Speckle Imaging Through a Coherent Fiber Bundle

Abstract

Speckle imaging is a well known method to achieve diffraction-limited (DL) imaging from ground-based telescopes. The traditional observing method for speckle has been to observe a single, unresolved, source per telescope pointing over a relatively small field of view (FOV). The need for large DL surveys of targets with high sky density motivates a desire for simultaneous speckle imaging over large FOVs, however it is currently impractical to attain this by covering the entire focal plane with fast readout detectors. An alternative approach is to connect a relatively small number of detector pixels to multiple interesting targets spanning a large FOV through the use of optical fibers, a technique employed in spectroscopy for decades. However, for imaging we require the use of coherent fiber bundles (CFBs). We discuss various design considerations for coherent fiber speckle imaging with an eye toward a multiplexed system using numerous configurable CFBs, and we test its viability with a prototype instrument that uses a single CFB to transport speckle images from the telescope focal plane to a traditionally designed, fast readout speckle camera. Using this device on University of Virginia's Fan Mountain Observatory 40-inch telescope we have for the first time successfully demonstrated speckle imaging through a CFB, using both optical and NIR detectors. Results are presented of DL speckle imaging of well-known close (including subarcsecond) binary stars resolved with this fiber-fed speckle system and compared to both literature results and traditional speckle imaging taken with the same camera directly, with no intervening CFB.