A Split-Frame Transfer CCD for the Tierras Observatory

The Tierras Observatory is a refurbished 1.3-m ultra-precise fully-automated precision photometer located at the F. L. Whipple Observatory atop Mt. Hopkins, Arizona. It is intended for the characterization of terrestrial planet transits orbiting < 0.3 R⊙ stars, as well as the potential discovery of exo-moons and exo-rings. In order to obtain high-cadence, ultraprecise time series of diverse M dwarf targets, the photometer’s CCD must have high quantum efficiency in the NIR (810-900 nm), as well as a large full well. A large area, deeply cooled backside-illuminated Charge-coupled device (CCD) was chosen as the sensor, but it was decided to operate the CCD camera system without an optical shutter due to both size limitations and because the large number of images acquired would greatly exceed the expected lifetime of any commercially available shutter. Instead, a commercially available 4096(H) × 4112(V) Teledyne e2v CCD231-84-*-F64 bulk silicon deep-depletion CCD was modified to add optical masks to the outer two sections of 1032 rows, and the camera electronics were modified to allow operation in a split-frame transfer mode. The central 4096(H) × 2048(V) area is used to accumulate image charge, which is then shifted under the masked storage regions for readout while the next image is being acquired. Rapid transfer of charge from the central image sections to the storage sections maximizes the time the CCD is actively acquiring image signal. The masks are fabricated from silicon and positioned close to, but not touching, the CCD surface. The outer surface of the mask is coated with an optical absorbing coating to minimize reflections back into the focal reducer and field-flattener lenses.

Details of the optical mask construction and its performance with respect to accuracy of placement, sharpness of the mask edge, and reflections from the mask surfaces will be presented. A description of the operation of the CCD in split-frame transfer mode, and performance of the camera system with respect to parameters especially relevant to this application, including frame rate, readout speed and noise, and baseline stability will be discussed.