Correlated double sampling (CDS) is a process used in many charge-coupled device readout systems to cancel the reset noise component that would otherwise dominate. CDS processing typically consists of subtracting the integrated video signal during a “signal” period from that during a “reset” period. The response of this processing depends, therefore, on the shape of the video signal with respect to the integration bounds. In particular, the amount of noise appearing in the final image and the linearity of the pixel value with signal charge are affected by the choice of the CDS timing intervals. We use a digital CDS readout system which highly oversamples the video signal (as compared with the pixel rate) to reconstruct pixel values for different CDS timings using identical raw video signal data. We use this technique to develop insights into optimal strategy for selecting CDS timings both in the digital case (where the raw video signal may be available) and in the general case (where it is not). In particular, we show that the linearity of the CDS operation allows subtraction of the raw video signals of pixels in bias images from those in illuminated images to directly show the effects of CDS processing on the final (subtracted) pixel values.
The Large Synoptic Survey Telescope instrument include four guiding and wavefront sensing subsystems called corner
raft subsystems, in addition to the main science array of 189 4K x 4K CCDs. These four subsystems are placed at the
four corners of the instrumented field of view. Each wavefront/guiding subsystem comprises a pair of 4K x 4K guide
sensors, capable of producing 9 frames/second, and a pair of offset 2K x 4K wavefront curvature sensors from which the
images are read out at the cadence of the main camera system, providing 15 sec integrations. These four
guider/wavefront corner rafts are mechanically and electrically isolated from the science sensor rafts and can be installed
or removed independently from any other focal plane subsystem. We present the implementation of this LSST
subsystem detailing both hardware and software development and status.
The Large Synoptic Survey Telescope (LSST) uses an Active Optics System (AOS) to maintain system alignment and surface figure on its three large mirrors. Corrective actions fed to the LSST AOS are determined from 4 curvature based wavefront sensors located on the corners of the inscribed square within the 3.5 degree field of view. Each wavefront sensor is a split detector such that the halves are 1mm on either side of focus. In this paper we describe the development of the Active Optics Pipeline prototype that simulates processing the raw image data from the wavefront sensors through to wavefront estimation on to the active optics corrective actions. We also describe various wavefront estimation algorithms under development for the LSST active optics system. The algorithms proposed are comprised of the Zernike compensation routine which improve the accuracy of the wavefront estimate. Algorithm development has been aided by a bench top optical simulator which we also describe. The current software prototype combines MATLAB modules for image processing, tomographic reconstruction, atmospheric turbulence and Zemax for optical ray-tracing to simulate the closed loop behavior of the LSST AOS. We describe the overall simulation model and results for image processing using simulated images and initial results of the wavefront estimation algorithms.
The Large Synoptic Survey Telescope (LSST) is a proposed ground based telescope that will perform a comprehensive
astronomical survey by imaging the entire visible sky in a continuous series of short exposures. Four special purpose
rafts, mounted at the corners of the LSST science camera, contain wavefront sensors and guide sensors. Wavefront
measurements are accomplished using curvature sensing, in which the spatial intensity distribution of stars is measured
at equal distances on either side of focus by CCD detectors. The four Corner Rafts also each hold two guide sensors. The
guide sensors monitor the locations of bright stars to provide feedback that controls and maintains the tracking of the
telescope during an exposure. The baseline sensor for the guider is a Hybrid Visible Silicon hybrid-CMOS detector. We
present here a conceptual mechanical and electrical design for the LSST Corner Rafts that meets the requirements
imposed by the camera structure, and the precision of both the wavefront reconstruction and the tracking. We find that a
single design can accommodate two guide sensors and one split-plane wavefront sensor integrated into the four corner
locations in the camera.