered an area of approximately 95 square degrees, a very long and narrow strip along the Galactic Plane two degrees wide and over 40 degrees long. The final maps now cover 420 square degrees, more than four times larger.

- The ATLASGAL survey is the single most successful APEX large program with nearly 70 associated science papers already published, and its legacy will expand much further with all the reduced data products now available to the full astronomical community. The data products are available through the ESO archive.

- At the heart of APEX are its sensitive instruments. One of these, LABOCA (LArge BOlometer Camera) was used for the ATLASGAL survey. LABOCA measures incoming radiation by registering the tiny rise in temperature it causes on its detectors and can detect emission from the cold dark dust bands obscuring the stellar light.

- The new release of ATLASGAL complements observations from ESA's Planck and Herschel satellites. The combination of the Planck and APEX data allowed astronomers to detect emission spread over a larger area of sky and to estimate from it the fraction of dense gas in the inner Galaxy. The ATLASGAL data were also used to create a complete census of cold and massive clouds where new generations of stars are forming.
Note: The Planck data cover the full sky, but with poor spatial resolution. ATLASGAL covers only the Galactic plane, but with high angular resolution. Combining both provides excellent spatial dynamic range. Herschel has mapped the full galactic plane, but at shorter wavelengths than ATLASGAL. These datasets are highly complementary.

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Figure 13: A spectacular new image of the Milky Way has been released to mark the completion of the ATLASGAL. The APEX data, at a wavelength of 0.87 mm, shows up in red and the background blue image was imaged at shorter infrared wavelengths by the NASA Spitzer Space Telescope as part of the GLIMPSE survey (image credit: ESO/APEX/ATLASGAL consortium/NASA/GLIMPSE consortium/ESA/Planck)

• February 10, 2016: A celebration was held at the APEX base station in Sequitor, San Pedro de Atacama, to mark ten years of astronomical research with the APEX telescope, which resides on Chile's Chajnantor Plateau, 5100 m above sea level. 11)

- From there, the telescope observes the sky in submillimeter wavelengths, giving astronomers access to the coldest regions of the Universe, which cannot be seen in visible light. As it looks forward to further discoveries at the forefront of observational astronomy, the APEX 12-metre telescope has already made significant contributions across a variety of astronomical fields, including the discovery of new interstellar molecules and deep imaging of the submillimetre sky, leading to new insights into star formation in the Milky Way and distant starburst galaxies in the early Universe.

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Figure 14: The APEX telescope and visitors on the occasion of the 10th anniversary (image credit: ESO)

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Figure 15: Party-goers gather outside the APEX Base Station (image credit: ESO)

• February 9, 2016: On Earth, hydrogen peroxide plays a key role in the chemistry of water and ozone in our planet's atmosphere, and is familiar for its use as a disinfectant or to bleach hair blonde. Now it has been detected in space by a team of astronomers from Sweden and Germany using the APEX telescope in Chile. APEX is a collaboration between Onsala Space Observatory (OSO), the Max Planck Institute for Radio Astronomy (MPIfR) and ESO. 12)

- An international team of astronomers made the discovery with the APEX (Atacama Pathfinder Experiment) telescope, situated on the 5000 m high Chajnantor plateau in the Chilean Andes. They observed a region in our galaxy close to the star Rho Ophiuchi, about 400 light-years away. The region contains very cold (around -250 degrees Celsius), dense clouds of cosmic gas and dust, in which new stars are being born. The clouds are mostly made of hydrogen, but contain traces of other chemicals, and are prime targets for astronomers hunting for molecules in space. Telescopes such as APEX, which make observations of light at millimeter- and submillimeter-wavelengths, are ideal for detecting the signals from these molecules.

- Now, the team has found the characteristic signature of light emitted by hydrogen peroxide, coming from part of the Rho Ophiuchi clouds.

- "We were really excited to discover the signatures of hydrogen peroxide with APEX. We knew from laboratory experiments which wavelengths to look for, but the amount of hydrogen peroxide in the cloud is just one molecule for every ten billion hydrogen molecules, so the detection required very careful observations," says Per Bergman, astronomer at Chalmers and Onsala Space Observatory. Bergman is lead author of the study, which is published in the journal Astronomy & Astrophysics. 13)

- Hydrogen peroxide (H2O2) is a key molecule for both astronomers and chemists. Its formation is closely linked to two other familiar molecules, oxygen and water, which are critical for life. Because much of the water on our planet is thought to have originally formed in space, scientists are keen to understand how it is created.

