CHARA offers two methods of applying for time. Consortium members may propose through an internal call for proposals. This process is advertised on the CHARA mailing list and coincides with the NOIR Lab proposal deadline. Each year there are two calls for proposals.
Non-consortium members may request community access time through NSF's NOIR Lab (formerly known as NOAO). We are offering 30 nights of community access time at the CHARA Array in the 2021A observing semester (March 1 - July 31). Proposals will be submitted through NOIR Lab using the standard NOAO proposal form. The proposal deadline is Wednesday, September 30, 2020. More information about this observing opportunity can be found below and on the NOAO-CHARA informational page. Time should be requested in half-night increments, with a minimum allocation of 0.5 nights (about 5 hours). For new open access obervers, the observations will be conducted by CHARA consortium staff at Mount Wilson Observatory, however, visitors are encouraged to travel to the Array 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. Those who prefer LaTeX can use the archived templates. Requests for engineering time should fill out the Engineering Form.
Proposals for semester 2021B (August - December) will be due on the last day in March 2021. 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 2021A: The available beam combiners in 2021A are CLASSIC, CLIMB, MIRC-X, and PAVO. The recently commissioned MIRC-X combiner is available 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. VEGA will be decommissioned to make way for the next generation optical instrument SPICA that is being built by Denis Mourard and collaborators and that should be available in 2022. The new K-band combiner MYSTIC is still under development by John Monnier and collaborators.
|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||6.5||7.5||50||Stellar Imaging, Binaries, Disks|
|VEGA (hi-res)||2 or 3||2 bands of 7nm (separation 30nm) in 480-850nm||4.0||5.0||30000||Spectral studies|
|VEGA (med-res)||2 or 3||2 bands of 35nm (separation 160nm) in 480-850nm||6.5||7.5||6000||Spectral studies, Diameters|
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 NOAO 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 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 45 min, so a CAL-SCI pair will take about 1.5 hours.
- 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.
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.