1999PASP..111..512L

We have measured chromatic differential refraction of astronomical images by the Earth's atmosphere, comparing visible light at 500-600 nm with infrared near 12 µm, and found the index of refraction near 12 µm to be greater than predicted by the dispersion relation in common use. The differential refraction between visible and 12 µm thus is smaller than the extrapolated difference. At Mauna Kea, the empirical differential refraction at 45° from zenith was measured at 0".28±0".28 (3 σ) on one occasion, compared to an expected differential refraction of 0".63-0".69. Refractivity scales linearly with barometric pressure, and thus the inaccuracy is more severe at lower altitude observatories. The pointing error from inaccurate correction for differential refraction is comparable to the point spread function width of an 8-10 m telescope in this wavelength range. Even on smaller telescopes, accurate correction for differential refraction is necessary to guide small-aperture spectrophotometry or to register mid-infrared imaging with visible-light structures.