The Dragonfly directional sensor was deployed at the Army’s Yuma Proving Grounds for preliminary field tests against rocket-propelled grenades. This wide-field (nonimaging) sensor’s purpose was to angularly locate the latter’s launch plume. These tests successfully demonstrated proof-of-concept.
This paper discusses the concept and hardware development of an all fiber-based, solid state, coherent array directional sensor that can locate and track bright objects against a darker background. This sensor is not an imager. It relies on the inherent structure of the global fiber distribution. Methods for characterizing and calibrating hardware embodiments are also presented.
We use the two-dimensional Chebyshev polynomials as the basis for decomposition of test data over rectangular apertures, particularly for anamorphic optics. This includes simple optics such as cylindrical lenses and mirrors as well as complex optics, such as aspheric cylindrical optics. The new basis set is strictly orthogonal over rectangles of arbitrary aspect ratio and they correspond well with the aberrations of systems containing such type of optics. An example is given that applies the new basis set to study the surface figure error of a cylindrical Schmidt corrector plate. It is not only an excellent fitting basis but also can be used to flag misalignment errors that are critical to fabrication.
We report initial results on designing and manufacturing a Schmidt-like corrector plate for a commercial off-the-shelf cylindrical lens, eliminating the cylindrical equivalent of its spherical aberration. The corrector is made by figuring the correction profile onto a precision glass window, which is subsequently aligned to the cylindrical lens. We have successfully fabricated the first plate and applied it in an interferometric test of a near-cylinder optic. The interferometric data from before and after applying the corrector demonstrates that the modified optic produces a cylindrical test wavefront with <1λ P-V of residual error at 632.8 nm, a >25× reduction compared to the uncorrected case.
We present a new approach of measuring the spectral transmission ratio of a lens under test (LUT). Three auxiliary optics are used to perform this test, including one focusing lens, one small prism, and one reflecting mirror, whose transmission or reflection properties need not be known. This method is able to measure the lens transmission ratio over the entire visible spectrum range, and covers a large portion of the lens pupil aperture.
We present a technique to characterize and quantitatively measure the vibrational mode shapes and amplitudes of mirrors concurrently with surface figure testing. The technique utilizes a fast interferometer that does not introduce any mass loading to the test structure. We present the fundamentals of the technique, discuss sevral modes of operation, such as resonant and transient response, and analyze the operational limits. The performance of the measurement system is characterized using a small ambient test mirror.
The successful augmentation of NASA's X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Demonstrator (SBMD) and NGST Mirror Sub-scale Demonstrator (NMSD) programs required significant modifications and enhancements to achieve reliable data. In addition to building and integrating both a helium shroud and a rugged, stable platform to support a wavefront sensor, a custom sensor suite was assembled and integrated to meet the test requirements. The metrology suite consisted of a high-resolution Shack-Hartmann sensor, a point diffraction interferometer, a point spread function camera, and a radius of curvature measuring device.
The evolution from the SBMD and NMSD tests to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD program's more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This paper will discuss the instrumentation changes made for AMSD, including the interferometer selection and null optics. The error budget for the tests will be presented using modeling and experimental data. We will show how the facility is ready to meet the test requirements.
An Optical Testing System (OTS) has been developed to measure the figure and radius of curvature of Next Generation Space Telescope (NGST) developmental mirrors in a vacuum, cryogenic environment using the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The OTS consists of a WaveScope Shack-Hartmann sensor from Adaptive Optics Associates as the main instrument and a Leica Disto Pro distance measurement instrument. Testing is done at the center of curvature of the test mirror and at a wavelength of 632.8 nm. The error in the figure measurement is <EQ(lambda) /13 peak-to-valley (PV). The error in radius of curvature is less than 5 mm. The OTS has been used to test the Subscale Beryllium Mirror Demonstrator (SBMD), a 0.532-m diameter spherical mirror with a radius of curvature of 20 m. SBMD characterization consisted of three separate cryogenic tests at or near 35 K. The first two determined the cryogenic changes in the mirror surface and their repeatability. The last followed cryo-figuring of the mirror. This paper will describe the results of these tests. Figure results will include full aperture results as well as an analysis of the mid-spatial frequency error results. The results indicate that the SBMD performed well in these tests with respect to the requirements of (lambda) /4 PV (full aperture), (lambda) /10 PV (mid-spatial, 1-10 cm), and +/- 0.1 m for radius of curvature after cryo-figuring.
The successful augmentation of NASA's X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Development (SBMD) and NGST Mirror Sub-scale Development (NMSD) programs required significant modifications and enhancements to achieve useful and meaningful data. In addition to building and integrating both a helium shroud and a rugged and stable platform to support a custom sensor suite, the sensor suite was assembled and integrated to meet the performance requirements for the program. The subsequent evolution from NMSD and SBMD testing to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD program's more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This overview paper will discuss the instrumentation changes made for AMSD, including the interferometer selection, null optics, and radius of curvature measurement method. The error budgeting process will be presented, and the overall test plan developed to successfully carry out the tests will be discussed.
An Integrated Product Team was formed to develop a detailed concept for optical test methodology for testing of the NGST individual primary, secondary and tertiary mirrors and the full telescope system on the ground. The large, lightweight, deployable primary mirror, and the cryogenic operating environment make optical testing of NGST OTA (Optical Testing Assembly) extremely challenging. A telescope of the complexity of NGST has never been built and tested on the ground in 1-g environment. A brief summary of the preliminary metrology test plan at the mirror component and telescope system level is presented.
