Star image appearance in large ground-based telescopes is determined by the properties of the Optical Path Difference (OPD) fluctuation associated with the image-forming wave potions collected by the telescope aperture. The principal properties are the root mean square (rms) OPD fluctuation and the autocorrelation function of the OPD fluctuation. The OPD properties ultimately depend on the combined effects of turbulence in the atmospheric path, the fixed aberrations of the telescope and, if appropriate, the corrective effects of Adaptive Optics (AO). The equations given in this paper relating star image properties to the OPD properties (and also the inverse relations) apply to all large ground-based reflector telescopes, including ELTs. They apply equally to telescopes with and without AO. The OPD properties can be obtained directly from an image of an unresolved star. This image represents the intensity Point Spread Function (PSF) corresponding to the entire end-to-end imaging path. To obtain the full OPD information compliment, however, the image must be formed at a wavelength that delivers the most general type of star image: a core and halo image. Once the OPD properties have been obtained from such an image, the intensity PSF for the telescope/atmosphere/AO combination can immediately be calculated for any other wavelengths of interest in the extended optical wavelength range, 0.3 μm – 1000 μm. There are numerous applications for the mathematical relationships set out in this paper, including characterization of atmospheric paths, assessment of telescope/AO imaging performance, establishing wave front tolerances for ELTs and other large ground-based telescopes, and the rapid identification of sweetspot wavelength regions where highest resolution is achieved and star images attain maximum central intensity.
An interferometric null Test Method is described for testing convex aspheric surfaces, such as found in secondary
mirrors of Cassegrain telescopes or variations thereof such as Mersenne and Ritchey-Chrétien. A family of test designs
is described covering a wide range of mirror diameters, radii of curvature, and aspheric shapes as described by conic
constants and/or polynomials. The Test Method has been used successfully for testing the convex hyperboloid surface
of the 244-mm diameter secondary mirror of the NASA 3-meter IRTF telescope. The Test Method is currently being
used to test the 120-mm diameter, convex paraboloid secondary mirrors of the Magdalena Ridge Observatory
Interferometer (MROI). Test designs exist on paper for both Keck secondary mirrors (0.53-m and 1.4-m diameter),
the HST secondary (0.3-meter diameter), and secondary mirrors of some of the extremely large telescopes of the
future, such as the TMT secondary (3.2-m diameter). In typical test embodiments, the simplicity of the Test enables
rapid implementation at a fraction of the cost of comparable Hindle-Sphere or Hindle-Simpson tests.
Fundamental measurement procedures are described that enable full end-to-end characterization of the entire imaging path between astronomical objects and their telescope images. The procedures are based on measurements of certain key properties of point-object (unresolved star) images, properties that carry the essential information about the integrated effects of all mechanisms in the imaging path that contribute to the OPD fluctuations. These mechanisms can include, but are not limited to, atmospheric turbulence, dome turbulence, telescope aberrations, the effect of wind-induced oscillations on large multiple-segment primary mirrors, and the (corrective) effects of Adaptive Optics (AO). The measurement procedures are fully general and take into account amplitude scintillation as well as phase variation in the wavefronts.
The effects of atmospheric turbulence are fully encrypted in the statistical properties of the OPD fluctuations in the telescope pupil. In turn, these OPD properties are fully encrypted in the measured point-object image properties. Consequently, there is no need to make a priori assumptions about the form of the turbulence structure function, Kolmogorov or otherwise; the measurement procedures naturally provide the appropriate structure function. Whereas telescope aberrations usually contribute fixed amounts to the total OPD fluctuation, other mechanisms, such as atmospheric turbulence, contribute time-varying amounts. The measurement procedures allow the fixed OPD contributions to be separated from the time varying contributions. If AO is used, it is appropriate to have the AO system running during the measurement procedures, which otherwise remain unchanged. The procedures described are fundamental to establishing appropriate end-to-end wavefront error budgets for ground-based telescopes and to establishing telescope resolution expectations as a function of wavelength.
Atmospheric turbulence is described in terms of a general formulation that does not assume any particular form for the turbulence structure function. The formulation allows for the possibility of Kolmogorov turbulence without bestowing any special favor to this type of turbulence. Expressions are obtained for the two-point correlation function of the complex amplitudes (i.e., Atmospheric MTF) and for the more general function, the two-point two-wavelength correlation function of the complex amplitudes. A cardinal set of measurement procedures naturally derives from these two functions that enables both the integrated strength and the average structure function of the turbulence in a propagation path to be characterized. Kolmogorov and non-Kolmogorov types of turbulence structure are measured impartially by these procedures. The measurement procedures are based on certain key properties of point-object images, properties that carry the essential information about the integrated effects of all mechanisms in the propagation path that affect the wavefronts. These mechanisms can include, but are not limited to, atmospheric turbulence, boundary layer turbulence, telescope aberrations, and the (corrective) effects of adaptive optics. The measurement procedures enable full end-to-end characterization of the entire propagation path between object and image. They take account of amplitude scintillation as well as phase variation in the wavefronts. Once the entire path has been characterized, certain wavelengths can be identified that lead to optimum image resolution. For HEL systems, optimum wavelengths lead to maximum irradiance at the target and maximum target lethality range. Large performance improvements are attained by use of optimum rather than non-optimum wavelengths.