- Hydrogen peroxide is thought to form in space on the surfaces of cosmic dust grains — very fine particles similar to sand and soot — when hydrogen (H) is added to oxygen molecules (O2). A further reaction of the hydrogen peroxide with more hydrogen is one way to produce water (H2O). This new detection of hydrogen peroxide will therefore help astronomers better understand the formation of water in the Universe.

- "We don't understand yet how some of the most important molecules here on Earth are made in space. But our discovery of hydrogen peroxide with APEX seems to be showing us that cosmic dust is the missing ingredient in the process," says Bérengère Parise, head of the Emmy Noether research group on star formation and astrochemistry at the Max-Planck Institute for Radio Astronomy in Germany, and a co-author of the paper.

- The new discovery of hydrogen peroxide may also help astronomers understand another interstellar mystery: why oxygen molecules are so hard to find in space. It was only in 2007 that oxygen molecules were first discovered in space, by a team of scientists (among them Chalmers researchers) using the satellite Odin.

- To work out just how the origins of these important molecules are intertwined will need more observations of Rho Ophiuchi and other star-forming clouds with future telescopes such as the ALMA (Atacama Large Millimeter/submillimeter Array) — and help from chemists in laboratories on Earth.

• July 18, 2012: An international team of astronomers has observed the heart of a distant quasar with unprecedented sharpness, two million times finer than human vision. The observations, made by connecting the Atacama Pathfinder Experiment (APEX) telescope to two others on different continents for the first time, is a crucial step towards the dramatic scientific goal of the "Event Horizon Telescope" project: imaging the supermassive black holes at the center of our own galaxy and others. 14)

- Astronomers connected APEX, in Chile, to the SMA (Submillimeter Array) in Hawaii, USA, and the SMT (Submillimeter Telescope) in Arizona, USA. They were able to make the sharpest direct observation ever, of the center of a distant galaxy, the bright quasar 3C 279, which contains a supermassive black hole with a mass about one billion times that of the Sun, and is so far from Earth that its light has taken more than 5 billion years to reach us. APEX is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), the Onsala Space Observatory (OSO) and ESO. APEX is operated by ESO.

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Figure 16: This is an artist's impression of the quasar 3C 279. Quasars are the very bright centers of distant galaxies that are powered by supermassive black holes. This quasar contains a black hole with a mass about one billion times that of the sun, and is so far from Earth that its light has taken more than 5 billion years to reach us. The team were able to probe scales of less than a light-year across the quasar — a remarkable achievement for a target that is billions of light-years away (image credit: ESO, M. Kornmesser)

- The telescopes were linked using a technique known as VLBI (Very Long Baseline Interferometry). Larger telescopes can make sharper observations, and interferometry allows multiple telescopes to act like a single telescope as large as the separation — or "baseline" — between them. Using VLBI, the sharpest observations can be achieved by making the separation between telescopes as large as possible. For their quasar observations, the team used the three telescopes to create an interferometer with transcontinental baseline lengths of 9447 km from Chile to Hawaii, 7174 km from Chile to Arizona and 4627 km from Arizona to Hawaii. Connecting APEX in Chile to the network was crucial, as it contributed the longest baselines.

- The observations were made in radio waves with a wavelength of 1.3 mm. This is the first time observations at a wavelength as short as this have been made using such long baselines. The observations achieved a sharpness, or angular resolution, of just 28 microarcseconds (marsec) — about 8 billionths of a degree. This represents the ability to distinguish details an amazing two million times sharper than human vision. Observations this sharp can probe scales of less than a light-year across the quasar — a remarkable achievement for a target that is billions of light-years away.

- The observations represent a new milestone towards imaging supermassive black holes and the regions around them. In future it is planned to connect even more telescopes in this way to create the so-called Event Horizon Telescope. The Event Horizon Telescope will be able to image the shadow of the supermassive black hole in the center of our Milky Way galaxy, as well as others in nearby galaxies. The shadow — a dark region seen against a brighter background — is caused by the bending of light by the black hole, and would be the first direct observational evidence for the existence of a black hole's event horizon, the boundary from within which not even light can escape.

