Scratch-dig is an acknowledged cosmetic specification that is often misused as a specification to limit
scattered light. In spite of its shortcomings, the stray light analyst is often called upon to write this
specification, or at least approve it. An analytic model that relates a bidirectional reflectance distribution
function (BRDF) to a scratch-dig specification is proposed. Applying scalar diffraction theory and some
idealizations for the shape, orientation, and number of scratches and digs; the magnitude and functional form
of the BRDF is derived. The effective BRDF associated with the scratch dig specification is compared with
the magnitude and functional form of the BRDF from surface roughness and contamination.
Image degradation due to scattered radiation is a serious problem in many short-wavelength (x-ray and EUV) imaging systems. Most currently available image analysis codes require the scattering behavior [data on the bidirectional scattering distribution function (BSDF)] as input in order to calculate the image quality from such systems. Predicting image degradation due to scattering effects is typically quite computation-intensive. If using a conventional optical design and analysis code, each geometrically traced ray spawns hundreds of scattered rays randomly distributed and weighted according to the input BSDF. These scattered rays must then be traced through the system to the focal plane using nonsequential ray-tracing techniques. For multielement imaging systems even the scattered rays spawn more scattered rays at each additional surface encountered in the system. In this paper we describe a generalization of Peterson's analytical treatment of in-field stray light in multielement imaging systems. In particular, we remove the smooth-surface limitation that ignores the scattered-scattered radiation, which can be quite large for EUV wavelengths even for state-of-the-art optical surfaces. Predictions of image degradation for a two-mirror EUV telescope with the generalized Peterson model are then numerically validated with the much more computation-intensive ZEMAX® and ASAP® codes.
Breault Research Organization has designed and built a stray light test station. The station measures the point
source transmission and background thermal irradiance of visible and infrared sensors. Two beam expanders, including
a large 0.89 meter spherical mirror, expand and collimate light from laser sources at 0.658 and 10.6 µm. The large
mirror is mounted on a gimbal to illuminate sensors at off-axis angles from 0° to 10°, and azimuths from 0° to 180°.
Sensors with apertures as large as 0.3 meters can be tested with the existing facility. The large mirror is placed within a
vacuum chamber so cryogenic infrared sensors can be tested in a vacuum environment. A dark cryogenic cold plate can
be translated into the field of view of a sensor to measure its background thermal irradiance.
Ultraviolet (UV) observations of the Earth's upper atmosphere are essential for meeting operational requirements for space weather specification and prediction. Such observations provide valuable information about neutral and ion density variations. Current operational sensors measure the limb profiles by mechanically scanning the field of view across the limb. This mechanical scan mechanism requires significant power and can fail, and the high counting rates during observations near the peak of the limb profile require high-speed detectors to accommodate the counting rates when using the high-sensitivity sensors. This paper describes an instrument that can provide limb observations of the UV airglow by aligning the slit perpendicular to the limb. To measure the limb profile without scanning requires a combination of wide field of view and high spatial resolution that previous instruments have been unable to provide. This approach would require significantly less resources (power, weight, etc.) than current sensors, while providing similar performance. A preliminary scattering analysis of the instrument is also included.
The development of sensors that are compact, lighter weight, and adaptive is critical for the success of future military initiatives. Space-based systems need the flexibility of a wide FOV for surveillance while simultaneously maintaining high-resolution for threat identification and tracking from a single, nonmechanical imaging system. In order to meet these stringent requirements, the military needs revolutionary alternatives to conventional imaging systems.
We will present recent progress in active optical (aka nonmechanical) zoom for space applications. Active optical zoom uses multiple active optics elements to change the magnification of the imaging system. In order to optically vary the magnification of an imaging system, continuous mechanical zoom systems require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of elements. By incorporating active elements into the optical design, we have designed, demonstrated, and patented imaging systems that are capable of variable optical magnification with no macroscopic moving parts.
Light that is scattered from lenses and mirrors in an optical system produces a halo of stray light around bright objects within the field of view. The angular distribution of scattered light from any one component is usually described by the Harvey model. This paper presents analytic expressions for the scattered irradiance at a focal plane from optical components that scatter light in accordance with the Harvey model. It is found that the irradiance is independent of the location of an optical element within the system, provided the element is not located at or near an intermediate image plane. It is also found that the irradiance has little or no dependence on the size of the element.
The Clouds and Earth Radiant Energy System (CERES) instrument was designed to make measurements of solar radiance reflected from the Earth (0.2 to 0.5 microns) and radiance emitted from the Earch (5.0 to 50+ microns) with 1% accuracies. The CERES design evolved from the Earth Radiation Budget Experiment instrument which had similar objectives. The CERES also had a channel to measure radiance in the 8 to 12 micron window emitted by the Earth for studying the effects of water vapor on the Earth's radiation budget. A CERES instrument flew on the Tropical Rainfall Measuring Mission and 2 are operating on the TERRA spacecraft. One instrument will map the geographical distribution of radiation and the other will measure the anisotrophy of the radiance field. Two CERES instruments will also fly on the AQUA spacecraft. The design features of the telescope and the rationales are described. These aspects of the instrument should be understood by users of the data for a number of purposes. Each channel has its separate telescope to gather radiation onto its detector, which is a thermistor-bolometer. There is a total channel which measures radiances over the range 0.2 to 50+ microns. The shortwave (0.2-5.0 micron) and window (8-12 micron) channel each have filters to provide the desired band. The emitted radiation is computed as the total minus the shortwave radiance.
