All Weather Calibration of Wide Field Optical and NIR Surveys

The science goals for ground-based large-area surveys, such as the Dark Energy Survey, Pan-STARRS, and the Large Synoptic Survey Telescope, require calibration of broadband photometry that is stable in time and uniform over the sky to precisions of a per cent or better. This performance will need to be achieved with data taken over the course of many years, and often in less than ideal conditions. This paper describes a strategy to achieve precise internal calibration of imaging survey data taken in less than photometric conditions, and reports results of an observational study of the techniques needed to implement this strategy. We find that images of celestial fields used in this case study with stellar densities of order one per arcmin-squared and taken through cloudless skies can be calibrated with relative precision of 0.5 per cent (reproducibility). We report measurements of spatial structure functions of cloud absorption observed over a range of atmospheric conditions, and find it possible to achieve photometric measurements that are reproducible to 1 per cent in images that were taken through cloud layers that transmit as little as 25 per cent of the incident optical flux (1.5 magnitudes of extinction). We find, however, that photometric precision below 1 per cent is impeded by the thinnest detectable cloud layers. We comment on implications of these results for the observing strategies of future surveys.Comment: Accepted for publication in The Astronomical Journal (AJ

The guider and wavefront curvature sensor subsystem for the Large Synoptic Survey Telescope

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.Astronom

A framework for modeling the detailed optical response of thick, multiple segment, large format sensors for precision astronomy applications

Near-future astronomical survey experiments, such as LSST, possess system requirements of unprecedented fidelity that span photometry, astrometry and shape transfer. Some of these requirements flow directly to the array of science imaging sensors at the focal plane. Availability of high quality characterization data acquired in the course of our sensor development program has given us an opportunity to develop and test a framework for simulation and modeling that is based on a limited set of physical and geometric effects. In this paper we describe those models, provide quantitative comparisons between data and modeled response, and extrapolate the response model to predict imaging array response to astronomical exposure. The emergent picture departs from the notion of a fixed, rectilinear grid that maps photo-conversions to the potential well of the channel. In place of that, we have a situation where structures from device fabrication, local silicon bulk resistivity variations and photo-converted carrier patterns still accumulating at the channel, together influence and distort positions within the photosensitive volume that map to pixel boundaries. Strategies for efficient extraction of modeling parameters from routinely acquired characterization data are described. Methods for high fidelity illumination/image distribution parameter retrieval, in the presence of such distortions, are also discussed. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.Astronom

A daytime measurement of the lunar contribution to the night sky brightness in LSST’s ugrizy bands–initial results

We report measurements from which we determine the spatial structure of the lunar contribution to night sky brightness, taken at the LSST site on Cerro Pachon in Chile. We use an array of six photodiodes with filters that approximate the Large Synoptic Survey Telescope’s u, g, r, i, z, and y bands. We use the sun as a proxy for the moon, and measure sky brightness as a function of zenith angle of the point on sky, zenith angle of the sun, and angular distance between the sun and the point on sky. We make a correction for the difference between the illumination spectrum of the sun and the moon. Since scattered sunlight totally dominates the daytime sky brightness, this technique allows us to cleanly determine the contribution to the (cloudless) night sky from backscattered moonlight, without contamination from other sources of night sky brightness. We estimate our uncertainty in the relative lunar night sky brightness vs. zenith and lunar angle to be 10 %. This information is useful in planning the optimal execution of the LSST survey, and perhaps for other astronomical observations as well. Although our primary objective is to map out the angular structure and spectrum of the scattered light from the atmosphere and particulates, we also make an estimate of the expected number of scattered lunar photons per pixel per second in LSST, and find values that are in overall agreement with previous estimates.Physic

The Effects of Commercial Airline Traffic on LSST Observing Efficiency

<p>The Large Synoptic Survey Telescope (LSST) is a ten-year survey that will map the southern sky in six different filters 800 times before the end of its run. In this paper, we explore the primary effect of airline traffic on scheduling the LSST observations in addition to the secondary effect of condensation trails, or contrails, created by the presence of the aircraft. The large national investment being made in LSST implies that small improvments observing efficiency through aircraft and contrail avoidance can result in a significant improvement in the quality of the survey and its science. We have used the Automatic Dependent Surveillance-Broadcast (ADS-B) signals received from commercial aircraft to monitor and record activity over the LSST site. We installed a ADS-B ground station on Cerro Pachón, Chile consiting of a1090Mhz antenna on the Andes Lidar Observatory feeding a RTL2832U software defined radio. We used dump1090 to convert the received ADS-B telementry into Basestation format, where we found that during the busiest time of the night there were only 4 signals being received each minute on average, which will have very small direct effect, if any, on the LSST observing scheduler. As part of future studies we will examin the effects of contrals on LSST observations. Gibson and this work is supported by the NOAO/KPNO Research Experiences for Undergraduates (REU) Program which is funded by the NSF REU Program (AST-1262829) and under US DOE grant SC0007881.</p