Christopher L. Morbey

Progress on Altair: The Gemini North Adaptive Optics System

The Gemini Adaptive Optics System, (Altair), under construction at the National Research Council of Canadas Herzberg Institute of Astrophysics is unique among AO systems. Altair is designed with its deformable mirror (DM) conjugate to high altitude. We summarize construction progress. We then describe Altair in more detail. Both the Wavefront sensor foreoptics and control system are unconventional, because the guide star footprint on an altitude-conjugated DM moves as the guide star position varies. During a typical nodding sequence, where the telescope moves 10 arcseconds between exposures, this footprint moves by half an actuator and/or WFS lenslet. The advantages of altitude conjugation include increased isoplanatic patch size, which improves sky coverage, and improved uniformity of the corrected field. Altitude conjugation also reduces focal anisoplanatism with laser beacons. Although the initial installation of Altair will use natural guide stars, it will be fully ready to use a laser guide star (LGS). The infrastructure of Gemini observatory provides a variety of wavefront sensors and nested control loops that together permit some unique design concepts for Altair.

GMOS: the GEMINI Multiple Object Spectrographs

The two Gemini multiple object spectrographs (GMOS) are being designed and built for use with the Gemini telescopes on Mauna Kea and Cerro Pachon starting in 1999 and 2000 respectively. They have four operating modes: imaging, long slit spectroscopy, aperture plate multiple object spectroscopy and area (or integral field) spectroscopy. The spectrograph uses refracting optics for both the collimator and camera and uses grating dispersion. The image quality delivered to the spectrograph is anticipated to be excellent and the design is driven by the need to retain this acuity over a large wavelength range and the full 5.5 arcminute field of view. The spectrograph optics are required to perform from 0.36 to 1.8 microns although it is likely that the northern and southern versions of GMOS will use coatings optimized for the red and blue respectively. A stringent flexure specification is imposed by the scientific requirement to measure velocities to high precision (1 - 2 km/s). Here we present an overview of the design concentrating on the optical and mechanical aspects.

Long-period radial-velocity variations of Arcturus

Precise measurements of the radial velocity of Arcturus were obtained on 43 occasions between 1981 and 1985. The measurements show a 500 m/s range, confirming the radial velocity variability of Arcturus. Multiperiod models which give a good representation of the velocities are derived using a least-squares technique. The models also fit the velocities of Smith et al. (1987). It is suggested that the small-amplitude short-period components of the models could be analogous to the solar five-minute oscillations. The best-fitting models require the largest-amplitude component to have a period of at least 640 days.

Megacam: the next-generation wide-field imaging camera for CFHT

MEGACAM is the next generation wide-field camera to be used at the prime focus of the 3.6 m CFHT telescope. This instrument, which will cover a full 1 square degree, is designed around a mosaic of 36 to 40 2K X 4K CCDs. such a large detector requires new approaches for the hardware as well as the software, and will have major impacts on the telescope structure, optics and operations. We present in this paper several novel ideas and techniques that will be implemented as part of this challenging project.

FUSE: fine error sensor optical performance

The Far Ultraviolet Spectroscopic Explorer mission imposes stringent requirements on the satellite attitude control system. Target acquisition accuracy and target tracking stability must each be no greater than 0.5 arcseconds FWHM. The data required by the attitude control system to meet these requirements are provided by two redundant Fine Error Sensors. Each Fine Error Sensor operates as a slit-jaw camera that provides either complete images of the star-field around the line of sight of the telescope, or centroided positions of selected guide stars in the field of view. The satellite pointing requirements must be met over a wide dynamic range of target or guide star brightness, for both sparse and crowded starfields, and for targets that may be either point sources or extended objects. We will describe the operational characteristics of the FES and present data on its performance. We also discuss the optical, mechanical, thermal, and electronic design challenges encountered in meeting the mission requirements, and how they were addressed in the context of a very tight development schedule.

Canadian very large optical telescope technical studies

A design is proposed for a 20 m Canadian Very Large Optical Telescope (VLOT). This design meets the science, schedule, and availability requirements of the Canadian astronomical community. The telescope could be operational by early in the next decade to complement the science discoveries of the Next Generation Space Telescope (NGST) and Atacama Large Millimeter Array (ALMA). This design is suitable for location on the Mauna Kea summit ridge, and could replace the current 3.6 m CFHT telescope. The telescope structure provides room for two vertically oriented Nasmyth instruments, implements a very stiff monocoque mirror cell, and offers a short and direct load path to the telescope mount. A Calotte style dome structure offers many advantages over current designs including lower and more even power requirements, and a circular aperture that will better protect the telescope structure from wind buffeting. The science requirements are presented, and the telescope optical design, primary mirror pupil segmentation options, including hexagonal segments and a radial segment design with a central 8 m mirror, are considered. Point spread function plots and encircled energy calculations show that there is no significant diffraction performance difference between the options except that hexagonal segments in the 1 m point-to-point range appear to deliver poorer PSFs as compared to 2 m and larger segments. Plans for implementation of a Matlab based integrated telescope model are discussed. A summary of adaptive optics system issues for large telescopes is presented along with plans for future research in AO.

The Gemini Multi-Object Spectrographs

Each GEMINI telescope will be equipped with a versatile optical/near-IR spectrographs which will fully exploit its large aperture and excellent image quality. The basic parameters of the GEMINI Multi-object Spectrographs (GMOS) are summarised in the table below.

Fifteen Degree Correcting Optics for a 10-Meter Liquid Mirror Telescope

An optical design of a corrector which permits a 10-metre, F/5 liquid mirror to be operated 7.5 degrees from the zenith is described. The corrector consists of three computer-controlled, actively stressed, moving mirrors.

The application of artificial neural networks for telescope guidance: a feasibility study for Lyman FUSE

Since the fine error sensor camera of the Lyman Far Ultraviolet Spectrographic Explorer (FUSE) has significant residual field curvature aberration, the star images over the field of view have a wide variety of shapes. A search for appropriate backup guiding techniques led to the investigation of artificial neural networks (ANN). Such a technique is shown to be capable of learning the image shapes of stars if they are sufficiently differnet. This study investigates the feasibility of using image patterns as positional references for telescope guidance to satisfy redundancy requirements for the mission. For this initial simulation, the ANN was trained to categorize images according to how far they were from the center of the field of view (radius). We found that a non-linear, single hidden layer ANN learned 90 percent of the training patterns, then correctly classified 89 percent of a set of patterns randomly spread over the field of view. This indicates that the network interpolates between training images. Half of the misclassifications are attributed to the image pattern degradation caused by the secondary support structure spider.

OCCULTATION STUDIES AT THE DOMINION ASTROPHYSICAL OBSERVATORY

During the last year studies of lunar occultations have begun at the Dominion Astrophysical Observatory. Since the acquisition of a Fabri-Tek (model FT 1074) signal averager and analyser, twelve occultations have been observed but only one has been reduced and a comparison made with a theoretical model. The observed diffraction pattern for this occultation was found to correspond with an almost point source model.