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 5-15 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, 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.  The table below gives estimates for the number of observations needed for typical science programs for each instrument:

 

Beam Combiner Number of Measurements in One Data Set Science Objective Recommended Number of Data Sets
CLASSIC, JOUFLU Each data set consists of one visibility measurement on a single baseline. Angular Diameter ~10 calibrated data sets on each baseline (2.5 - 7.5 hours per baseline).  Observations on two separate nights and on two different baselines are recommended to minimize systematics.  If the star is oblate, then a few baselines at different position angles should be selected.
Binary ~10 calibrated data sets on each baseline (2.5 - 7.5 hours per baseline).  To solve for the binary separation and position angle, then data should be collected on at least two perpendicular baselines.
PAVO Each data set consists of ~ 20 visibility measurements (in each spectral channel) on a single baseline. Angular Diameter 5-10 calibrated data sets on each baseline (2 - 7 hours per baseline).  Observations on two separate nights and on two different baselines are recommended to minimize systematics.  If the star is oblate, then a few baselines at different position angles should be selected.
Binary 5-10 calibrated data sets on each baseline (2 - 7 hours per baseline).  To solve for the binary separation and position angle, then data should be collected on at least two perpendicular baselines.
CLIMB Each data set consists of 3 visibility measurements on each of the 3 baselines and one closure phase. Angular Diameter Obtain ~ 10 calibrated data sets using three telescopes simultaneously (2.5 - 7.5 hours).  Although data is collected more efficiently with CLIMB, the visibility precision isn't quite as good as the 2-telescope combiners such as CLASSIC or JOUFLU. 
Binary Obtain 5-10 calibrated data sets using three telescopes simultaneously (2.5 - 7.5 hours).  In addition to the visibilities, the closure phase measurements provide an additional constraint on the binary separation.
Disks / Imaging Obtain ~ 5 calibrated data sets on each 3-telescope configuration (2 - 4 hours on each configuration).  Select several different 3-telescope configurations to fill in the sky coverage and to sample different spatial frequencies.
VEGA For calibrated V2 observations, each data set provides two visibility measurement on each baseline (one baseline for 2T, three for 3T, and six for 4T).  For differential measurements, visibilities and phases are measured as a function of wavelength across the spectral line relative to the continuum. Angular Diameter Obtain ~ 10 calibrated data sets on each selected configuration.
Spectral Studies Obtain a few repeated measurements on the selected configurations.
MIRC Each data set will consist of visibility measurements on up to 15 baselines across 8 spectral channels and closure phase measurements on up to 20 triangles also across 8 spectral channels. Angular Diameters, Circumstellar Disks A diameter or limb darkening measurement could be obtained with two calibrated data sets using all 6 telescopes simultaneously (~ 3 hours).  The second set can serve as a check on systematics.  For the size and orientation of circumstellar disks, 2-4 calibrated data sets are recommended to improve sky coverage (3 - 6 hours)
Binary 2-3 calibrated data sets using all 6 telescopes simultaneously (3.0 - 4.5 hours).  To solve for the binary position, a minimum of one calibrated 6T data set is required.  However,  to test systematics, two data sets are recommended.  For faint targets or for detecting faint companions, 2-3 sets are recommended to improve the signal-to-noise.
Imaging Complex Sources Imaging stellar surface features or structure within circumstellar disks requires collecting many calibrated data sets on multiple baselines during the night to fill in the sky coverage (~ 1 night per target or the number of hours in a night when the target is above ~ 30 deg elevation).