CHARA offers two methods of applying for time: open access through NOIRLab and internal time for consortium members. Each year there are two calls for proposals.
Community open access time is available through NSF's NOIR Lab. We are offering 45 nights of open access time at the CHARA Array in the 2024A observing semester (March 1 through July 31, 2024). The proposal deadline is Monday October 2, 2023. Proposals can be submitted using the NOIRLab proposal form. More information about this observing opportunity can be found below. Time should be requested in half-night increments, with a minimum allocation of 0.5 nights (about 5 hours). For open access observers with no prior experience, the observations will be conducted by CHARA staff, however, visitors are encouraged to travel to the Array or connect remotely to participate in the observations. P.I.'s can apply for travel support once the time allocation process is complete. The community access program is funded through the National Science Foundation.
The observing request form for internal proposals is available as an MS Word document (or as LaTeX templates) Requests for engineering time should fill out the Engineering Form (or in LaTeX templates). Non-GSU consortium members can apply for time either internally or through NOIRLab, but we ask that you do not submit the same proposal to both Time Allocation Committees.
Proposals for semester 2024B (August - December) will be due on the last day in March 2024. The Call for Proposals is usually announced about a month before the deadline.
The CHARA Array enables scientific exploration in many areas of contemporary astronomy. It is particularly suited to stellar astrophysics where it is used to measure the diameters and effective temperatures of stars, the masses and distances to binary systems, and to image stellar surfaces. Other science topics have included measuring the fundamental parameters of exoplanet host stars, imaging circumstellar disks, and resolving 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.
Instrument Availability in 2024A:The available beam combiners in 2024A are CLASSIC, CLIMB, MIRC-X/MYSTIC, and PAVO. The MIRC-X and MYSTIC combiners operate simultaneously for six-telescope observations in the H and K bands. Publications resulting from MIRC-X/MYSTIC observations must include the instrument team as co-authors. The new 6T visible combiner SPICA is available in shared-risk mode. Observing time in shared-risk mode is not guaranteed. The SPICA instrument team will be involved with approving proposals, planning observations, reducing data, and publishing papers to ensure basic data quality and to allow use of the data for engineering and data validation purposes. For internal applicants, please arrange time to be trained on SPICA in advance of your run and have two observers available (one to run MIRCX/MYSTIC for fringe tracking and one to run SPICA). On sky commissioning of the new high sensitivity, 3T near-IR combiner Silmaril is on-going, and we hope to offer time on Silmaril in 2024B.
|Mode||Telescopes||Band||Typical limit Mag=||Best performance Mag=||Spectral Resolution R=||Science Drivers|
|Acquisition, AO, Tiptilt Tracking||6||V-R||10.0||12.0|
|CLASSIC||2||H or K band||7.0||8.5||Broad band||Diameters|
|CLIMB||3||H or K band||6.0||7.0||Broad band||Binaries, Disks|
|MIRC-X||6||H-PRISM50||6.5||7.5||50||Stellar Imaging, Binaries, Disks|
|MIRC-X||6||H-GRISM190||5.5||6.5||190||Stellar Imaging, Binaries, Disks|
|MYSTIC||6||K-PRISM49||6.5||7.5||49||Stellar Imaging, Binaries, Disks|
* Shared Risk
|6||650–950 nm||5.5*||5.5*||150||Diameters, Imaging|
Note: The most frequently used spectral modes for MIRC-X are R=50 prism (best sensitivity) and R=190 grism (bright targets). MIRC-X is also available using R=102 prism and R=1170 grism. Most observations for MYSTIC have been collected using the R=49 prism, although MYSTIC is now also available using R=22 prism, R=100 prism, R=278 grism, R=981 grism, and R=1724 grism. Some data have been collected in these new modes. Please consult with the MIRC-X/MYSTIC instrument teams to learn about expected sensitivity limits if you are interested in using these additional modes.
Note: Limiting performance is achieved 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 fringe visibility amplitude). Tools referenced below on the Data Analysis pages can be useful for this estimation, and CHARA or NOIRLab staff can provide guidance.
The six 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. For binary stars, see the Table of Companion Detection Limits for the range of separations and magnitude differences that can be measured with each instrument.
Time Required 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 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 MIRC-X/MYSTIC, an observation on an individual target will take about 30-45 min, so a CAL-SCI pair will take about 1 to 1.5 hours. This includes telescope acquisition, alignments, recording fringes (10 min), and shutter sequence (10 min).
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.
Please see the Planning an Observation page for guidelines on selecting the beam combiner, telescopes, and calibrators, and for detailed instructions on how to plan observations. Observations may be planned using either Aspro2 developed by JMMC or the chara_plan2 software distributed with the CLASSIC/CLIMB data reduction software. An example of how to use Aspro2 and chara_plan2 to plan observations is available here. Interferometric calibrators may be found using SearchCal or getCal.
For internal proposals, Benjamin Setterholm has created a python script for automatically generating the Object Catalog given a list of objects and calibrators. The routine, cattle.py, is publicly available on CHARA GitLab.
Data Reduction and Analysis
CHARA staff and consortium members will support data reduction to Optical Interferometry FITS format (OIFITS), 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 that is available to the community is described in the data analysis pages.
Has a target already been observed at the CHARA Array?
A preliminary version of the CHARA Online Database Portal is available and can be used to search for prior observations on a target. More details about using the database can be found on the database page. Some targets might be missing from the current version of the database, so contact the CHARA staff listed below if you have any questions about finding observations. A table of published measurements from CHARA can be found through the following link.
The JMMC Optical Interferometry DataBase (OIDB) contains some observing log information for the Classic, CLIMB, and VEGA beam combiners.
For publications based upon CHARA data, please use the following acknowledgment guidelines.
Steve Ridgway (