Victor L. Krabbendam

Early performance and present status of the Hobby-Eberly Telescope

The Hobby-Eberly telescope (HET) is a recently completed 9- meter telescope designed to specialize in spectroscopy. It saw first light in December 1996 and during July 1997, it underwent its first end-to-end testing acquiring its first spectra of target objects. We review the basic design of the HET. In addition we summarize the performance of the telescope used with a commissioning spherical aberration correlator and spectrograph, the status of science operations and plans for the implementation of the final spherical aberration corrector and facility class instruments.

LARGE SYNOPTIC SURVEY TELESCOPE: FROM SCIENCE DRIVERS TO REFERENCE DESIGN

In the history of astronomy, major advances in our understanding of the Universe have come from dramatic improvements in our ability to accurately measure astronomical quantities. Aided by rapid progress in information technology, current sky surveys are changing the way we view and study the Universe. Next-generation surveys will maintain this revolutionary progress. We focus here on the most ambitious survey currently planned in the visible band, the Large Synoptic Survey Telescope (LSST). LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: constraining dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. It will be a large, wide-field ground-based system designed to obtain multiple images covering the sky that is visible from Cerro Pachon in Northern Chile. The current baseline design, with an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg{sup 2} field of view, and a 3,200 Megapixel camera, will allow about 10,000 square degrees of sky to be covered using pairs of 15-second exposures in two photometric bands every three nights on average. The system is designed to yield high image quality, as well as superb astrometric and photometric accuracy. The survey area will include 30,000 deg{sup 2} with {delta} < +34.5{sup o}, and will be imaged multiple times in six bands, ugrizy, covering the wavelength range 320-1050 nm. About 90% of the observing time will be devoted to a deep-wide-fast survey mode which will observe a 20,000 deg{sup 2} region about 1000 times in the six bands during the anticipated 10 years of operation. These data will result in databases including 10 billion galaxies and a similar number of stars, and will serve the majority of science programs. The remaining 10% of the observing time will be allocated to special programs such as Very Deep and Very Fast time domain surveys. We describe how the LSST science drivers led to these choices of system parameters.

Development and performance of Hobby-Eberly Telescope 11-m segmented mirror

The Hobby Eberly Telescope features a unique eleven-meter spherical primary mirror consisting of a single steel truss populated with 91 ZerodurTM mirror segments. The 1 meter hexagonal segments are fabricated to 0.033 micron RMS spherical surfaces with matched radii to 0.5 mm. Silver coatings are applied to meet reflectance criteria for wavelengths from 0.35 to 2.5 micron. To support the primary spectroscopic uses of the telescope the mirror must provide a 0.52 arc sec FWHM point spread function. Mirror segments are co-aligned to within 0.0625 ar sec and held to 25 microns of piston envelope using a segment positioning system that consists of 273 actuators (3 per mirror), a distributed population of controllers, and custom developed software. A common path polarization shearing interferometer was developed to provide alignment sensing of the entire array from the primary mirrors center of curvature. Performance of the array is being tested with an emphasis on alignment stability. Distributed temperature measurements throughout the truss are correlated to pointing variances of the individual mirror segments over extended periods of time. Results are very encouraging and indicate that this mirror system approach will prove to be a cost-effective solution for large optical collecting apertures.

Simulating the LSST system

Extracting science from the LSST data stream requires a detailed knowledge of the properties of the LSST catalogs and images (from their detection limits to the accuracy of the calibration to how well galaxy shapes can be characterized). These properties will depend on many of the LSST components including the design of the telescope, the conditions under which the data are taken and the overall survey strategy. To understand how these components impact the nature of the LSST data the simulations group is developing a framework for high fidelity simulations that scale to the volume of data expected from the LSST. This framework comprises galaxy, stellar and solar system catalogs designed to match the depths and properties of the LSST (to r=28), transient and moving sources, and image simulations that ray-trace the photons from above the atmosphere through the optics and to the camera. We describe here the state of the current simulation framework and its computational challenges.

Project status of the 8.4-m LSST

The 8.4m Large Synoptic Survey Telescope (LSST) is a wide-field telescope facility that will add a qualitatively new capability in astronomy. For the first time, the LSST will provide time-lapse digital imaging of faint astronomical objects across the entire sky. The LSST has been identified as a national scientific priority by diverse national panels, including multiple National Academy of Sciences committees. This judgment is based upon the LSSTs ability to address some of the most pressing open questions in astronomy and fundamental physics, while driving advances in data-intensive science and computing. The LSST will provide unprecedented 3-dimensional maps of the mass distribution in the Universe, in addition to the traditional images of luminous stars and galaxies. These mass maps can be used to better understand the nature of the newly discovered and utterly mysterious Dark Energy that is driving the accelerating expansion of the Universe. The LSST will also provide a comprehensive census of our solar system, including potentially hazardous asteroids as small as 100 meters in size. The LSST facility consists of three major subsystems: 1) the telescope, 2) the camera and 3) the data processing system. The baseline design for the LSST telescope is a 8.4m 3-mirror design with a 3.5 degree field of view resulting in an A-Omega product (etendue) of 302deg2m2. The camera consists of 3-element transmisive corrector producing a 64cm diameter flat focal plane. This focal plane will be populated with roughly 3 billion 10μm pixels. The data processing system will include pipelines to monitor and assess the data quality, detect and classify transient events, and establish a large searchable object database. We report on the status of the designs for these three major LSST subsystems along with the overall project structure and management.

