Images of spectroscopic binaries with the Navy Prototype Optical Interferometer

Abstract

We discuss reduction, calibration, and imaging of data from the multi-baseline, multi-channel Navy Prototype Optical Interferometer (NPOI). Images utilizing closure phase are presented for Mizar A and several other binaries on scales from a few milliarcseconds (mas) to several tens of mas. A complete orbital solution has been obtained for Mizar A with a median residual of 0.08 mas. A correlation between the visibility amplitude and a temporal seeing indicator is used to calibrate the amplitudes with an error of about 15 percent; closure phase offsets vary slowly with time and can be calibrated with an error of a few degrees. However, unmodeled visibility variations remain at a level above the design goals. Earth rotation aperture synthesis is very fast with the 520 nm to 850 nm wavelength coverage of the NPOI, so that as few as four scans result in images with dynamic range of 100:1 of simple binaries with negligible color difference between the components. Mapping was done in AIPS, a package developed for radio interferometry with the VLA and VLBI, but fundamental differences between radio and optical data are the availability from NPOI of an independently computed triple product of the complex visibilities, and the availability of the square of the visibility amplitude on single baselines. The latter quantity needs bias correction, whereas the former is practically unbiased if independent detectors contribute to the signal. Broad band optical interferometry mapping algorithms therefore present a new development challenge.