Design of the Longitudinal Dispersion Compensation System for the CHARA Array

David Berger

Department of Physics and Astronomy, Georgia State University

The Center for High Angular Resolution Astronomy (CHARA) has constructed an array of six alt-az telescopes at Mount Wilson Observatory in southern California. Together with the central beam combining facility, the telescopes operate as an optical/near-IR interferometer with a maximum baseline of 330 meters. Due to practicality and cost constraints, some of the long path delay required for path length compensation occur out of vacuum. A consequence is a spectrally dispersed beam along the optical axis which decreases fringe contrast. To combat this visibility loss, wedges of glass are placed in the beam to chromatically equalize path lengths. Each set of glass wedges is called a Longitudinal Dispersion Compensator (LDC).

All of the construction phases for the LDC systems are described. Beginning with the material selection process, a glass with similar dispersive qualities to air within the observing bandwidths was selected. Next was the optomechanical design which included custom engineered optical mounts for the glass wedges, high precision translation stages for automated thickness variation, and calibration adjustments. Following this, the hardware driver, software controls, and the user interface were written. Finally, the LDC systems were assembled, integrated into the Beam Synthesis Facility, and tested. The quantified results are presented, but overall, there was an improvement to the interferometric measurements.

Construction of the CHARA Array was made possible by grants from the National Science Foundation, the W. M. Keck Foundation, and the David and Lucile Packard Foundation, and by the generous support of Georgia State University. This research was also funded in part by the Michelson Fellowship Program sponsored by Jet Propulsion Laboratory.