This paper will summarize the stray-light study commissioned by USRA from BRO (Breault Research Organization) to estimate the level of dynamic background that might be observable at SOFIA's focal plane. This dynamic background is due to cavity and aircraft motions with respect to the inertially fixed telescope. BRO used their ASAP program to trace rays emitted from the Earth, aircraft engines, and telescope cavity to the focal plane through reflection and scatter off a number of surfaces (including Level 500 contaminated optics).
Faster, better, and cheaper computers make it seem as if any optical calculation can be performed. However, in most cases brute force stray light calculations are still impossible. This paper discusses why this is so, and why it is unlikely to change in the near future. Standard software techniques for solving this problem are then presented, along with a discussion of how old techniques are used to take advantage of the new features that are available in the latest generation of optical analysis software.
Stray light analysis has been carried out for the main laser section of the National Ignition Facility main laser section using a comprehensive non-sequential ray trace model supplemented with additional ray trace and diffraction propagation modeling. This paper describes the analysis and control methodology, gives examples of ghost paths and required tilted lenses, baffles, absorbers, and beam dumps, and discusses analysis of stray light 'pencil beams' in the system.
The European Space Agency (ESA) X-Ray Mirror Module (XMM) telescope is a set of three multiple shell grazing incidence Wolter Type I telescopes that will study astronomical x-ray sources from Earth orbit. There are a total of 58 nested mirror shells within each telescope. Each shell consists of a paraboloid primary and hyperboloid secondary that together focus x rays that are incident at grazing incidence onto a detector, known as the EPIC. Two of the telescopes also have grazing incidence diffraction gratings that disperse a portion of the focused x rays across a second detector, known as the RFC. The complex geometry of XMM makes stray light design and analysis of this telescope a unique and difficult challenge. The fundamental problem is that the detectors collecting the x-rays are also sensitive to visible and near-infrared radiation from outside sources such as the Sun and the Earth. This paper is an overview of the approach used to perform a stray light analysis of this visible radiation, and a presentation of four of the stray light problems that are unique to XMM and related grazing incidence telescopes. For each problem, a summary of the technique that was used to calculate the magnitude of the stray light is given.
MERIS is a passive optical instrument, that will fly on the first polar orbiting Earth observation mission ENVISAT. The development of this instrument is currently carried by an international team led by AEROSPATIALE. The instrument primary mission goal is to monitor bio-optical ocean parameters on a large scale. Secondary goals of MERIS include atmospheric investigation on cloud and aerosols parameters and on land surface processes. The instrument will acquire 15 spectral images, programmable in width and position with a spectral sampling interval of 1.25 nm within the visible spectral range of 390 nm to 1040 nm. MERIS images will have, a swath width of 1100 km and spatial resolution of 300 m. The high radiometrical (1 to 5%) and spectrometrical performances (1 nm) will be provided by an on board calibration system. This paper describes the straylight design of the instrument and associated performances in order to obtain the high level of radiometric resolution.
The stray light rejection properties of the Lockheed-Stavroudis reflective baffle geometry as implemented in the BeCOAT telescope are described in terms of PST and baffle efficiency. PST is dominated by scatter from diffuse baffle surfaces (aperture stop, etc.) and mirror scatter and reflective baffle edge specular reflections. Manufacturing and alignment tolerances for this system are discussed.
Baffles are placed on the front of telescopes to shade the telescope interior from solar and earth radiation that is outside the instrument's field of view. Benefits and drawbacks of specular baffles, which are being investigated as an alternative to the standard diffuse black baffle, are summarized. An overview of four specular baffles designs, the elliptical baffles, Linlor baffle, bielliptical baffle, and Lockheed-Stavroudis baffle, is given. A stray light analysis of the Lockheed-Stavroudis baffle is reported.
Diffraction-limited beam operation at high output power levels (0.5 W cw and 1.5 W pulsed) has been demonstrated from resonant-optical-waveguide (ROW) array structures. Uniphase mode operation is achieved without the need for active phase control. As a result, a reliable monolithic device capable of watt-range coherent output power is obtained.
Diffraction-limited beam operation at high output power levels (0.36 W cw and 1.5 W pulsed) have been demonstrated from resonant-optical-waveguide array structures. Uniphase mode operation is achieved without the need for active phase control. As a result, a reliable monolithic device capable of watt-range coherent output power is obtained.
Stabilized in-phase mode oscillation is demonstrated from large-aperture 20-element antiguided diode laser arrays. 20-element resonant optical waveguide arrays emit 160 mW total power at 2.8 x I(th) in a diffraction-limited beam and have high spatial coherence across the entire array. CW operation of nonresonant 20-element array structures is demonstrated to high output power levels.
A first-principle method for calculating reflection, transmission, and scattering of light at a boundary between a diode laser and an adjoining waveguide is presented. The electric field on each side of the boundary is expanded in terms of the trapped modes and radiation modes of the laser and waveguide. A matrix equation for the coefficients of each mode is then obtained by matching the fields and their longitudinal derivatives at the boundary. This equation is solved numerically to obtain a reflection and scattering matrix for the fraction of the incident light reflected into each trapped mode of the diode laser, scattered into radiation modes, and transmitted into an adjoining waveguide. Results obtained from this method are compared with calculations using the effective index of refraction technique and an overlap integral analysis to determine when these simpler approaches are valid.