D'nardo Colucci

Perception in HMDs: what is it in head-mounted displays (HMDs) that really make them all so terrible?

Head mounted displays (HMDs) have disappointed real world users in their inability to live up to over-hyped expectations. This does not, however, mean that HMDs are useless. While still technologically lacking in some areas, appropriately designed HMDs can be extremely useful tools. We will look at the limitations of current HMDs and ways around them. Rather than approach the problem from the optical, electrical and mechanical engineers point of view, we will approach it from the physiology point of view, answering the question; what is needed to create a useful HMD. The paper is divided into two separate sections. The first, is a description of the performance of the human visual system. The second, addresses how designers attempt to mimic the human visual system in an HMD. This second section will discuss applications that need the specific performance described in section one, current solutions to those needs and finally ideal solutions not yet implemented. Finally, a summary of these findings is presented in a table format.

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″

Direct 75 milliarcsecond images from the Multiple Mirror Telescope with adaptive optics

We report results from an adaptive optics system designed to provide imaging at the diffraction limit of resolution in the near-infrared at the Multiple Mirror Telescope (MMT). For the present experiment, the aperture consisted of five of the six primary mirrors of the MMT, operating as a coherently phased array. The largest components of the atmospherically induced wave-front aberration are the fluctuations in mean phase between the segments. These errors were derived in real time from the Fourier transform of short-exposure stellar images at 2.2 microns and corrected at an image of the telescope pupil with piston motion from a segmented adaptive mirror. At a correction rate of 43 Hz, this level of adaptive control resulted in an integrated image with a clear diffraction-limited component of 0.075 arcsec FWHM. This stabilized component is present directly in the light arriving at the detector and is not the result of postprocessing. We discuss future improvements to our adaptive wave-front control and its application to astronomical observations.

Neural network adaptive optics for the Multiple Mirror Telescope

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.

Adaptive optics for array telescopes using piston-and-tilt wave-front sensing.

A near-infrared adaptive optics system operating at approximately 50 Hz has been used to control phase errors adaptively between two mirrors of the Multiple Mirror Telescope by stabilizing the position of the interference fringe in the combined unresolved far-field image. The resultant integrated images have angular resolutions of better than 0.1 arcsec and fringe contrasts of >0.6. Measurements of wave-front tilt have confirmed the wavelength independence of image motion. These results show that interferometric sensing of phase errors, when combined with a system for sensing the wave-front tilt of the individual telescopes, will provide a means of achieving a stable diffraction-limited focus with segmented telescopes or arrays of telescopes.

A REFLECTIVE SHACK-HARTMANN WAVEFRONT SENSOR FOR ADAPTIVE OPTICS

We describe a new wavefront sensor based on the efficient Shack-Hartmann quad cell system. The key improvement to existing designs is a modified lenslet array. The traditional refractive lenslet array is replaced with a segmented reflective system that allows individual control of each subaperture. This system has produced diffraction limited slope correction of the six 1.83 m mirrors of the MMT using a 9.9 V magnitudeguide star and significant image improvement on guide stars as faint as 11.3 V magnitude, limited by the readout noise of the CCD quad cell detector. Recent experiements with an improved detector diffraction limited imaging on guide stars as faint as 15th magnitude. This optimized wavefront sensor, equally applicable to filled aperture telescopes, promises to extend the amount of sky coverage available for adaptive correction in the near IR.

High resolution imaging at the Multiple Mirror Telescope using adaptive optics

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.

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.

Diffraction-limited K-band imaging at the Multiple Mirror Telescope with adaptive optics

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 telescopes 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.

Real-time adaptive wavefront correction by an artificial neural network at the multiple mirror telescope

Summary form only given. The authors reported the first results from an adaptive system operating online at the Multiple Mirror Telescope (MMT), in which the wavefront is sensed by a neural network, implemented on an array of 20 transputers. Star images were formed at a wavelength of 2.2 mu m by two coherently phased apertures of the MMT, and analyzed by the net. Outputs from the net were used to drive piezoelectric actuators on two segments of an adaptive mirror in the optical beam train, controlling five degrees of freedom of the wavefront. With the servo loop in operation, the corrected image shows significant power at the diffraction limit of 0.1 arcseconds (0.5 mu rad) and a much improved peak intensity.<<ETX>>