KEYWORDS: Acquisition tracking and pointing, Missiles, Line of sight stabilization, Mirrors, Laser marking, Sensors, Control systems, Fiber optic illuminators, Cameras, Software development
The High Altitude Balloon Experiment (HABE) is being developed by the U.S. Air Force Research Laboratory, Space Vehicle Directorate at Kirtland Air Force Base, to investigate technologies needed to perform acquisition, tracking, and pointing (ATP) functions against boosting missiles in near-space environments. HABE is designed to demonstrate ATP sequence steps that start with acquisition of a missile plume, transition through passive IR tracking of the plume, and handover to precision tracking, which employs an active laser illuminator and imaging camera to image and track the missile nose. The Inertial Pseudo Star Reference Unit provides inertially stabilized line-of-sights (LOSs) for the illuminator laser, active fine track camera, and the marker scoring. The latter serves to measure and score the payload's pointing performance. The payload will be operated and carried aloft under a large, scientific balloon. The engagement parameters and timelines for the HABE ATP payload are consistent with scenarios encountered in space-based missile defense applications. In HABE experiments, target missiles will pass at ranges from 50 to 200 km. The performance goals of the ATP payload's LOS stabilization and marker laser pointing are required to exceed 1 microradian RMS or better in jitter, drift, and accuracy (two-axis, one sigma metrics), a requirement which stresses testing capabilities.
Wavefront sensors have been used to make measurements in fluid-dynamics and for closed loop control of adaptive optics. In most common Shack-Hartmann wavefront sensors, the light is broken up into series of rectangular or hexagonal apertures that divide the light into a series of focal spots. The position of these focal spots is used to determine the wavefront slopes over each subaperture. Using binary optics technology, we have developed a hierarchical or fractal wavefront sensor that divides the subapertures up on a more optimal fashion. We have demonstrated this concept for up to four tiers and developed the wavefront reconstruction methods for both segmented adaptive optics and continuous wavefront measurement.
Geosynchronous satellites use solar arrays as their primary source of electrical power. During earth eclipse, which occurs 90 times each year, the satellites are powered by batteries, but the heavy charge-discharge cycle decreases their life expectancy. By beaming laser power to satellites during the eclipses, satellite life expectancy can be significantly increased. In this paper, we investigate the basic system parameters and trade-offs of using reactor pumped laser technology to beam power from the Nevada Test Site. A first order argument is used to develop a consistent set of requirements for such a system.
Optical measurement techniques are extremely useful in fluid mechanics because of their non- invasive nature. However, it is often difficult to separate measurement effects due to pressure, temperature and density in real flows. Using a variation of a Shack-Hartmann wavefront sensor, we have made wavefront measurements that have extremely large dynamic range coupled with excellent sensitivity at high temporal and spatial resolution. These wavefront variations can be directly related to density perturbations in the fluid. We have examined several classes of flow including volumetrically heated gas, grid turbulence and droplet evaporation.
A prediction is made that, for small values of turbulence (L sub 0) in the range of 20-40 cm, ground-based laser beams at near infrared can be focused to diffraction limited accuracy on targets in space without using adaptive optics or other wavefront correction procedures. It is found that, at infrared wavelengths, resolution is more likely to be limited by the telescope size and aberrations than by atmospheric turbulence. By building large enough telescopes to diffraction limited standards rather than currently accepted standards which fall well short of this limit, there is almost no limit to achievable resolution.
It is generally thought that the resolution of large ground-based telescopes is limited by atmospheric turbulence rather than by diffraction from the telescope aperture. However, longer wavelengths are less affected by atmospheric turbulence than shorter wavelengths and, conversely, longer wavelengths are more affected by diffraction from the telescope aperture. An optimum wavelength exists where these two counteracting effects balance. At this wavelength, maximum (diffraction-limited) resolution is obtained. In night seeing conditions at typical telescope sites, the optimum wavelength is in the range 1-2.5 microns. For a 5-m telescope, it should be possible to obtain resolution of the order 0.05-0.15 arcsec routinely at these wavelengths. However, to facilitate such precise resolution the telescope must be diffraction-limited.
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