A critical component in the 2-micrometer coherent spaced-based lidar system (SPARCLE) is the compact, off-axis, 25-cm aperture telescope. The stressing optical performance demanded from this telescope coupled with the difficulty associated with aligning such a fast, off-axis system; has created the need for a multiple-axis alignment stage for the secondary mirror. Precision micrometer kinematic mounts were used in the laboratory to demonstrate the ability to successfully align the telescope. For the flight configuration, a more robust and considerably smaller stage (both in size and weight) had to be designed in order to fit within the space shuttle packaging constraints. The new stage operates with multiple degrees of freedom of motion to achieve micrometer precision alignment and then uses a mechanical multiple point support to lock-in the alignment and provide stability. The optomechanical design of the flight stage is described.
This paper presents the test results on a compact, off-axis telescope which is the precursor projector/receiver for a NASA Shuttle-based coherent lidar system operating at a wavelength of 2 microns to measure atmospheric wind profiles. The afocal telescope has an entrance pupil diameter of 25 cm, and an angular magnification of 25x. To determine the transmitted and returned optical wavefront quality, the telescope was tested in a Twyman-Green configuration at the operational wavelength. Interferograms were obtained via an infrared camera, and analyzed using a digitizing tablet and WYKO WISP software. Interferograms were obtained with and without an 11.7 degree wedged silicon window located in the entrance pupil. This window, which rotates orthogonal to the telescope optical axis, serves as the lidar system scanner. The measured wavefront information from the interferometer was used in a GLAD heterodyne receiver model to predict the effect of the optical system on the lidar performance. The experimental setup and procedures will be described, and the measurement results of the coherent lidar optical subsystem will be presented in this paper.
A laboratory prototype of a passive optical lane position monitor has been designed, built, and tested. The sensor head is simple and consists of two parts: a cylindrical lens, and a position sensitive detector. The amplifier/processing electronics which provides the position signal is compact and lightweight. No complex software or computer is needed. Sensor performance was validated both in the laboratory and in the field. The prototype was tested in sunlight over a range of solar angles from dawn to dusk. It was even tested at night with illumination provided by headlights. The bottom line is that, for such a simple system, the sensor worked quite well. This opens up possibilities for its use as a practical tool in vehicle/highway management.
Axial intensity scans provide a noninterferometric method for measuring aberration content of a focused beam. Applications of the technique include the determination of the conic constant of aspheric mirrors (without the need for a null lens).
The point diffraction interferometer (PDI) is modeled using the GLAD diffraction code. Behavior of the interference fringe pattern is examined as a function of F-number, aberration type, and lateral and axial translation of the PDI.
Standard diffractive estimates based on the system f/# were at odds with geometric predictions concerning beam footprint size at a relayed pupil image plane due to an object point at infinity. The reason for this was that the relay optic was well within the defractive depth of focus of an intermediate star image. This invalidated geometric predictions for the system f/# and the location and size of the system impulse response. Accurate predictions emerged when Fourier transforms of the highly apodized planar wavefront incident on the relay optic were calculated.
Several wavefront sensing techniques were tested against a common aberrated beam. The error on the beam was predominantly spherical aberration. The techniques included Shack- Hartmann Test, Lateral Shear interferometry, Point Diffraction Interferometry, and a Knife Edge Test. Beam calibration was accomplished using Fizeau Interferometry.
An f/10.3 lens with 6 waves of spherical aberration was tested on a Fizeau interferometer using a standard ZYGO retro sphere. It was found that this configuration led to retrace error difficulties. A longer radius retro sphere yielded results in much better agreement with theory.
Infrared presensitization photography (IRPP) is used to acquire images of the solar disk in the 1- to 2-μm regime. This is the first time IRPP has been employed at such short IR wavelengths, used against a thermal (instead of a laser) source, and applied to astronomy. The experiments demonstrate the feasibility of IRPP for solar imaging, but the image quality needs improvement for solar research
An investigation was made of five common 35-mm black and white Kodak films showing their relative sensitivity to the IR presensitization photographic effect. The basis of comparison was the density versus IR energy (DIRE) curve. It was found that fine-grain films are more sensitive than coarse-grain films to the presensitization.
Aberrations introduced due to focus and tilt adjustments between the optical fiber cylindrical reference and line focus formed by a cylindrical optic in a Fizeau interferometer arrangement are examined.
Hartmann wavefront sensors rely on an exact correlation between a transverse ray aberration measurement in the focal plane and the sampling hole position in the pupil identifying that ray. lf this relationship breaks down, the reconstructed wavefronts will not correspond to the actual incident wavefronts. This occurs if rays yielding certain transverse errors are misidentified, e.g., if sampling is in a plane not conjugate to the pupil. A case study is made of the SHAPE wavefront sensor in which such ray misidentification can arise.
The principles involved in the IR presensitization photography (IRPP) are discussed, and the feasibility of using IRPP in solar IR astronomy is investigated. Images of the solar disk were acquired in the 1- to 2-micron regime, demonstrating that IRPP is a feasible technique for solar imaging. However, in order to use IRPP images for solar research, the image quality needs improvement in terms of resolution and photometric accuracy.
Performance degradations due to polarization mismatch between subapertures of an optical array (Young system) are explored. Two tools are utilized: The first is a mathematical model for a linear array of N subapertures. The second makes use of a wavefront propagation code. The former aids our understanding of polarization mismatch on the interference
pattern, and the latter ties in the diffraction envelope along with 2-D array behavior.
This is a practical "how to" course in optical testing. Emphasis is on techniques, procedures, and instruments. Mathematics is kept to a minimum. This course provides a basic understanding of the kinds of tools used for measurements and how to perform them.