David G. Sandler

Adaptive optics for diffraction-limited infrared imaging with 8-m telescopes

When equipped with adaptive optics, the coming generation of large 6–10-m telescopes can combine huge light grasp with very sharp images. We describe a specific design concept for recovery of diffraction-limited images in the 1.6- and the 2.2-μm atmospheric windows, yielding 0.05-arcsec resolution for an 8-m telescope. Our goal has been to achieve this performance routinely by not requiring above-average atmospheric conditions or the use of unusually bright nearby stars. Atmospheric blurring is sensed with a sodium laser beacon of a few watts. Image motion is sensed by starlight, with a quadrant detector that is sensitive to the broad infrared band in which photon flux is typically largest and the field star has been sharpened by laser-beacon correction that is shared with the science target. A detailed performance analysis shows that for typical conditions Strehl ratios of >25% are expected at 2.2 μm, with the probability of finding a sufficiently bright field star exceeding 50%.

Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets

A recent study by Angel (1994) using simplified analytical models indicated the feasibility of imaging extrasolar planets from the ground, making use of adaptive optics correction of a large telescope. We have performed detailed simulations of the method using computer codes that model propagation through atmospheric turbulence, adaptive correction, and broadband imaging. We confirm that high-resolution correction at the limit of photon noise errors reduces the halo intensity to 10-6 of the peak star flux. Our work shows how to avoid systematic errors. Thus, we find that time delays between sensing the wave front and its detection lead to persistent structure in the stellar halo, which uncorrected would prevent rapid averaging of the residual halo speckle structure. A local wave front reconstructor that extrapolates ahead in time has been devised to remove this problem. We find the chromatic differences in wave front structure are small enough that the signal-to-noise ratio can be improved by wave front sensing and imaging in separate adjacent bands. We verify that correction of amplitude scintillation is needed and the optimum level of clipping is derived. A simulated image of a twin of the solar system at 8 pc is presented for the new 6.5 m telescope and the measured turbulence at the Multiple Mirror Telescope site. The Jupiter twin shows up at the 5 σ level in a 5 hr integration.

Shearing interferometry for laser-guide-star atmospheric correction at large D/r 0

Shearing interferometry offers a possible method to scale wave-front sensors to the large number of subapertures that will be required for correction of the new generation of large 6–10-m-class telescopes at visible wavelengths. We discuss static shearing interferometers, which are prototype wave-front sensors for use with laser guide stars at large values of D/r0. The dc interferometers utilize low-noise detector array technology and can be implemented with a variety of shear lengths to accommodate different atmospheric conditions. We discuss the optical design and the noise sensitivity of two versions of dc interferometer. Atmospheric tests of the interferometers for correcting a 500-subaperture adaptive system are presented, including fringe data obtained with ultraviolet and visible laser stars and compensated ultraviolet images of Vega obtained at our test facility in San Diego.

First results of an on-line adaptive optics system with atmospheric wavefront sensing by an artificial neural network

The first results from an adaptive optics system operating on-line at the telescope with the wavefront aberration sensed by a trained artificial neural network are presented. Star images were formed at 2.2 μm wavelength by two coherently phased apertures of the Multiple Mirror Telescope (MMT), and analyzed by the neural net. The net derives wavefront parameters in a few milliseconds, and the system performance is fast enough that the aberration is nearly frozen during the time needed to make a correction. With the servo loop in operation, the corrected image shows significant power at the diffraction limit of 0.1″

Lightweight mirror technology using a thin facesheet with active rigid support

The next generation of space telescopes will require primary mirrors that push beyond the current state of technology of mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/m2 and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes--diffraction limited optical performance of a 0.5-m diameter mirror and launch survival of a 1-m diameter mirror. We have also built and tested a prototype actuator that achieves 25 nm resolution at cryogenic temperatures. We are now building a 2-m mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators and control electronics. It will be actively controlled and interferometrically measured at 35 K.

High-order adaptive secondary mirrors: where are we?

