Fig. 9
From: A low-dispersion spectral video camera for observing lunar impact flashes
a Atmospheric dispersion due to the wavelength dependence of the refraction angle of light rays from a star in the terrestrial atmosphere. The dispersion makes the blue image up and red image down relatively and leads to a vertically elongated star image. b When the horizontal direction of the frame, that is, the dispersion direction of our spectral camera, is parallel to the local horizon, the horizontal distance, ξb, ξ0, and ξr are related to wavelengths based on the dispersion relation of the camera described in section "Wavelength calibration". c When the dispersion direction of the camera is perpendicular to the local horizon, as shown here, the 1st-order spectral image elongates, though ξb, ξ0, and ξr do not change. d In general cases, the dispersion direction makes an angle θ with the local vertical. See text for further explanation