Proc. SPIE. 4836, Survey and Other Telescope Technologies and Discoveries
KEYWORDS: Telescopes, Stereoscopy, Spectroscopy, Tomography, Solids, Galactic astronomy, Galaxy groups and clusters, Large Synoptic Survey Telescope, Surface conduction electron emitter displays, 3D image processing
We explore a possible "killer app" for the LSST and similar surveys: imaging mass in three dimensions. We describe its scientific importance, practical techniques for realizing it, the current state of the art and how it might scale to the LSST.
The Deep Lens Survey (DLS) is a deep BV Rz' imaging survey of seven 2°×2° degree fields, with all data to be made public. The primary scientific driver is weak gravitational lensing, but the survey is also designed to enable a wide array of other astrophysical investigations. A unique feature of this survey is the search for transient phenomena. We subtract multiple exposures of a field, detect differences, classify, and release transients on the Web within about an hour of observation. Here we summarize the scientific goals of the DLS, field and filter selection, observing techniques and current status, data reduction, data products and release, and transient detections. Finally, we discuss some lessons which might apply to future large surveys such as LSST.
The Big Throughput Camera (BTC) recently celebrated its first anniversary as a user instrument on the Blanco 4-meter telescope at Cerro Tololo Interamerican Observatory (CTIO), where it collects more photons per second than any other nighttime astronomical camera in the world. We offer a look at the successes and lessons learned during the first year of operation. After an overview of the hardware, we describe the software from the user's point of view, and then offer examples of the observing targets and strategies used. BTC has become very popular among CTIO observers -- more than one- third of dark time is now assigned to BTC -- but the large field of view leads to some new data reduction challenges which we discuss in the final section.
A new adaptive optics system has been constructed for moderately high resolution in the near infrared at the Multiple Mirror Telescope (MMT). The system, called FASTTRAC II, has been designed to combine the highest throughput with the lowest possible background emission by making the adaptive optical element be an existing and necessary part of the telescope, and by eliminating all warm surfaces between the telescope and the science camera's dewar. At present, only natural guide stars are supported, but by the end of 1995, we will add the capability to use a single sodium resonance beacon derived from a laser beam projected nearly coaxially with the telescope. In this paper, we present a description of FASTTRAC II, and show results from its first test run at the telescope in April 1995.
A tip-tilt secondary system has been developed on a 2.3 m telescope. The system, called FASTTRAC, stabilizes image motion to less than 0.1' rms at closed loop corrective frequencies of <EQ 100 Hz. Resolutions of 0.19' and Strehl ratios of 0.14 have been obtained in long exposure images at 1.6 micrometers in seeing conditions of D/ro approximately 4. FASTTRAC is unique in its ability to use infrared guide stars (K <EQ 8). Recently FASTTRAC was upgraded to accommodate a low read noise CCD, allowing faint visible guide stars to be utilized. We anticipate that guide stars of R equals 17 will be adequate at correction frequencies of 30 Hz.
A sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Telescope (MMT). For the first run in 1993 May, the artificial star was created by a 0.5 W beam from a continuous- wave dye laser tuned to the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1.8 m mirrors. Scattering by the mesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and of angular extent 1'.2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used to generate an artificial guide star of visual magnitude 10.4, and 1'.1 FWHM. In each case, the beacon was used by the MMT adaptive optics system to compensate in real time for atmospherically- induced differential image motion between the six mirror elements, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star was added, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon show significantly reduced width and an increase in Strehl ratio of almost a factor of two.
Low spatial frequencies of atmospheric turbulence are specially troublesome to astronomers because the phase distortions they cause have large amplitude. We have begun experiments at the Multiple Mirror Telescope (MMT) to remove these errors with tip, tilt, and piston control of pieces of the wave front defined by the telescope's six 1.8 m primary mirrors. We show long exposure images taken at the telescope with resolution as high as 0.08 arcsec under piston control, and 0.26 arcsec under tilt control, using an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. We also present preliminary results from analysis of images of the pre-main sequence star T Tauri taken with tilt control of the six beams only, at three infrared wavelengths. The resolution is between 0.35 and 0.4 arcsec, higher than has previously been achieved with direct imaging. The faint red companion to T Tau is clearly revealed, and is seen to be undergoing an energetic outburst.
The next generation of 6 to 10 m class telescopes is being planned to include the capability for adaptive wavefront correction. The MMT with its 7-m baseline, provides an ideal testbed for novel techniques of adaptive optics. Using a new instrument based on a six-segment adaptive mirror, a number of wavefront sensing algorithms including an artificial neural network have been implemented to demonstrate the high resolution imaging capability of the telescope. These algorithms rely on a freely available property of starlight, namely, its coherence over large scales, to sense directly atmospheric and instrumental phase errors across large distances. In this paper, we report results obtained so far with resolutions between 0.08 and 0.3 arcsec at 2.2-micron wavelength. We also show data indicating that at the level of 0.1-arcsec imaging in good seeing, the isoplanatic patch at this wavelength is at least 20 arcsec across.
The MMT consists of six comounted 1.8 m telescopes from which the light is brought to a combined coherent focus. Atmospheric turbulence spoils the MMT diffraction-limited beam profile, which would otherwise have a central peak of 0.06 arcsec FWHM, at 2 microns wavelength. At this wavelength, the adaptive correction of the tilt and path difference of each telescope beam is sufficient to recover diffraction-limited angular resolution. Computer simulations have shown that these tilts and pistons can be derived by an artificial neural network, given only a simultaneous pair of in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We describe such an adaptive optics system for the MMT, as well as some successful tests of neural network wavefront sensing on images, and initial real-time tests of the adaptive system at the telescope; attention is given to a demonstration of the adaptive stabilization of the mean phase errors between two mirrors which resulted in stable fringes with 0.1 arcsec resolution.