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. Twenty-five nights will be available during the 2018A observing semester (February 2017 - July 2018)

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 minimum allocation of 0.5 nights (about 5 hours). Observations will be carried out by CHARA staff, however, we encourage new observers to participate in making observations at Mt. Wilson observatory, and some travel support from GSU will be available on request for those who are awarded time.

What is the purpose of this call for proposals?

GSU/CHARA was awarded funding from the NSF Mid-Scale Innovations Program to provide community access to the CHARA observing program and data archive. This is intended to be an introductory opportunity, and previous experience with interferometry is not required. The number of available nights is expected to remain at about 25 per semester through semester 2021B.

CHARA capabilities and proposal preparation

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.

Mode Telescopes Band Typical limit Mag= (2) Best performance Mag= (2) At Spectral Resolution R=
Acquisition 6 V-R 10.0 12.0 Broad band
Tilt tracking 6 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
JouFLU 2 K 4.5 5 Broad band
MIRC(1) 6 H 5.5 6.5 50
PAVO 2 630-900 nm 7.0 8.0 30
VEGA (hi-res) 2 or 3 2 bands of 7nm (separation 30nm) in 520-850nm 4.0 5.0 30000
VEGA (med-res) 2 or 3 2 bands of 35nm (separation 160nm) in 520-850nm 6.5 7.5 6000
1 - Following major upgrades, MIRC is currently in shared-risk mode, which means that the instrument team needs to be involved in the data reduction and resulting publications to ensure basic data quality and to allow use of the data for engineering and data validation purposes. There is also a possibility that the observations cannot be carried out this semester.
2 - Limiting performance is achieved only on point-source targets. For resolved sources, the limiting performance should be scaled lower proportional to the fraction of flux that is correlated (as measured by the visibility for the observing parameters). Tools referenced below in the Data Reduction and Analysis paragraph can be useful for this estimation, and CHARA or NOAO staff can provide guidance.

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. According to current CHARA consortium policy, "Members of the CHARA collaboration may participate as collaborators on NOAO proposals and be listed as such in the text of proposals (but not as P.I. or Co-I. on the cover sheet)."

How much time is needed for an observation?

A single "snapshot", including calibrators, requires ~30-90 minutes. This may produce between one and several dozen UV points, depending on the instrument. Typically, several calibrated snapshots are taken on a target during the night to improve sensitivity, precision, and sky coverage. This set 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 encouraged to travel to the Array - however, observation dates may not adhere to an advance schedule. P.I.'s can apply for travel support once the time allocation process is complete -- contact CHARA Array Director Dr Theo ten Brummelaar (

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 19, 2017

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