Matthew Cheselka

STELLAR SCINTILLATIONS AS A REMOTE ATMOSPHERIC WAVE-FRONT SENSOR

Stellar scintillations are considered noise in adaptive-optics sensors and are measured for calibration purposes only. We propose to use scintillations to provide direct instantaneous information about the structure of the atmosphere. As a result it will be possible to increase the field of view provided by adaptive optics. The scintillation pattern is created when stellar light is diffracted by high-altitude turbulence. Alternatively, this pattern can be viewed as a Laplacian of this turbulence and can thus be inverted to estimate it. The measurement is limited by the intensity and the angular size of the reference star, by the height distribution of the atmospheric turbulence, and by the detector resolution and spectral response.

Implementation of the Chicago sum frequency laser at Palomar laser guide star test bed

Work is underway at the University of Chicago and Caltech Optical Observatories to implement a sodium laser guide star adaptive optics system for the 200 inch Hale telescope at Palomar Observatory. The Chicago sum frequency laser (CSFL) consists of two pulsed, diode-pumped, mode-locked Nd:YAG lasers working at 1.064 micron and 1.32 micron wavelengths. Light from the two laser beams is mixed in a non-linear crystal to produce radiation centered at 589 nm with a spectral width of 1.0 GHz (FWHM) to match that of the Sodium-D2 line. Currently the 1.064 micron and 1.32 micron lasers produce 14 watts and 8 watts of TEM-00 power respectively. The laser runs at 500 Hz rep. rate with 10% duty cycle. This pulse format is similar to that of the MIT-Lincoln labs and allows range gating of unwanted Rayleigh scatter down an angle of 60 degrees to zenith angle. The laser system will be kept in the Coude lab and will be projected up to a laser launch telescope (LLT) bore-sited to the Hale telescope. The beam-transfer optics, which conveys the laser beam from the Coude lab to the LLT, consists of motorized mirrors that are controlled in real time using quad-cell positioning systems. This needs to be done to prevent laser beam wander due to deflections of the telescope while tracking. There is a central computer that monitors the laser beam propagation up to the LLT, the interlocks and safety system status, laser status and actively controls the motorized mirrors. We plan to install a wide-field visible camera (for high flying aircraft) and a narrow field of view (FoV) IR camera (for low-flying aircraft) as part of our aircraft avoidance system.

First results of nulling interferometry with the Multiple-Mirror Telescope

We have successfully used nulling interferometry at 10 μm wavelength to interferometrically suppress a stars radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star α Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects.

Simulation of aperture synthesis with the Large Binocular Telescope

The large binocular telescope (LBT) will have two 8.4 m apertures spaced 14.4 m from center to center. Adaptive optics will be used to recover deep, long exposure diffraction-limited images in the infrared. The LBT configuration has a diffraction-limited resolution equivalent to a 22.8 m telescope along the center-to-center baseline. Using simulated LBT images and an iterative blind deconvolution algorithm (IBD--Jefferies and Christou, 1993) a sequence of three exposures, at sufficiently different parallactic angles, allows recovery of imagery nearly equivalent to that of the circumscribing 22.8 m circular aperture. To establish a credibility basis for these simulations we have studied the performance of IBD for image constructions of several examples of atmospherically perturbed and partially corrected stellar and galactic data. IBD is robust against influences of real, non-ideal data obtained from large astronomical telescopes, including partial anisoplanicity and Poisson noise from object, sky, and thermal background. For faint objects, which are sky-background and photon-statistics limited, the use of adaptive optics is presumed in these simulations. IBD removes the dilute aperture point spread function effects in the set of parallactic angle-diverse images linearly combined to produce the circumscribed aperture result. Optimal image combination strategy is considered for multi-aperture imaging array configurations.

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.

Verification of a System to Prevent Aircraft Illumination by Adaptive Optics Laser Beacons

Laser beams directed into the sky from astronomical observatories to generate guide beacons for adaptive optics image-sharpening systems are a potential hazard to aircraft. Detection systems are needed to sense aircraft and interrupt the laser beam to prevent accidental illumination. We describe here a system designed for this task. A computer examines CCD images of the sky over a wide field of view and uses a motion detection algorithm to find tracks made by aircraft lights. The results of tests, conducted with simultaneous radar coverage from Tucson International Airport, are reported. A test with an actual laser beacon at the Multiple Mirror Telescope in which Mars was used as a simulated aircraft is also described.

Atmospheric limitations on speckle astrometry with large telescopes

Traditional differential astrometric techniques are limited in precision by the atmosphere in a way that does not show much improvement with increased telescope aperture. However, greatly improved astrometric precision may be obtainable by exploiting the strong aperture dependence of the spatial correlation between simultaneously recorded specklegrams within the speckle isoplanatic angle. The cross-correlation of two speckle iages of a binary star pair may yield higher astrometric precision in the measurement of the binary separation than centroid differences. The degree of this improvement, however, depends strongly upon the effective thickness of the turbulence in the atmosphere. A 5- minute observation using a large-format, rapid-readout CCD at a 2.3-m telescope has demonstrated 1-milliarcsec precision in the determination of the separation of a 7.3 arcsec binary star pair when processed with speckle techniques.

Physically constrained iterative deconvolution of adaptive optics images

Adaptive Optics produces diffraction-limited images but does not fully compensate for the atmospheric degradation of the incoming signal. Post processing is important to fully restore the image. The results of applying a physically constrained iterative deconvolution algorithm to adaptive optics data are presented here for different types of simulated data with different signal-to-noise ratios.

Infrared adaptive optics system for the 6.5-m MMT: system status and prototype results

The Multiple Mirror Telescope (MMT) is now being replaced by a single 6.5m telescope which will have an integral adaptive optics system optimized for near IR operation. We illustrate key results obtained with low-order adaptive optics at the MMT and report on progress on the major components of the system for the new telescope. We also give a brief update on the status of the telescope itself. First light for the adaptive system is expected in mid 1999.

Point-spread-function calibration of dilute aperture imagery

Astronomical speckle holographic methods are shown to calibrate the image blurring effects of the large fraction of the energy in the side-lobes of the point spread function of a dilute aperture imaging system. This self calibration method works for imagery which contains a local point-like reference within the partially isoplanatic field of view. The reference may be a physical object within the (partially isoplanatic) field of view or it may be reconstructed by iterative deconvolution. Data reductions with an iterative deconvolution algorithm show even more striking performance than speckle holography. Atmospheric modeling was used to simulate multiple observations of the same target object with a 5 m dilute aperture pupil with different point spread functions. The iterative deconvolution algorithm recovers Fourier interpolated results for the equivalent 25 m filled aperture without requiring independent observations of a point-like reference source.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.