LSST IR Camera for Cloud Monitoring and Observation Planning

The LSST project has acquired an all sky IR camera and started to investigate its effectiveness in cloud monitoring. The IR camera has a 180-degree field of view. The camera uses six filters in the 8-12 micron atmospheric window and has a built in black body reference and visible all sky camera for additional diagnostics. The camera is installed and in nightly use on Cerro Pachon in Chile, between the SOAR and Gemini South telescopes. This paper describes the measurements made to date in comparison to the SOAR visible All Sky Camera (SASCA) and other observed atmospheric throughput. The objective for these tests is to find an IR camera design to provide the survey scheduler with real-time measured conditions of clouds, including high cirrus to better optimize the observing strategy.

Hobby-Eberly Telescope: commissioning experience and observing plans

Experience in bringing into operation the 91-segment primary mirror alignment and control system, the focal plane tracker system, and other critical subsystems of the HET will be described. Particular attention is given to the tracker, which utilizes three linear and three rotational degrees of freedom to follow sidereal targets. Coarse time-dependent functions for each axis are downloaded to autonomous PMAC controllers that provide the precise motion drives to the two linear stages and the hexapod system. Experience gained in aligning the sperate mirrors and then maintaining image quality in a variable thermal environments will also be described. Because of the fixed elevation of the primary optical axis, only a limited amount of time is available for observing objects in the 12 degrees wide observing band. With a small core HET team working with McDonald Observatory staff, efficient, reliable, uncomplicated methodologies are required in all aspects of the observing operations.

Active optics in Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) has a 3.5º field of view and F/1.2 focus that makes the performance quite sensitive to the perturbations of misalignments and mirror surface deformations. In order to maintain the image quality, LSST has an active optics system (AOS) to measure and correct those perturbations in a closed loop. The perturbed wavefront errors are measured by the wavefront sensors (WFS) located at the four corners of the focal plane. The perturbations are solved by the non-linear least square algorithm by minimizing the rms variation of the measured and baseline designed wavefront errors. Then the correction is realized by applying the inverse of the perturbations to the optical system. In this paper, we will describe the correction processing in the LSST AOS. We also will discuss the application of the algorithm, the properties of the sensitivity matrix and the stabilities of the correction. A simulation model, using ZEMAX as a ray tracing engine and MATLAB as an analysis platform, is set up to simulate the testing and correction loop of the LSST AOS. Several simulation examples and results are presented.

The Large Synoptic Survey Telescope concept design overview

The Large Synoptic Survey Telescope Project is a public-private partnership that has successfully completed the Concept Design of its wide-field ground based survey system and started several long-lead construction activities using private funding. The telescope has a 3-mirror wide field optical system with an 8.4 meter primary, 3.4 meter secondary, and 5 meter tertiary mirror. The reflective optics feed three refractive elements and a 64 cm 3.2 gigapixel camera. The telescope will be located on the summit of Cerro Pachón in Chile. The LSST data management system will reduce, transport, alert, archive the roughly 15 terabytes of data produced nightly, and will serve the raw and catalog data accumulating at an average of 7 petabytes per year to the community without any proprietary period. This survey will yield contiguous overlapping imaging of 20,000 square degrees of sky in 6 optical filter bands covering wavelengths from 320 to 1080nm. The project continues to attract institutional partners and has acquired non-federal funding sufficient to construct the primary mirror, already in progress at the University of Arizona, and fund detector prototype efforts, two of the longest lead items in the LSST. The project has submitted a proposal for construction to the National Science Foundation Major Research Equipment and Facilities Construction (MREFC) program and is preparing for a 2011 funding authorization.

Adaptive periodic error correction for Heidenhain tape encoders

Heidenhain position tape encoders are in use on almost all modern telescopes with excellent results. Performance of these systems can be limited by minor mechanical misalignments between the tape and read head causing errors at the grating period. The first and second harmonics of the measured signal are the dominant errors, and have a varying frequency dependant on axis rate. When the error spectrum is within the mount servo bandwidth it results in periodic telescope pointing jitter. This paper will describe an adaptive error correction using elliptic interpolation of the raw signals, based on the well known compensation technique developed by Heydemann [1]. The approach allows the compensation to track in real time with no need of a large static look-up table, or frequent calibrations. This paper also presents the results obtained after applying this approach on data measured on the SOAR telescope.