Combined Array for Research in Millimeter-wave Astronomy

According to the CARMA observatory catalog, the median height of all telescope pads was at an elevation of {{convert|2196.223|m|ft|3|sp=us}}. The observatory was located in the Inyo Mountains to the east of the Owens Valley Radio Observatory, at a site called Cedar Flat (after relocating the [https://www.fs.usda.gov/recarea/inyo/recreation/camping-cabins/recarea/?recid=20274&actid=33 Cedar Flat Group Camps] to the west of Hwy-168), accessed through Westgard Pass. The high elevation site was chosen to minimize millimeter wave absorption and phase decoherence by atmospheric water vapor.{{Cite web

|url=https://newsarchive.berkeley.edu/news/media/releases/2004/10/29_carma.shtml

|title=Out with the BIMA, in with the ATA

|publisher=UC Berkeley

|quote=The high-elevation Cedar Flat site is better for millimeter-wave astronomy than Hat Creek, Plambeck said, because there is less water vapor to interfere with radio observations.

|date=October 29, 2004

|access-date=March 7, 2025}}

== Features ==

This array was unique for being a heterogeneous collection of radio telescopes of varying sizes and design. There were three types of telescopes, all Cassegrain reflector antennas with parabolic primary mirrors and hyperbolic secondary mirrors:{{cite journal |title=Leo Blitz (1945–2022) |journal=Bulletin of the AAS |date=2023 |volume=55 |issue=1 |url=https://doi.org/10.3847/25c2cfeb.008e4e7b |access-date=March 5, 2025 |quote=The next step was to expand the number of galaxies studied, but BIMA wasn't sensitive enough...Enter the Combined Array for Research in Millimeter-wave Astronomy (CARMA), a combination of six 10.4-meter telescopes (from Caltech), nine 6.1-meter (from BIMA), and eight 3.5-meter (from University of Chicago) telescopes.}}

• Six Leighton Radio Telescopes in diameter. These were part of the Millimeter Array at the OVRO site operated by Caltech. They were moved to Cedar Flat in the Spring of 2005.
• Nine telescopes each {{convert|6.1|m|ft|abbr=on}} in diameter. These were formerly located at the Hat Creek Radio Observatory and operated by the [https://web.archive.org/web/20110411000816/http://bima.astro.umd.edu/ Berkeley-Illinois-Maryland-Association] (BIMA) consortium. These were moved from HCRO in the spring of 2005 to Cedar Flat.
• Eight telescopes each {{convert|3.5|m|ft|abbr=on}} in diameter. These were built as an instrument for cosmology and are also known as the Sunyaev-Zel'dovich Array (SZA), a project led by John Carlstrom at the University of Chicago. The SZA spent three years on the valley floor at the Owens Valley Radio Observatory observing the cosmic microwave background (CMB) and galaxy clusters. In the summer of 2008 it was moved up to Cedar Flat.

File:CARMA 2012 17.jpg

According to Leo Blitz, when in 1996 he took over directorship of Berkeley's Radio Astronomy Laboratory, the giant millimeter-wave astronomy project now in Atacama (ALMA) was already in prospect, although far from completion.{{Cite web

|url=https://www.aspbooks.org/publications/356/127.pdf

|work = ASP Conference Series, Vol. 356: Revealing the Molecular Universe: One Antenna Is Never Enough

|publisher = Astronomical Society of the Pacific

|title=The Future of the UC Berkeley Radio Astronomy Laboratory

|last=Blitz

|first=Leo

|quote=When I became RAL director in 1996...To further the work being done at the millimeter wavelengths, with ALMA looming on the horizon (then known at the Millimeter Array (MMA)), I thought it important to join the BIMA array with the Owens Valley Radio Observatory (OVRO) millimeter array to form what is now known as California Array for Millimeter Astronomy (CARMA), which is currently in the process of being commissioned.

|date=2006

|access-date=March 7, 2025}} Over the next decade-plus, RAL moved the millimeter-wave telescopes that had been at their Hat Creek site -- six from OVRO and nine from BIMA -- to the higher-altitude at Cedar Flat, and coordinated them in what became known as CARMA. (They were replaced at Hat Creek by the beginnings of the Allen Telescope Array.)

{{As of|2006|11}}, the first fifteen telescopes were working together to gather scientific data. Pioneering work on compensating for the image distortion resulting from turbulent water vapor distributions in the troposphere started in the fall of 2008.

The most extended configurations of the array, up to {{convert|2|km|mi|sp=us}}, were required for viewing the finest details in astronomical images.{{Citation needed|date=November 2018}} Over these distances the variation in the time of arrival of signals at the different telescopes as they pass through different amounts of water vapor severely limits the quality of images.The temporal power spectrum of atmospheric fluctuations due to water vapor (aanda.org)

By siting an SZA antenna near each of the CARMA antennas and observing a compact astronomical radio source near the source under study, the properties of the atmosphere could be measured on time scales as short as a couple of seconds. This information could be used in the data reduction process to remove a significant fraction of the degradation caused by the atmospheric scintillation.{{Cite web|title=Beating atmospheric scintillation at millimeter and submillimeter wavelengths|url=https://spie.org/news/1685-beating-atmospheric-scintillation-at-millimeter-and-submillimeter-wavelengths|access-date=2021-12-01|website=spie.org}}

Observations using the SZA (operating at 30 GHz) to make the atmospheric measurements started in November 2008. The SZA has also participated directly in the science operations of CARMA during experiments where all three types of telescopes were attached to the same correlator.

