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 NOAO proposal deadline.
Non-consortium members may request community access time through the National Optical Astronomy Observatory. We are offering 25 nights of community access time at the CHARA Array in the 2018B observing semester (August - December). Proposals will be submitted through NOAO and are due at the end of March. More information about this observing opportunity can be found 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). 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 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 2018B: 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. All other combiners are available in normal observing mode.
|Mode||Telescopes||Band||Typical limit Mag=||Best performance Mag=||Spectral Resolution R=||Science Drivers|
|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|
|JouFLU||2||K band||4.5||5.0||Broadband||Precision Diameters|
|MIRC||6||H||5.0||6.0||40||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.
Time Required for an Observation
A single interferometric observation consists of a calibrator-science-calibrator sequence. 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, JOUFLU, and PAVO, an observation on an individual target will take 5-10 min, so a single CAL-SCI-CAL set will take between 15 to 30 min.
- For MIRC, 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:
- For an angular diameter or measurement of limb darkening, several (5-10) calibrated sets of data are required on at least one or two baselines. Observations on two separate nights and on two different baselines are recommended to minimize systematics.
- To resolve a binary system it is recommended to collect several (3-5) calibrated sets of data using three or more baselines simultaneously. If a two-telescope combiner is required to observe the binary, then collecting more calibrated sets on a couple of different baselines would be suitable.
- Imaging more complex sources requires many calibrated observations on multiple baselines during the night to fill in the sky coverage.
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.
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.