Selecting a Beam Combiner

The choice of beam combiner will depend on the science objectives and the brightness of the target.  Please see the Applying for Time page for a table of available beam combiners that lists the number of telescopes combined by the instrument, available filters, current sensitivities, and spectral resolution.  Below is a brief overview of the main capabilities of each combiner:

  • CLASSIC: Two-telescope near infrared combiner with highest sensitivity.
  • CLIMB: Three-telescope version of CLASSIC in the near infrared, also with high sensitivity.
  • JouFlu: Two-telescope near infrared combiner with high precision visibilities.
  • MIRC: The main imaging instrument that combines light from all six telescopes in the near infrared.
  • PAVO: Two-telescope visible light combiner with high sensitivity.
  • VEGA: Two to three telescope visible light combiner with high spectral resolution (e.g., Halpha emission).

 

Selecting 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, a baseline should be selected to match the size of the object being measured.  A table of the available baselines and their lengths is given on the CHARA Facility page.  Here are tips for selecting the number of baselines to be used:

  • For a simple diameter measurement of a stellar disk, a single baseline consisting of two telescopes should be sufficient.
  • 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 and asymmetric structures, use a beam combiner that combines the light from 3 or more telescopes simultaneously to provide multiple baseline projections as well as closure phase measurements.  Closure phases can measure the departure from point-symmetry.
  • For detailed imaging of stellar surfaces, it is recommended to measure fringe amplitudes beyond the first null in the visibility curve (at 1.22 λ/B) to provide information on the source structure at high spatial frequencies, including limb-darkening, gravity darkening, and starspots.  By using a multi-telescope configuration, the shorter baselines can be used to help track the low-amplitude fringes on the longer baselines.

 

Selecting Calibrator Stars

An interferometer measures the amplitude and phase of interference fringes.  An unresolved point source will have a normalized visibility amplitude of 1 for a perfect system.  For a spatially resolved star, light from across the stellar surface combines incoherently causing a drop in the visibility amplitude (the normalized amplitude will be less than 1).  Instrumental and environmental effects can also cause a loss of coherence.  To calibrate these instrumental effects, astronomers typically observe an unresolved point source before and after the science target in order to measure the "system visibility," which is a ratio of the measured visibility of the calibrator divided by its expected visibility.  The calibrated visibility of the science target is calculated from its observed visibility divided by the system visibility.

Calibrator stars should be:

  • Unresolved or of a known angular diameter. 
  • Within ~ 5-10 degrees on the sky from the science target.  For beam combiners with a long observing cadence like MIRC, it can be useful to find a range of calibrators spanning about 30 min to 1 hour before and after the science target in Right Ascension. 
  • Preferably within 1 to 2 magnitudes in brightness from the science target and similar in color so they can be observed with the same instrument readout mode and tip/tilt settings.
  • Avoid binary stars.  If possible, avoid rapidly rotating stars if the angular diameter might be marginally resolved.
  • Select about 2 to 3 possible calibrators.  One calibrator can be used if it is known to be good based on previous interferometric observations.  Unknown companions are sometimes discovered.

Useful tools for selecting calibrators:

  • SearchCal developed by JMMC.
  • getCal developed by NExSci.
  • For beam combiners with bright magnitude limits, it is sometimes necessary to observe calibrator stars that are marginally resolved.  Selecting a bright star whose diameter has been measured interferometrically can be useful in these cases.   Follow the link to view a table of stars with angular diameters that have been measured at the CHARA Array.  This is not a complete listing of stars that have been observed at CHARA, but contains the results from several survey programs.

 

Selecting Delay Settings (POP Configuration)

To obtain fringes, the distance that the star light travels along each beam must be the same.  To compensate for the difference in path length between each telescope and the beam combination lab, we add delay to the path in two stages.  The first stage occurs inside the vacuum tubes where a mirror assembly can be inserted into the vacuum pipe at fixed delay intervals of 0, 37, 73, 110, and 143 meters.  These are called the POP mirrors.  The second stage is a continuously variable delay added by movable carts that travel along delay line rails.  The rails are 46 meters in length from front to back.  

