M. Rademacher

ARGOS - The laser guide star system for the LBT

ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding wavefront sensors, driving LBTs adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial beacons are generated via Rayleigh scattering in earths atmosphere. ARGOS will project a set of three guide stars above each of LBTs mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.

Scientific goals for the MMT's multi-laser-guided adaptive optics

The MMTs five Rayleigh laser guide star system has successfully demonstrated open loop wavefront sensing for both ground-layer and laser tomography adaptive optics (AO). Closed loop correction is expected for the first time in the autumn of 2006. The program is moving into its second phase: construction of a permanent facility to feed AO instruments now used with the telescopes existing natural star AO system. The new facility will preserve the thermal cleanliness afforded by the systems adaptive secondary mirror. With the present laser power of 4 W in each of the Rayleigh beacons, we will first offer ground-layer correction over a 2 arcmin field in J, H, and K bands, with expected image quality routinely 0.2 arcsec or better. Later, we will also offer imaging and spectroscopy from 1.5 to 4.8 μm with a tomographically corrected diffraction limited beam. The development of these techniques will lead to a facility all-sky capability at the MMT for both ground-layer and diffraction-limited imaging, and will be a critical advance in the tools necessary for extremely large telescopes of the future, particularly the Giant Magellan Telescope. We describe the present state of system development, planned progress to completion, and highlight the early scientific applications.

Full-system laboratory testing of the F/15 deformable secondary mirror for the new MMT adaptive optics system

We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 cm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the really optics at the test wavelengths of 0.594 and 1.5 micrometers . Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.

Construction and testing of the wavefront sensor camera for the new MMT adaptive optics system

This paper describes the construction and testing of the Shack-Hartmann wavefront sensor camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 X 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 by 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 by 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 micrometers rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5-5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec squares field stop matched to the quall-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 micrometers /arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10 percent compared to an ideal quad-cell with perfectly sharp boundaries.

Status report on the Large Binocular Telescope's ARGOS ground-layer AO system

ARGOS, the laser-guided adaptive optics system for the Large Binocular Telescope (LBT), is now under construction at the telescope. By correcting atmospheric turbulence close to the telescope, the system is designed to deliver high resolution near infrared images over a field of 4 arc minute diameter. Each side of the LBT is being equipped with three Rayleigh laser guide stars derived from six 18 W pulsed green lasers and projected into two triangular constellations matching the size of the corrected field. The returning light is to be detected by wavefront sensors that are range gated within the seeing-limited depth of focus of the telescope. Wavefront correction will be introduced by the telescopes deformable secondary mirrors driven on the basis of the average wavefront errors computed from the respective guide star constellation. Measured atmospheric turbulence profiles from the site lead us to expect that by compensating the ground-layer turbulence, ARGOS will deliver median image quality of about 0.2 arc sec across the JHK bands. This will be exploited by a pair of multi-object near-IR spectrographs, LUCIFER1 and LUCIFER2, with 4 arc minute field already operating on the telescope. In future, ARGOS will also feed two interferometric imaging instruments, the LBT Interferometer operating in the thermal infrared, and LINC-NIRVANA, operating at visible and near infrared wavelengths. Together, these instruments will offer very broad spectral coverage at the diffraction limit of the LBTs combined aperture, 23 m in size.

Status of the MMT Observatory multiple laser beacon projector

The laser guidestar system at the MMT Observatory has produced its first closed loop results and should be producing ground-layer corrected closed loop images within a few months. The LGS system at the MMT is one of few in the world that uses atmospheric Rayleigh scattering from reliable, low-cost lasers, and is unique in its use of a dynamic refocus technique to increase the telescope depth of field for increased return flux. The resulting 10 km depth of field introduces additional constraints on the minimum spot size for the beam projector design. The short exposure spot size as measured at the telescope cassegrain focus is 0.65 arcseconds in 0.59 arcsecond seeing in the visible. Additionally, a method to correct for image motion due to telescope vibrations using accelerometer data has been successfully implemented.

MEDI: An instrument for direct-detection of massive extrasolar planets

We have developed an instrument, MEDI (Massive Exoplanet Differential Imager), that takes advantage of a unique method of starlight rejection, simultaneous differential imaging, in order to directly image massive planets around nearby stars. Using this technique we expect to achieve suppression of starlight to the photon-noise limit, which means that increased exposure time will translate into higher sensitivities. This is in contrast to past sequential and two-color simultaneous studies that reach a sensitivity floor due to speckle-noise limitations. MEDI is currently installed in ARIES, the infrared camera that will be commissioned at the newly refurbished 6.5 MMT in January 2003, with the world’s first adaptive secondary. This should allow us to take Nyquist sampled, diffraction-limited images in the near-IR. The adaptive secondary will also give us unprecedented throughput while minimizing the thermal background and providing a smooth PSF. Based on lab results, we expect to be able to detect objects 106 times fainter than their primaries at 0.5” separations in 2 hours, limited only by photon noise. This suggests that we will be sensitive to objects with masses as small as ~5 MJupiter at separations of greater than ~5 AU for G2 V stars that are ~300 Myr old and within about 10 pc. Therefore, we will probe a unique search space compared with current radial velocity methods, which are so far restricted to close-in (<6 AU) orbits.

Thermal characterization of the support plate for the 6.5-m MMT thin-shell adaptive secondary mirror

The Multiple Mirror Telescope on Mt. Hopkins will soon be upgraded to a single 6.5 m primary mirror.An adaptive- optical system, featuring a thin-shell, adaptive-secondary mirror with 330 voice-coil actuators is being developed for this new telescope. The thin-shell mirror is supported by a thick, concave aluminum substrate which also holds the actuator control and monitoring electronics and serves as the reference surface. With the actuator electronics dissipating heat into the substrate, the thermal behavior of the aluminum reference plate becomes as important issue. This paper presents results form tow experiments designed to determine the thermal behavior of the reference plate.