Observations were primarily in the 3 mm range (80–115 GHz) and the 1 mm range (210–270 GHz). These frequencies are useful for detecting many molecular gases, including the second most abundant molecule in the universe, carbon monoxide (CO).

Observing CO is an indirect indicator of the presence of molecular hydrogen gas (the most abundant molecule in the universe) which is difficult to detect directly. Cold dust is also detectable in this wavelength range and can be used to study planet-forming disks around stars, for example. In 2009, the OVRO 10.4 m antennas were instrumented with 27–35 GHz receivers and made observations in the centimeter band in concert with the SZA antennas.{{Citation needed|date=November 2018}}

The telescopes ceased operation in 2015 and were relocated to the Owens Valley Radio Observatory for storage.{{Cite web

|url=https://www.astro.umd.edu/~mpound/mmarray.org/intro.html

|title=Combined Array for Research in Millimeter-wave Astronomy

|publisher=University of Maryland Astronomy Department

|quote=Observations finished on April 3, 2015, and the Observatory was shut down. Decommissioning is complete. The equipment has been removed from all the buildings. Antenna removal started on June 16, 2015.

|date=2015

|access-date=March 7, 2025}}

File:CARMA 2012 17.jpg

CARMA was an array element in the early proof-of-concept observations by the Event Horizon Telescope project, and in 2007 participated in observations which showed that event-horizon-scale structures could be seen in the Milky Way's supermassive black hole, Sgr A*.{{cite journal |last1=Doeleman |first1=Shepard S. |last2=Weintroub |first2=Jonathan |last3=Rogers |first3=Alan E.E. |last4=Plambeck |first4=Richard |last5=Tilanus |first5=Remo P.J. |last6=Friberg |first6=Per |last7=Ziurys |first7=Lucy M.|author7-link= Lucy Ziurys |last8=Moran |first8=James M. |last9=Corey |first9=Brian |last10=Young |first10=Ken H. |last11=Smythe |first11=Daniel L. |last12=Titus |first12=Michael |last13=Marrone |first13=Daniel P. |last14=Cappallo |first14=Roger J. |last15=Bock |first15=Douglas C.J. |last16=Bower |first16=Geoffrey C. |last17=Chamberlin |first17=Richard |last18=Davis |first18=Gary R. |last19=Krichbaum |first19=Thomas P. |last20=Lamb |first20=James |last21=Maness |first21=Holly |last22=Niell |first22=Authur E. |last23=Roy |first23=Alan |last24=Strittmatter |first24=Peter |last25=Werthimer |first25=Daniel |last26=Whitney |first26=Alan R. |last27=Woody |first27=David |title=Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre |journal=Nature |date=4 September 2008 |volume=455 |issue=7209 |pages=78–80 |doi=10.1038/nature07245 |pmid=18769434 |url=https://www.nature.com/articles/nature07245|accessdate=21 November 2020|arxiv=0809.2442 |s2cid=4424735 }}

CARMA was a consortium composed of three primary groups.

California Institute of Technology, Berkeley-Illinois-Maryland Association (BIMA), University of Chicago

• California Institute of Technology
• University of California, Berkeley, Radio Astronomy Laboratory
• University of Chicago
• University of Illinois at Urbana-Champaign, Laboratory for Astronomical Imaging{{Cite web

|url=https://web.archive.org/web/20050412085632/http://www.astro.uiuc.edu/projects/lai/

|title=What is LAI?

|publisher=University of Illinois

|quote=The Laboratory for Astronomical Imaging (LAI) is located in the department of Astronomy at the University of Illinois. LAI is the Illinois contingent of the Berkeley-Illinois-Maryland-Association (BIMA), the consortium which runs the BIMA array (a 10-element mm-wave radio interferometer) in Hat Creek, California.

|date=April 12, 2005

|access-date=March 31, 2025}}

• University of Maryland, College Park, Laboratory for Millimeter-wave Astronomy{{Cite web

|url=https://web.archive.org/web/20020811173534/http://www.astro.umd.edu/rareas/lma/

|title=Laboratory for Millimeter-Wave Astronomy

|publisher=University of Maryland

|quote=The Laboratory for Millimeter-Wave Astronomy (LMA) is an organization set up within the Department of Astronomy to manage Maryland's 20% participation in the Berkeley Illinois Maryland Array (BIMA), a millimeter-wave radio array in Hat Creek, California.

|date=August 11, 2002

|access-date=March 7, 2025}}

• Atacama Large Millimeter Array
• Interferometry
• Owens Valley Radio Observatory
• Radio astronomy
• Sunyaev–Zeldovich effect

{{reflist}}

{{commons category|Combined Array for Research in Millimeter-wave Astronomy}}

• [http://www.mmarray.org CARMA Website] {{Webarchive|url=https://web.archive.org/web/20200711121152/http://www.mmarray.org/ |date=2020-07-11 }}
• [http://www.ovro.caltech.edu OVRO Website]
• [https://web.archive.org/web/20110411000816/http://bima.astro.umd.edu/ BIMA Website]
• [https://web.archive.org/web/20090809152930/http://science.nasa.gov/newhome/headlines/ast22feb99_1.htm Sunyaev-Zeldovich observations at BIMA and OVRO]
• [http://astro.berkeley.edu/~welch/public_html/ursi/CARMA2.pdf CARMA: Specifications & Status], 2002
• [https://www.youtube.com/watch?v=CMIKx97e5qA "Narrow passage"] Moving radio telescopes from CARMA site to the Owens Valley Radio Observatory (Video, June 22, 2015)

{{Radio-astronomy}}

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Category:Radio telescopes

Category:Interferometric telescopes

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