The position of the POP mirrors needs to be selected prior to the night of observation.  Different POP settings will provide access to different areas of the sky.  The POP settings can be switched during the night to follow a particular star across the sky or to provide access to stars in different parts of the sky.  Because of overheads in changing the POP mirrors and finding new fringe offsets, it is recommended to use no more than 1-3 different POP configurations during the night.  

There are two software packages available for selecting the POP configurations and determining the time when each target is available for a given configuration:

  • ASPRO 2 developed and maintained by JMMC.  Java-based program for planning a full night of observation with multiple stars.
  • chara_plan2 is part of the reduceir software package developed at CHARA/GSU.  This is a linux-based program that is a duplicate of the control software and provides the most accurate timing information for one star at a time.

Please follow this link to see examples of how to plan observations using aspro2 and chara_plan2.

 

Observing Strategies

  • Select a bright star to align the instrument and use as a fringe finder at the start of the night.  Good fringe finders should be bright (2 to 3 magnitudes brighter than the sensitivity limit of the beam combiner), have strong fringes (within the first lobe of the visibility curve), and be close to the target star (within about 10 degrees, preferably earlier in right ascension).  It is often useful to have a bright alignment star when switching baseline configurations or slewing to a new target.  Sometimes a calibrator or the target itself is bright enough to use as the alignment star.
  • Interferometric observations alternate between observing calibrator stars and science targets.  The observing cadence will depend on the instrument being used, the brightness of the target, and the seeing conditions.  The table below lists the typical amount of time needed to observe an individual target (either a calibrator or science target).

    Beam Combiner

    Time Required for Observation Alignment  Notes
    CLASSIC 5-10 min Align on bright star at the beginning of an observing sequence.  Re-align every 45-60 minutes or if slewing to a different part of sky.
    CLIMB 5-10 min Align on bright star at the beginning of an observing sequence.  Re-align every 45-60 minutes or if slewing to a different part of sky.
    JOUFLU 5-10 min Align using a raster scan on every target.
    MIRC 45 - 60 min Align using fiber explorer on every target (alignment overhead is included in time estimate).
    PAVO 5-10 min Align on bright star at the beginning of an observing sequence.  Re-align every 1-2 hours or if slewing to different part of sky.
    VEGA 5-20 min  
  • Therefore a calibrator-science-calibrator sequence requires 30-90 minutes.  This sequence is usually called a calibration "bracket".
  • Several repeated calibration brackets are typically obtained for each science target.  If using two calibrators, typical observing sequences for "fast" instruments (CLASSIC, CLIMB, JOUFLU, PAVO) are as follows:
    • Cal1 - Obj - Cal2 - Obj - Cal1 - ....
    • Cal1 - Obj - Cal2 - Cal1 - Obj - Cal2 - ....
    • The first sequence is faster and provides more time on target.  The second sequence is slower but provides a better opportunity to calibrate the calibrators against each other to check for systematic calibration errors and bad calibrators.
    • Another strategy is to separate out the two calibrators, but this should only be used if the calibrators are known to be good.  Because the calibrators are observed in separate sequences, they cannot be checked directly against each other:
    • Cal1 - Obj - Cal1 - Obj - Cal1 - .... and Cal2 - Obj - Cal2 - Obj - Cal2 - ....
  • For the slower instruments, the number of observations on a science target is typically set by the length of the observing window defined by the baseline configuration.  With MIRC, it is possible to continue observing a target outside of the 6-telescope fringe window (dropping down to 4 or 5 telescope fringes).  This improves the uv coverage on imaging targets after one or more carts go out of delay.
  • While conducting observations, it is useful to keep an observing log to note when instrument alignments were done and when the position of the reference cart was changed.  Both of these can affect the visibility calibration, so it is recommended not to make these changes until after a calibration bracket is finished (ending on a calibrator).  Following any change, it is useful to start another observing sequence on a calibrator star.

 

Submitting an Observing Request

Please follow instructions on the Logistics of Observing page to submit your observing request at least two days prior to the beginning of your scheduled observing time.