VOLT: the Victoria Open Loop Testbed

Abstract

The Victoria Open Loop Testbed (VOLT) serves as a demonstration of open loop control both on-sky (at the Dominion Astrophysical Observatory's 1.2m telescope) and in the lab in order to facilitate the future development of Multi-Object Adaptive Optics (MOAO). MOAO, when combined with multiple deployable integral field units, is a concept which promises to deliver near diffraction-limited images over a large field of view. Astronomers will be able to use the multiplex advantage of MOAO instruments to mount large, detailed surveys of galaxies and star formation regions. However, several challenges await MOAO instrument designers. The greatest of these is implementing open loop control in an astronomical adaptive optics (AO) system. Almost all astronomical AO systems to date have used some form of closed loop control, in which the wavefront sensors (WFSs) measure a residual wavefront error after the deformable mirror (DM) has taken on its commanded shape. WFSs in an open loop system can be spatially separated from the DM, but doing so creates new challenges, some known and some unknown. Uncertainties springing from open loop control pose the greatest risk to the design of a MOAO instrument. To mitigate this risk, we have designed and built VOLT, a simple on-axis open loop adaptive optics system. We describe several sources of open loop error, and our measurements of their expected contribution to the VOLT performance. Finally we present observations of a bright star, showing that VOLT, operating in open loop, was able to significantly improve the image quality from 2.5 arcseconds to 0.5 arcseconds in I-band, consistent with our estimates of the wavefront errors. We also present the open loop rejection transfer function for VOLT based on both on-sky and lab measurements.