Selecting the Telescopes and Baselines

The angular resolution of an interferometer is given by 0.5 λ/B where λ is the wavelength of light and B is the baseline length between two telescopes.  On the longest 331 meter baseline between the S1 and E1 telescopes, the CHARA Array can achieve an angular resolution of 0.66 mas in the K-band (2.13 μm), 0.52 mas in the H-band (1.67 μm), and 0.20 mas in the visible at 650 nm.

At a given wavelength, the baseline should be selected to match the size of the object being measured.  A table of the available baseline lengths is given on the CHARA Facility page.

For a simple diameter measurement of a stellar disk, a single baseline consisting of two telescopes would be sufficient.  However, if the source is not circular, such as a rotationally flattened star, inclined circumstellar disk, or binary star, then using multiple baselines with different position angles on the sky are needed to map the two-dimensional structure of the source.  The rotation of each baseline as the earth rotates, provides additional coverage on the sky.  For imaging complex structures like starspots on the surfaces of stars or asymmetries in circumstellar disks, then using beam combiners that combine the light from 3 to 6 telescopes provide multiple baseline projections as well as closure phase measurements.  Measuring the fringe amplitudes beyond the first null in the visibility curve (at 1.22 λ/B), provides information on the source structure at high spatial frequencies such as limb-darkening, gravity darkening, and spots.

Proposal methods

CHARA offers two methods of applying for time. Consortium members may propose through an internal annual call for proposals. This process is advertised on the CHARA mailing list and coincides with the NOAO proposal deadline.

Non-consortium members may request community access time through the National Optical Astronomy Observatory. We are offering 15 nights of community access time at the CHARA Array in the 2017B observing semester (August - December).  Proposals will be submitted through NOAO and are due on March 31, 2017.  More information about this observing opportunity can be found on the NOAO-CHARA informational page.  Observations will be carried out by CHARA staff, however, we encourage new observers to participate in the observations at Mount Wilson Observatory, and some travel support from GSU will be available on request for those who are awarded time. Consortium members will support data reduction to Optical Interferometry FITS format, although most users find it interesting (and not that difficult) to run the reduction software themselves. Information about the data reduction pipelines can be found on the data reduction and individual instrument pages. Modeling and image reconstruction software may be found in the data analysis section of this website, the Jean-Marie Mariotti Center, and the Exoplanet Science Institute.

CHARA capabilities

The CHARA array is applicable to problems in almost all areas of contemporary astronomy. It is particularly suited to stellar astrophysics where it is used to measure the diameters, effective temperatures, masses, and distances of stars and binary systems, as well as to image features such as spots and flares on stellar surfaces. Other science topics have included measuring the fundamental parameters of  exoplanet host stars, imaging circumstellar disks, and studies of bright transient phenomena like novae.

All objects observed must satisfy all magnitude requirements for acquisition, tracking, and beam combination listed in the table below. At present, CHARA has no off-axis guiding capabilities.  More information about the baselines and beam combiners can be found on the CHARA Facility and Instrumentation pages.

ModeTelescopesBandTypical 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 4.5 5.5 40
PAVO 2 630-900 nm 7.0 8.0 30
JouFLU 2 K band 4.5 5.0 Broadband

Time required for one observation

A single interferometric “snapshot” is often taken in a calibrator-science-calibrator sequence. This process requires ~30-90 minutes. Each observation may produce between one to 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. More complex tasks like time variable studies or imaging complex sources typically require larger time allocation.

Proposal tools

Observations may be planned using either the chara_plan2 software distributed with the CLASSIC/CLIMB data reduction software or usingAspro2 from the JMMC. Interferometric calibrators may be found using SearchCal or getCal.

Booking a room on the mountain

Please send your request for accommodation on the mountain to Larry Webster at This email address is being protected from spambots. You need JavaScript enabled to view it. .

Sending your setup to the CHARA team

In order to streamline nightly operations, observers should send their configuration requests to This email address is being protected from spambots. You need JavaScript enabled to view it. at least two days prior to the beginning of your scheduled observing time.  Please include the following information:

  • date of program
  • program PI/designation
  • beam combiner(s)
  • telescopes (and changes during the night)
  • beam order (if you have a preference)
  • PoP combination(s)
  • first target (or designated check star for first target)
  • any special requirements you might have
  • skype or phone contact if remote (in case we do not already have it)

Links for detailed observing procedures


Transportation to Mount Wilson: There is no public transportation to Mount Wilson. It is recommended that visitors arrange for a rental car during their stay. Transportation from LAX to Pasadena: Some visitors may stop in Pasadena during their stay. There are limousines and shared-ride vans that can take passengers from LAX to Pasadena. Alternatively, one can take the La FlyAway Bus from LAX to downtown LA. The Metro Gold Line train can then be taken from downtown LA to Pasadena. Note the La FlyAway Bus, does not accept cash.Tickets may be purchased at the bus with Visa, MasterCard, or American Express credit or debit cards only. The Observatory is an additional 25 miles from Pasadena, and visitors will need a rental car.

Driving Directions to Mount Wilson From the 210 freeway, follow the Angeles Crest Highway (California State Highway 2) north out of La Canada-Flintridge. Follow Route 2 past the Angeles Forest Highway turnoff. Continue on Route and turn right onto Mount Wislon Red Box Road. This is about 15 miles from the 210 Freeway. Drive about 5 miles more to Mount Wilson Circle. You will arrive at a small intersection near a number of transmitter towers. Follow the “Observatory” sign on the one way road to the far right around the towers until you reach the Forest Service gate, which is the entrance to Skyline Park and the Observatory.

Remember to turn on your headlights when driving on CA Route 2, beginning at La Cañada and when driving back down the mountain from the Observatory; drivers not using their headlights may be stopped and fined.

A map of the route to Mount Wilson can be found here: Map of Route to Mount Wilson

Alternate Route via Sunland Directions

This route is only valid if the main portion of the Angles Crest Highway (California Route 2) is closed . From Pasadena, take the I-210 west to Sunland Blvd. Turn right on Sunland Blvd to Oro Vista and turn left. Oro Vista Ave turns right and becomes Big Tujunga Canyon Road. Stay on this road for 12.9 miles. At Angeles Forest Highway turn right for 3.8 miles. On Angeles Crest Highway (California Route 2) turn left and continue on for 4.6 miles. Turn right on Mt. Wilson/Red Box Road and continue approximately 5 miles to Mt. Wilson Circle. Resources via Sunland Travel Route: There are many restaurants and stores in Sunland. These include a Ralphs Supermarket right off the freeway a Sizzler and Panda restaurant, a Yum Yum Donuts and a 31 Flavors Ice Cream, plus a Starbucks nearly on the corner of Oro Vista and Sunland Blvd. There are also plenty of gas stations. A driving map is available here Mt Wilson Map via Sunland If the map shows driving on the freeway, select Options, make sure Avoid Freeways is selected and hit Get Directions to see the Big Tujunga route through the mountains.

Arrival after hours

The Observatory gates are open from 10:00 a.m. to 4:00 p.m. daily from the first weekend in April through the last weekend in November. If you are planning to arrive when the gate will not be open, be sure that you know the gate code to the Observatory grounds a couple of days before your trip. Call Larry Webster (626-796-3730) or Theo during business hours for the code, as the code cannot be sent by email for security reasons. For last-minute arrangements, there is a pay phone just outside the Observatory gate (the charge is fifty cents), although there is no guarantee that anyone will be available to receive your call or that the pay phone will be operational.