Proceedings Article | 29 June 2006
Proc. SPIE. 6268, Advances in Stellar Interferometry
KEYWORDS: Observatories, Stars, Interferometers, Calibration, Error analysis, Interferometry, Atmospheric modeling, Prototyping, Visibility, Michelson interferometers
Visibility measurements with Michelson interferometers, particularly the measurement of fringe contrast, are
affected by various atmospheric and instrumental effects, all of which reduce the measured contrast. To compensate
for this, stars with known or predictable diameters (calibrators) are observed so that the overall reduction
in the visibility can be measured. Objects with the smallest possible diameters are preferred as calibrators, since
the predicted visibilities become less sensitive to any uncertainties. Therefore, unreddened, early type stars are
usually chosen if they are available because they are relatively bright for a given angular diameter. However,
early type stars bring additional complications. Rapid rotation, common with these stars can cause variations
in the visibility amplitudes due to oblateness and surface brightness asymmetries that are larger than implied by
the usual error estimates. In addition, rotation can introduce significant phase offsets. Using Roche models, von
Zeipel theory, and the observed constraints of V, B-V, and <i>v</i> sin <i>i</i>, it is possible to put limits on the size of these
effects and even estimate the distribution of possible visibilities. To make this easily available to the community,
we are in the process of creating a catalog of possible calibrators, including histograms of the visibilities, calculated
for configurations used at a number of observatories. We show the examples of several early type stars
which are potential calibrators using parameters appropriate for the Navy Prototype Optical Interferometer.