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Figure 17: Illustration of the VLBI configuration of APEX, SMA and SMT (image credit: ESO, L. Calçada)

• November 11, 2008: The RCW120 is a Milky Way Nebula towards the Constellation Scorpius, some 4500 light years away from Earth (Figure 18). 15)

- A hot, massive star in its center is emitting huge amounts of ultraviolet radiation, which ionizes the surrounding gas, stripping the electrons from hydrogen atoms and producing the characteristic red glow of so-called H-alpha emission. As this ionized region expands into space, the associated shock wave sweeps up a layer of the surrounding cold interstellar gas and cosmic dust. This layer becomes unstable and collapses under its own gravity into dense clumps, forming cold, dense clouds of hydrogen where new stars are born.

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Figure 18: Color composite image of RCW120. It reveals how an expanding bubble of ionized gas about ten light-years across is causing the surrounding material to collapse into dense clumps where new stars are then formed. The 870 µm submillimeter-wavelength data were taken with the LABOCA camera on the 12 m APEX (Atacama Pathfinder Experiment) telescope. Here, the submillimeter emission is shown as the blue clouds surrounding the reddish glow of the ionized gas (shown with data from the SuperCosmos H-alpha survey). The image also contains data from the Second Generation Digitized Sky Survey (I-band shown in blue, R-band shown in red), image credit: ESO/APEX/DSS2/ SuperCosmos/ Deharveng(LAM)/ Zavagno(LAM)

• December 22, 2005: In November 2005, CONDOR (CO N+ Deuterium Observation Receiver), opened its eye to the universe for the first time. CONDOR was installed at APEX (Atacama Pathfinder EXperiment) in the Chilean Andes and detected hot gas in the vicinity of young massive stars from radiation at the extremely high radio frequency of 1.5 terahertz (THz). The CONDOR detections are the first THz-frequency observations acquired with a large telescope (12 m diameter). The observations reveal several surprises, and the expectation that THz astronomy would yield valuable scientific results has been met. The success of CONDOR is a combined effort of researchers from the First Physical Institute of the University of Cologne and the Max Planck Institute for Radio Astronomy. 16)

- "CONDOR has fully met our expectations," said Martina Wiedner, the CONDOR project leader. "We prepared the receiver well, we had an excellent team at the site, but we also were lucky that the weather was so good." Because of the difficulty in detecting electromagnetic waves at such high frequencies (a thousand times higher than those of a cellular phone and a million times higher than "short wave" radio), state-of-the-art receivers have to be used. A special type of device called a Hot Electron Bolometer, developed at the University of Cologne by Karl Jacobs and his colleagues, was essential for CONDOR's success. This device converts the THz frequency radiation into frequencies around 1 GHz, which are much easier to manipulate. To achieve high sensitivity, the receiver is cooled to a temperature of -269°C, only 4°C above absolute zero.

- The CONDOR observations require that the amount of water vapor in the Earth atmosphere is exceptionally small, because water vapor readily absorbs THz radiation. Located in the Atacama Desert of Chile at an elevation of 5100 m, the site of the APEX telescope (Figure 19) is extraordinarily dry. APEX has a 12 m primary mirror that resembles a perfect paraboloid to within 15 µm (7 times thinner than a human hair). The telescope is currently equipped with receivers between 300 and 900 GHz. CONDOR, which requires different technology, is the first APEX receiver operational above 1 THz. "The CONDOR observations are done at the highest frequencies that APEX expects to ever reach," explains the APEX project manager, Rolf Güsten. "At even higher frequencies, the Earth's atmosphere becomes opaque until one reaches the infrared wavelengths."

- The CONDOR observations on APEX open up the nearly unknown THz universe for exploration. "If one could only see blue things, one would never know about trees and grass," explains Martina Wiedner. "Similarly, one discovers new things in the universe by looking at it in different frequencies. The spectral signatures of hot gas (high rotational transitions of the carbon monoxide (CO) molecule) are seen at THz frequencies. Since hot gas is an essential component of massive star formation, regions that give birth to massive stars can be observed at these frequencies." CONDOR detected emission from CO at 1.5 THz during its first observations (Figure 20). The line width is surprisingly narrow, suggesting that the gas is heated by ultraviolet (UV) radiation from stars, rather than collisions within gases as originally expected. Astronomers on the CONDOR project team are eager to make further observations.

