Brooke Gregory

Thirty Meter Telescope Site Testing I: Overview

As part of the conceptual and preliminary design processes of the Thirty Meter Telescope (TMT), the TMT site-testing team has spent the last five years measuring the atmospheric properties of five candidate mountains in North and South America with an unprecedented array of instrumentation. The site-testing period was preceded by several years of analyses selecting the five candidates: Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro Martir in Baja California, Mexico; and the 13 North (13N) site on Mauna Kea, Hawaii. Site testing was concluded by the selection of two remaining sites for further consideration, Armazones and Mauna Kea 13N. It showed that all five candidates are excellent sites for an extremely large astronomical observatory and that none of the sites stands out as the obvious and only logical choice based on its combined properties. This is the first article in a series discussing the TMT site-testing project.

Design of ground-layer turbulence compensation with a Rayleigh beacon

The adaptive optics instrument for the SOAR 4.1-m telescope will improve the spatial resolution by 2-3 times at visible wavelengths, over a field of 3 arcmin, by sensing and correcting low-altitude turbulence selectively. We will use a Rayleigh laser guide star to accomplish this. We present the laser guide star design with predictions of system performance based on real turbulence statistics and telescope properties, sky coverage and some opto-mechanical aspects of the AO module. Various design trade-offs are discussed.

The Cerro Tololo Inter-American Observatory infrared spectrometer

Inter-American Observatory. The instrument has a slit of modest length and a selection of gratings that allow resolutions (λ/{DELTA}λ) from ~150 to ~3000 over the wavelength range covered by its photovoltaic InSb array: 0.9 microns to 5 microns. The design and operating characteristics of the IRS are described, and observing procedures and data-reduction techniques are outlined for potential users.

Implementation of a laser traffic control system supporting laser guide star adaptive optics on Mauna Kea

The idea of achieving Adaptive Optics over the majority of the sky using sodium laser guide stars is reaching maturity on Mauna Kea. However, Mauna Kea is a shared astronomical site with 13 institutions and 11 telescopes. Coordination between observatories with laser guide stars and facilities without laser guide stars must be accomplished to prevent sodium light (Rayleigh scatter and the laser guide star itself) from interfering with science observations at the non-laser facilities. To achieve this goal, a technical working group was organized with participation from several Mauna Kea observatories to discuss and agree upon an automated system for avoiding laser “beam” collisions with other telescopes. This paper discussed the implementation of a Laser Traffic Control System (LTCS) for Mauna Kea including a brief history of the coordination effort, technical requirements and details surrounding implementation of laser beam avoidance software, critical configuration parameters, algorithmic approaches, test strategies used during deployment, and recommendations based upon experiences to date for others intending to implement similar systems.

A visible-light AO system for the 4.2-m SOAR telescope

Pushing the adaptive compensation of turbulence into the visible range remains a challenging task, despite the progress of AO technology. An AO system for SOAR, now under conceptual study, will be able to reach diffraction-limited resolution at 0.5-0.7 microns with natural guide stars as faint as magnitude 12, enabling studies of stellar vicinities for faint companions, nebulosity, etc. During the second stage of the project a Rayleigh laser guide star will be implemented. In this mode, only the lowest turbulent layers will be compensated. The angular resolution will be only two times better than natural seeing, but, in exchange, the uniformly compensated field will reach 2-3 arc-minutes, offering unique capabilities in crowded fields (clusters, nearby galaxies).

Temporal variability of the seeing of TMT sites

Seeing stability is an important criterion of site characterization. Two sites, with the same seeing statistics, could in principle differ in their temporal stability and hence have their observatories perform differently. Temporal variability can, however, be defined in several ways, all of which may determine the performance of the observatories in different manner. In this paper, we propose three methods to measure variability each focusing on different applications: Selection (maximization of observation time), Image quality (seeing variation within a given integration time) and finally Scheduling (prediction of seeing fluctuation on a given time scale). We apply these methods to the seeing of the TMT candidate sites to determine their stability properties.

Status of the Thirty Meter Telescope site selection program

The Thirty Meter Telescope (TMT) project has been collecting data on five candidate sites since 2003. This paper describes the site testing portion of the TMT site selection program and the process and standards employed by it. This includes descriptions of the candidate sites, the process by which they were identified, the site characterization instrument suite and its calibration and the available results, which will be published shortly.

Dark energy camera installation at CTIO: overview

The Dark Energy Camera (DECam) has been installed on the V. M. Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. This major upgrade to the facility has required numerous modifications to the telescope and improvements in observatory infrastructure. The telescope prime focus assembly has been entirely replaced, and the f/8 secondary change procedure radically changed. The heavier instrument means that telescope balance has been significantly modified. The telescope control system has been upgraded. NOAO has established a data transport system to efficiently move DECams output to the NCSA for processing. The observatory has integrated the DECam highpressure, two-phase cryogenic cooling system into its operations and converted the Coudé room into an environmentally-controlled instrument handling facility incorporating a high quality cleanroom. New procedures to ensure the safety of personnel and equipment have been introduced.

Active Tangent Link System for Transverse Support of Large Thin Meniscus Mirrors

An active tangent link system was developed to provide transverse support for large thin meniscus mirrors. The support system uses six tangent links to control position and distribute compensating force to the mirror. Each of the six tangent links utilizes an electromechanical actuator and an imbedded lever system working through a load cell and flexure. The lever system reduces the stiffness, strength and force resolution requirements of the electromechanical actuator and allows more compact packaging. Although all six actuators are essentially identical, three of them are operated quasi statically, and are only used to position the optic. The other three are actively operated to produce an optimal and repeatable distribution of the transverse load. This repeatable load distribution allows for a more effective application of a look up table and reduces the demands on the active optics system. A control system was developed to manage the quasi static force equilibrium servo loop using a control matrix that computes the displacements needed to correct any force imbalance with good convergence and stability. This system was successfully retrofitted to the 4.3 meter diameter, 100 mm thick SOAR primary mirror to allow for more expeditious convergence of the mirror figure control system. This system is also intended for use as the transverse support system for the LSST 3.4 meter diameter thin meniscus secondary mirror.

Improving the Blanco Telescope's Delivered Image Quality

The V. M. Blanco 4-m telescope at Cerro Tololo Inter-American Observatory is undergoing a number of improvements in preparation for the delivery of the Dark Energy Camera. The program includes upgrades having potential to deliver gains in image quality and stability. To this end, we have renovated the support structure of the primary mirror, incorporating innovations to improve both the radial support performance and the registration of the mirror and telescope top end. The resulting opto-mechanical condition of the telescope is described. We also describe some improvements to the environmental control. Upgrades to the telescope control system and measurements of the dome environment are described in separate papers in this conference.