We discuss the current developments and the perspective performances of adaptive secondary mirrors for high order adaptive a correction on large ground based telescopes. The development of the basic techniques involved a large collaborative effort of public research Institutes and of private companies is now essentially complete. The next crucial step will be the construction of an adaptive secondary mirror for the 6.5 m MMT. Problems such as the fabrication of very thin mirrors, the low cost implementation of fast position sensors, of efficient and compact electromagnetic actuators, of the control and communication electronics, of the actuator control system, of the thermal control and of the mechanical layout can be considered as solved, in some cases with more than one viable solution. To verify performances at system level two complete prototypes have been built and tested, one at ThermoTrex and the other at Arcetri. The two prototypes adopt the same basic approach concerning actuators, sensor and support of the thin mirror, but differ in a number of aspects such as the material of the rigid back plate used as reference for the thin mirror, the number and surface density of the actuators, the solution adopted for the removal of the heat, and the design of the electronics. We discuss how the results obtained by of the two prototypes and by numerical simulations will guide the design of full size adaptive secondary units.

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.

Progress in diffraction-limited imaging at the Multiple Mirror Telescope with adaptive optics

Low spatial frequencies of atmospheric turbulence are especially troublesome to astronomers because the phase distortions that these frequencies cause have a large amplitude. We have begun experiments at the Multiple Mirror Telescope to remove these errors with tip, tilt, and piston control of pieces of the wave front that are defined by the telescope’s six 1.8-m primary mirrors. We present long-exposure images that were recorded at the telescope with a resolution of as high as 0.08 arcsec under piston control, and 0.32 arcsec under tilt control, by use of an adaptive instrument designed to restore diffraction-limited imaging in the near infrared. Of particular importance for high-resolution imaging is the control of the piston or the mean phase errors between the segments. These errors can be calculated from the Fourier transform of the short-exposure combined-focus image, but the accuracy of the reconstructed wave front depends critically on the signal-to-noise ratio of the data. We present a theoretical analysis of the effects of photon and detector read noise on the derived piston errors and computer simulations of wave-front reconstructor algorithms. We find that a Wiener filter combined with nonlinear weighting of the piston errors minimizes the impact of noise. Finally, we summarize expected improvements to our system and discuss the application of these techniques to forthcoming large telescopes.

Adaptive optics spectroscopy: preliminary theoretical results

Diffraction-limited spectroscopy with adaptive optics (AO) has several advantages over traditional seeing-limited spectroscopy. First, high resolution can be achieved without a large loss of light at the entrance slit of the spectrograph. Second, the small AO image width allows the cross-dispersed orders to be spaced closer together on the detector, allowing a large wavelength coverage. Third, AO spectrograph optics are slow and small, costing much less than for a traditional spectrograph. Fourth, small AO images provide high spatial resolution. Fifth, scattered light is less problematic. And last, the small entrance slit of the spectrograph can get rid of much of the sky background to obtain spectra of faint objects. We have done theoretical calculations and simulations for infrared spectroscopy at the MMT 6.5 m with laser guide star AO, which provides almost full sky coverage. The results show we can expect 40-60% of the photons from a unresolved source within 0.2 arcsec diameter circle for J, H, K, L and M bands under typical atmospheric seeing condition at 2.2 micron (ro = 1.0 m, to = 21 ms, ?o = 15 arcsec and d0 = 25 m). Therefore, the spectrograph entrance slit size should match the 0.2 arcsec image to obtain high throughput. Higher resolution can be achieved by narrowing down the slit size to match the diffraction-limited image core size of about 0. 1 arcsec in the infrared. However, the throughput will be correspondingly reduced by a factor of two. Due to the limited atmospheric isoplanatic angle in the J, H and K bands, the encircled photon percentage within 0.2 arcsec diameter drops from 40-60% when the object is at the laser pointing direction to 20-40% when the object is about 30 arcsec away from the laser direction. Therefore, the useful field of view for AO multiple object spectroscopy is about 60 arcsec. Further studies of JR background (sky and thermal) and JR detector performance show that spectral resolution of R = 2,000 can take full advantage of AO images without much penalty due to the dark current of the JR detector and JR OH sky emission lines. We have also studied natural guide star AO spectroscopy. Though sky coverage for this kind of spectroscopy at the MMT 6.5 m is very limited, a bright star provides much better performance than the laser guide star AO spectroscopy. About 40-70% photons are concentrated within 0.1 arcsec diameter for guide stars brighter than 13 magnitude. Therefore, higher resolution and high throughput can be obtained simultaneously, given a bright enough natural guide object. The field-of-view for multiobject spectroscopy using a natural guide star is similar to that for laser guiding.

FASTTRAC II near-IR adaptive optics system for the Multiple Mirror Telescope: description and preliminary results

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 cameras 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.