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The strategies for image reconstruction from optical synthetic-aperture data can be divided into two camps according to their historical legacies: those coming from the field of more conventional optical image processing, where the data are relatively complete in terms of Fourier coverage, have concentrated on trying to recover images using “brute force” methods. They make use of the massive size of the typical datasets to try and derive an image without having to use any a-priori constraints. The size of the datasets to some extent restricts the algorithms used towards being as simple and hence fast as possible. Radio-astronomical imaging strategies, on the other hand, have always been designed to cope with sparse and missing data, and many sophisticated algorithms have been designed to make maximum use of a-priori constraints with iterative global fitting techniques.

We used the Mark III Optical Interferometer on Mt. Wilson in 1991 and 1992 to measure the diameters of 42 stars with 10 nm wide filters centered at 712 nm and at 754 nm. These filters probe the stellar atmosphere in a strong TiO band (712 nm) and in a "continuum" band relatively free of TiO absorption (754 nm). The data were taken on a North-South baseline that could be configured to lengths between 3.0 m and 31.5 m. Observations of the target stars were interleaved with frequent measurements of calibrator stars. The square of the calibrated visibility amplitude V 2 was determined as a function of baseline length, and a uniform disk model fitted to these data (for details see Mozurkewich et al. 1991). The average formal error of the diameter measurements -derived from the X 2 of the model fits -is 2.5%. Data for a subset of 12 stars have already been published (Quirrenbach et al. 1993). Results and DiscussionK stars, MO stars, and carbon stars have identical diameters at 712 nm and at 754 nm. For stars with spectral types later than MO, the diameters are systematically larger at 712 nm than at 754 nm (see Figure 1). The diameter ratio increases with decreasing effective temperature (or increasing R -I color index), and it is larger for luminosity class I than for luminosity class II and III stars. For C and S stars, which do not have TiO absorption bands, the diameter ratio is consistent with one, as expected.The variation of the uniform disk diameter between 712 nm and 754 nm cannot be due to wavelength-dependent variations of the limb darkening, which is a much smaller effect. The observations can be qualitatively explained by the different TiO opacities in the two filters: the T == 1 surface occurs at a 304 use, available at https://www.cambridge.org/core/terms. https://doi