Wavelength-dependent Image Quality Predictions and Other Considerations for Site Selection and Instrument Design

Abstract

Stellar image motion measured differentially with small telescopes can be converted to image quality predictions for large telescopes at the same site. To demonstrate this methodology, multi-wavelength seeing distributions are predicted for Las Campanas Observatory based on the Differential Image Motion Monitor (DIMM) results from the Giant Magellan Telescope site survey. The observed seeing distribution at 0.5 μm is used to compute a family of predicted seeing distributions from 0.32 to 4.7 μm. Tokovinin’s formula is used to transform the DIMM seeing distributions (based on Kolmogorov theory) to the large telescope image sizes expected in the von Karman theory. The latter framework includes an outer scale, or largest size, for the turbulent eddies above a telescope. The impact of seeing distributions on site selection and on telescope and instrument design are discussed. Other considerations in selecting an excellent site are also listed and explained.