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Figure 19: Photo of the new APEX Telescope (image credit: ESO, MPIfR)

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Figure 20: CONDOR detected emissions of CO at 1.5 THz during its first observations (image credit: ESO, MPIfR)

 


1) "APEX - Reaching new heights in submillimeter astronomy," ESO, URL: http://www.eso.org/public/teles-instr/apex/

2) "The Atacama Pathfinder Experiment —Reaching New Heights in Submillimeter Astronomy," ESO, 28 Oct. 2010, URL: http://www.eso.org/public/archives/handouts/pdf/handout_0015.pdf

3) "Atacama Pathfinder EXperiment APEX Telescope," APEX Web Team, 24 May 2015, URL: http://www.apex-telescope.org/telescope/

4) "Atacama Pathfinder EXperiment APEX Instrumentation," URL: http://www.apex-telescope.org/instruments/

5) V. Belitsky, I. Lapkin, M. Fredrixon, D. Meledin, E. Sundin, B. Billade, S.-E. Ferm, A. Pavolotsky, H. Rashid, M. Strandberg, V. Desmaris, A. Ermakov, S. Krause, M. Olberg, P. Aghdam, S. Shafiee, P. Bergman, E. De Beck, H. Olofsson, J. Conway, C. De Breuck, K. Immer, P. Yagoubov, F. M. Montenegro-Montes, K. Torstensson, J.-P. Pérez-Beaupuits, T. Klein, W. Boland, A. M. Baryshev, R. Hesper, J. Barkhof, J. Adema, M. E. Bekema, A. Koops, "SEPIA – a new single pixel receiver at the APEX telescope," Astronomy & Astrophysics, Vol. 23, A23, 2018, https://doi.org/10.1051/0004-6361/201731458 ESO, 2018, URL: https://www.aanda.org/articles/aa/pdf/2018/04/aa31458-17.pdf

6) "Our galaxy's heart," ESA Space Science Image of the Week, 14 May 2018, URL: http://m.esa.int/spaceinimages/Images/2018/05/Our_galaxy_s_heart

7) Ancient Galaxy Megamergers — ALMA and APEX discover massive conglomerations of forming galaxies in early Universe," ESO 1812 Science Release, 25 April 2018, URL: https://www.eso.org/public/news/eso1812/"

8) M. de Val-Borro, S. N. Milam, M. A. Cordiner, S. B. Charnley, I. M. Coulson, A. J. Remijan, G. L. Villanueva, "Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope," MNRAS 000, 1{10 (2017), DOI: 10.1093/mnras/stx2802, URL: https://arxiv.org/pdf/1710.11117.pdf

9) "APEX Extension Agreement Signed," ESO ann17026, 12 May 2017, URL: https://www.eso.org/public/announcements/ann17026/

10) "ATLASGAL Survey of Milky Way Completed," ESO 1606 Photo Release, 24 Feb. 2016, URL: https://www.eso.org/public/news/eso1606/

11) "APEX Celebrates Ten Years Exploring the Cold Universe — Atacama Pathfinder Experiment in Chile looks forward to further success at the cutting edge of astronomy," ESO ann16008, 10 Feb. 2016, URL: https://www.eso.org/public/announcements/ann16008/

12) "Hydrogen Peroxide Found in Space — Molecules of hydrogen peroxide have been found for the first time in interstellar space. The discovery gives clues about the chemical link between two molecules critical for life: water and oxygen," Chalmers, 09 Feb. 2016, URL: https://www.chalmers.se/en/news/Pages
Hydrogen-Peroxide-Found-in-Space.aspx

13) P. Bergman, B. Parise, R. Liseau, B. Larsson, H. Olofsson, K. M. Menten, R. Güsten, "Astronomy & Astrophysics Letters, Volume 531, July 2011, DOI: 10.1051/0004-6361/201117170, URL: https://www.aanda.org/articles/aa/pdf/2011/07/aa17170-11.pdf

14) "APEX telescope takes part in sharpest observation ever — Telescopes in Chile, Hawaii, and Arizona reach sharpness two million times finer than human vision," ESO 1229 Organization Release, July 18 2012, URL: https://www.eso.org/public/news/eso1229/

15) "Glowing stellar nurseries," ESO Press Release eso0840a, 11 Nov. 2008, URL: http://www.eso.org/public/images/eso0840a/

16) "CONDOR's First Look at the Universe — First light of a state-of-the-art terahertz receiver at the APEX telescope in Chile," MPG (Max Planck Gesellschaft), 22 Dec. 2005, URL: https://www.mpg.de/514249/pressRelease20051222
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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