We present results from our two year study of ground-layer turbulence as seen through the 6.5-meter Magellan
Telescopes at Las Campanas Observatory. The experiment consists of multiple, moderate resolution, Shack-
Hartmann wavefront sensors deployed over a large 16 arcminute field. Over the two years of the experiment,
the ground-layer turbulence has been sampled on eleven nights in a variety of seeing and wind conditions. On
most nights the ground-layer turbulence contributes 10% to the total visible-band seeing, although a few nights
exhibit ground-layer contributions up to 30%. We present the ground-layer turbulence on the sampled nights as
well as a demonstration of its strength as a function of field size. This information is combined with data from a
MASS-DIMM seeing monitor adjacent to the Magellan Telescopes to infer the annual ground-layer contribution
to seeing at Las Campanas.
It has recently been suggested that up to half of the wavefront variance can be removed from the total atmospheric distortion by correcting only the lowest seeing layer (Rigaut 2000, 2001). This Ground-Layer AO (GLAO) correction could provide improved image quality over a very wide field of view; however, no development work has been done on existing telescopes. The implications are profound for optical designs of future AO optimized telescopes (e.g. the ELTs) as accurately compensating for this ground-layer strongly favors an adaptive element conjugated to the median height of the ground-layer. The gains of GLAO are tantalizing but substantially unproven, and thus, the Giant Magellan Telescope (GMT) project has developed a multi-phased study with the goal of providing an on-sky demonstration of GLAO technology at the Magellan Telescopes.
The first phase of this experiment is to measure the the height and
boundary of the ground-layer through multiple, fixed wavefront sensors
on very bright cluster fields over the full 24 arcminute Magellan
field of view. With a typical wind speed of 9 m/s and a presumed secondary ground-layer conjugation error of 100 m, the equivalent decoherence time is approximately 0.04 seconds. Therefore, we have designed and constructed high resolution Shack-Hartmann sensors running at 100 frames per second with coarse, 0.6m sub-apertures.
We present a technical description of the wavefront sensors and image
analyzer, as well as current results from the first deployment of
this instrument at Magellan. In addition, we discuss the implications
for ground-layer modeling and describe the next phases of the GMT's
GLAO experiment.
The Magellan Inamori Kyocera Echelle (MIKE) is a double echelle spectrograph designed for use at the Magellan Telescopes at Las Campanas Observatory in Chile. It is currently in the final stages of construction and is scheduled for commissioning in the last quarter of 2002. In standard observing mode, the blue (320-480 nm) and red (440-1000 nm) channels are used simultaneously to obtain spectra over the full wavelength range with only a few gaps in wavelength coverage at the reddest orders. Both channels contain a three-group set of all-spherical, standard optical glass and calcium fluoride lenses which function as both camera and collimator in a double pass configuration. A single, standard echelle grating is used on each side and is illuminated close to true Littrow. Prism cross-dispersers are also used double-pass, and provide a minimum separation between orders of 6 arcsec. Spectral resolution is 19,000 and 25,000 on the red and blue sides, respectively, with a 1 arcsec slit. Typical rms image diameter is less than 0.2 arcsec, so that resolution increases linearly with decreasing slit width. The standard observing mode will use a slit up to 5" long, however a fiber-fed mode will also be available using blocking filters to select the desired orders for up to 256 objects at a time. In this paper, we describe the optical and mechanical design of the instrument.
We present the design for an optical spectrograph for the 6.5-meter Magellan II Telescope. The spectrograph covers the full visible spectrum in a single exposure at very high efficiency through a dual-channel design and the use of volume phase holographic (VPH) gratings in lieu of traditional surface gratings. A pair of symmetric fold mirrors about the grating keep the spectrograph in Littrow configuration, eliminating the need for an articulated camera. Efficient VPH prescriptions have been developed for all resolution modes up to R=11,000. The resulting design is, mechanically and optically, relatively simple, compact, and inexpensive.
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