CHARA - Community Access

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. Thirty nights will be available during 2020B observing semester (1 August 2020 - 20 December 2020)

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 30 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
MIRC-X 6 H 6.5 7.5 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
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 interferometric observation consists of a calibrator-science-calibrator sequence. Typically, several of these sequences are collected to fill in the UV coverage and improve precision. The amount time to collect data on a star depends on the instrument, the seeing, and the brightness of the target. Here are some guidelines for how long the observations will take:

  • For fast instruments like CLASSIC, CLIMB, and PAVO, an observation on an individual target will take 5-15 min, so a single CAL-SCI-CAL set will take between 15 to 45 min. Use the longer integration time for targets near the typical magnitude limit, the shorter integration time for stars ~ 2 mag brighter than the typical limit.
  • For MIRCX, an observation on an individual target will take about 30 min, so a CAL-SCI pair will take about 1 hour.
  • For VEGA, an observation on an individual target will take 10-20 min, so a CAL-SCI-CAL set will take 30 min to 1 hour.

Each observation may produce between one to several dozen UV points, depending on how many telescopes are combined and the number of spectral channels in the instrument. The number of calibrated observations needed to complete a program depends on the science objectives. See the Table of Recommended Number of Data Sets for examples of the number of calibrated data sets to collect for different science objectives using different beam combiners. These examples include measuring stellar angular diameters, resolving binary stars, and imaging more complex sources.

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.

Last updated or reviewed March 6, 2020.