Design and Development of an Atmospheric Dispersion Compensator for the LLAMAS instrument on the 6.5m Baade Magellan Telescope

Abstract

Atmospheric Dispersion is a phenomenon caused by the wavelength dependent refraction of incoming light by Earth’s atmosphere. For non-perpendicular angle of incidence, higher energy light (shorter wavelength) such as blue and violet are refracted more than its lower energy counterparts, for example, red light. As such, when a telescope is pointed at a non-zero zenith angle, there is a varying vertical angular separation between the different wavelengths of incoming light. In order to mitigate this separation and improve the spectral response of a scientific instrument, an Atmospheric Dispersion Compensator (ADC) is employed. At its simplest, this is a device consisting of two zero-deviation prisms that counter-rotate to counteract the unwanted atmospheric dispersion. At zenith, their dispersion axes cancel each other out, and near the horizon, their axes are parallel such that their net dispersion is opposite to that of the atmosphere. Here, a novel otpo-mechanical realization of an Atmospheric Dispersion Compensator is explored involving two hollow stepper motors with the appropriate diameters to house near-athermally RTV bonded powered optic lenses in order to meet dimensional constraints that prevented a more conventional ADC design. Using Hall-effect sensors, the ADC is able to reliably home without the need of motor encoders for positioning. Upon the ADC’s installation on the Large Lenslet Array MAgellan Spectrograph (LLAMAS) and the latter’s subsequent successful commissioning on the 6.5m Baade Magellan Telescope, the ADC has been operating and helping astronomers every night.