Atmosphyre: Modelling Atmospheric Chromatic Dispersion for Multi-Object Spectrographs

Abstract

The wavelength dependent refraction of light in the atmosphere causes the chromatic dispersion of a target on the focal plane of an instrument. This is known as atmospheric dispersion, with one of the consequences being wavelength dependent flux losses which are difficult to minimise, requiring analysis in both instrument design and operations. We present Atmosphyre, a novel python package developed to characterise the impact of atmospheric dispersion on a spectrograph, with a focus on fibre multi-object spectrographs (MOS) which will be at the forefront of ground-based astronomy for the next few decades. We show example simulations and provide recommendations for minimising fibre MOS flux losses. We conclude that the guiding wavelength should typically be bluer than the observing band mid-wavelength, around 25-45% of the way through the band. The aperture should be centred on this wavelength's location on the focal plane. This wavelength/position remains constant for all reasonable declinations and target hour angles. We also present an application of the package to MOSAIC, the ELT's multi-object spectrograph. We find that differential losses greater than 10% are unavoidable for 1h observations that are a) after a local hour angle of 2.5h, or b) at declinations below -60 degrees and above 10 degrees. We identify that the introduction of an atmospheric dispersion corrector (ADC) would result in the significant reduction of spectral distortions, a gain in survey speed for many observations, and enable the implementation of wider visible observing bands; as a result, there has been a proposal to adopt ADCs at a positioner level for MOSAIC. Future work includes adding field differential refraction to Atmosphyre, important for future wide-field multi-object spectrograph projects such as the proposed WST.