CHARA -- Community Access

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National Optical Astronomy Observatory

Community Access to the CHARA Interferometer on Mt. Wilson

Announcement of Opportunity

NOAO and Georgia State University are announcing an opportunity for observations with the Center for High Angular Resolution Astronomy (CHARA) Array at Mt. Wilson Observatory. About 5 nights will be available during the 2017A observing semester (February - July). Note that this call covers the six-month 2017A semester, not a full year as in past cycles. We anticipate a similar amount of time to be available in 2017B, making the total time available in 2017 twice that of past years.

Requests should be submitted using the standard NOAO proposal form by selecting "CHARA" in the telescope list. Time should be requested in half-night increments, with a mimimum allocation of 0.5 nights (about 5 hours). Observations will be carried out by CHARA staff.

What is the purpose of this call for proposals?

ReSTAR and ALTAIR reports have identified community interest in optical interferometry. Foreseeing increasing community access to the CHARA Array in the future, GSU would like to gain experience with visitor access to its Mt Wilson facility. NOAO would like to learn about reviewing interferometry proposals and allocating time on an optical array. This is intended to be an introductory opportunity, and previous experience with interferometry is not required.

CHARA capabilities and proposal preparation

CHAMP is temporarily withdrawn from operation for upgrades, and may be available for the 2017B application period. MIRC will be available only between Feb 1 and Apr 15 2017 in 2017A. It will then be withdrawn from operation for engineering work for a new camera.

The best way to study the capability of the instruments is to look over some of the science papers from the array. A bibliography of CHARA Array science is available:

The following table gives a high level view of the performance for the system and the most mature beam combiners. Please note that CHARA does not have offset tracking capability, and the science target must satisfy acquisition, tilt tracking, and beam combiner magnitude limits. CHARA has additional capabilities in various stages of commissioning - these will be made available where possible for approved programs.

Mode Telescopes Band Typical limit Mag= Best performance Mag= At Spectral Resolution R=
Acquisition 2 V-R 10.0 12.0 Broad band
Tilt tracking 2 V-R 10.0 12.0 Broad band
CLASSIC 2 H or K band 7.0 8.5 Broad band
CLIMB 3 H or K band 6.0 7.0 Broad band
VEGA (hi-res) 2 or 3 2 bands of 7nm (separation 30nm) in 480-850nm 4.0 5.0 30000
VEGA (med-res) 2 or 3 2 bands of 35nm (separation 160nm) in 480-850nm 6.5 7.5 6000
MIRC 6 H 4 6 42
PAVO 2 630-900 nm 7.0 8.0 30

Steve Ridgway ( is the NOAO point of contact for proposal preparation, and he can steer you to more expert advice as needed.

The 6 CHARA telescopes provide 15 baselines, listed here. Normally a two telescope combiner can be used with any two telescopes (one baseline), a 3-telescope combiner with any 3 telescopes (3 baselines), etc. The selection of telescopes can be changed during the night, within some limitations, provided it is part of the observing request and plan - please inquire for more specific information.

If you decide to prepare a proposal, you will probably want to look at the optical interferometry planning tools supported by the NASA Exoplanet Science Institute at

The Jean-Marie Mariotti Center in Grenoble offers an interferometry planning tool Aspro which supports CHARA instruments. The JMMC also offers SearchCal, for selecting calibrator stars.

There are no reserved targets or science, though proposers may optionally be put in contact with groups pursuing similar programs. NOAO policy on data proprietary period will apply.

How much time is needed for an observation?

A single "snapshop", including calibrators, requires ~30-90 minutes. This may produce between one and several dozen UV points, depending on the instrument. This amount of data can determine, for example, an angular diameter, a limb darkening strength, a binary separation, or the fraction of emission in a shell.

This may not be well suited for survey programs, for time variable studies, or for imaging of complex sources, which typically might require larger observing allocations.

The observations

CHARA cannot guarantee productive observations, but is prepared to devote more telescope time than the allocated total in order to increase the odds of success.

All observing will be done by CHARA consortium staff. Visitors are welcome to travel to the Array - however, observation dates may not adhere to an advance schedule.

Data reduction and analysis

Consortium members will also support data reduction to Optical Interferometry FITS format, though users will probably find it interesting and not difficult to run the reduction suites, either on a CHARA computer, or on their own Linux or Mac systems. Visibility modeling tools are available from the Exoplanet Science Institute at, and from the Jean-Marie Mariotti Center at

A very complete bibliography of interferometry science is available at the OLBIN website, , and may be the best guide for conventional ways to interpret data. However, optical interferometry is a young field and is wide open for new approaches.

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NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation.
Last updated September 6 2016

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