Spectrum of the blue sky

This image is an exposure of the blue sky, taken with the HR fiber set, showing two spectra of the sky (scattered sunlight with absorption features due to the Earth's atmosphere), one from each of the A and B fibers. Red is at the top and blue at the bottom. The throughput of the two fibers in a given set (HR or HE) is approximately the same to within 2-5%, depending on the set. Fiber A is used for the star and fiber B for either the sky (to correct the cross-correlation profile for the effects of contamination by moonlight) or for a Thorium simultaneous wavelength calibration exposure for improved radial velocity accuracy. Fiber B can also be masked. Click on the image above for a large-scale view.