Diffraction and reflection from the optical edges is the dominant source of stray light from a starshade. However, recent progress in optical edge design has led to much reduced predictions in this source. Secondary sources now also play a role; these sources arise from two or more reflections from the starshade structure. These multiple reflections allow light to reach the telescope from parts of the structure that are shaded from direct sunlight. Here we analyze the secondary sources for the starshade model developed as part of the S5 technology development and show the effects of optical edge mechanical design variants and mitigations.
Stray light analyses have been a serious consideration in the development of optomechanical instrumentation from the time of Galileo. In the late 1960’s and 1970’s, NASA and the US Air Force realized that software modeling was the key to predictive development and analysis of optical systems and funded the first efforts to automate the analysis process with the development of the GUERAP and APART computer software programs.. The development of multi-wavelength scatterometers in that same era made it possible to more fully characterize the scatter properties of paints and other surface treatments.. These two improvements advanced stray light modeling capabilities and a well-developed modeling approach. In the past twenty years demands for modeling complex structures (e.g., individual screws and small vanes edges) coupled with requirements for ever increasing sensitivity have placed a tremendous computational burden on the stray light analyst. In this paper, we discuss the approaches to modeling today’s complex telescope systems that frequently include stray light artifacts traceable to the complete observatory. Scatter modeling and data reduction has become major components of the stray light analysis task and recent work has identified instances where scatter measurements performed by skilled technicians are inaccurate and useless without additional analysis and interpretation. Finally we discuss common metrics for stray light characterization of large systems and how distributed computing, cloud computing, and GPU-enabled software allow the analyst to compute levels and uniformity of stray light in these systems to levels heretofore considered impossible.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe, using the Baryon Acoustic Oscillation technique and the growth of structure using redshift-space distortions (RSD). The spectra of 40 million galaxies over 14000 square degrees will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe modeling and mitigation of stray light within the front end of DESI, consisting of the Mayall telescope and the corrector assembly. This includes the creation of a stray light model, quantitative analysis of the unwanted light at the corrector focal surface, identification of the main scattering sources, and a description of mitigation strategies to remove the sources.
In the field of exoplanet detection and characterization, the use of a starshade, an external occulter in front of a telescope
at large separations, has been identified as one of the highly promising methods to achieve the necessary high contrast
imagery. Control of scattered sunlight from the edges of the starshade into the telescope has been identified as one of the
key technology development areas in order to make the starshade feasible. Modeling of the scattered light has resulted
in very different results so a campaign of experimentation with edge samples was undertaken to attempt to understand
the discrepancies.
Here, we present our results from the measurement of select samples of materials which would be suitable for manufacturing
the starshade edge, and related models. We have focused on coating metallic samples for ease of fabrication:
Titanium, Aluminum, and a Beryllium Copper alloy. Using standard machine shop methods, we fabricated samples
which had sharp edges with radius of curvature (RoC) between 15 and 20 μm. We then had these samples coated by two
suppliers to evaluate how well these coating types would conform to the edge and provide scatter suppression. The
results of scatter measurements of these coated edge samples are presented. These scatter results have been incorporated
into a new geometrical model in FRED which includes the details of the starshade mechanical model. This model
predicts both the magnitude and distribution of the scattered sunlight in the image plane of a nominal telescope. We
present these results, including a first effort at modeling the Solar System at 10 pc as seen by this mission architecture.
A class of optical surfaces called 'Swept Conics' or 'Conical Spheroids' can be used to transform wavefronts from point
sources to diffraction limited line or arc images, and vice versa.
Stray light modeling and analysis play a key role in new technology assessment, system engineering, and the overall
performance assessment of telescope/instrument systems under real use conditions. It also is a key tool in risk reduction
as stray light problems that appear late in the program are usually severe, expensive to fix, and often compromise final
system performance. This paper will review the current stray light software and testing tools of value to the astronomical
community and their capabilities/ limitations for general and specialized telescope systems. We will describe the role of
stray light analysis in end-to-end modeling and integrated modeling for a number of systems we have analyzed and
discuss in detail the stray light modeling and analysis cycle for different types of programs. A key issue is how managers
might deal with the issues revealed by an analysis as well as the risks of an incomplete or improperly-timed analysis.
The importance of stray light analysis for end-to-end performance assessment and whether such an analysis can reduce
life-cycle costs will also be discussed. The paper will use examples from ground and space-based astronomical
telescope/instrument systems.
The Galileoscope student telescope kit was developed by a volunteer team of astronomers, science education experts,
and optical engineers in conjunction with the International Year of Astronomy 2009. This refracting telescope is in
production with over 180,000 units produced and distributed with 25,000 units in production. The telescope was
designed to be able to resolve the rings of Saturn and to be used in urban areas. The telescope system requirements,
performance metrics, and architecture were established after an analysis of current inexpensive telescopes and student
telescope kits. The optical design approaches used in the various prototypes and the optical system engineering tradeoffs
will be described. Risk analysis, risk management, and change management were critical as was cost management
since the final product was to cost around $15 (but had to perform as well as $100 telescopes). In the system engineering
of the Galileoscope a variety of analysis and testing approaches were used, including stray light design and analysis
using the powerful optical analysis program FRED.
The Multi-Object Broadband Imaging Echellette (MOBIE) is the seeing-limited, optical spectrograph planned for the
first generation of Thirty Meter Telescope (TMT) instruments1. An end-to-end stray light analysis of the full optical path
(telescope to detector array) has been undertaken as a first step towards validating the design concept with regard to stray
light requirements. The geometric, stray light model includes the TMT Calotte-style dome structure, telescope optics,
telescope support structures, and the MOBIE instrument itself. The stray light calculations, including assumptions,
methodology, and conclusions, are described. Particular emphasis is placed on the stray light contributions from the
telescope, atmospheric dispersion corrector, and spectrograph optics. Recommendations for stray light controls internal
to the MOBIE instrument are discussed.
The Large Synoptic Survey Telescope (LSST) is a proposed large, ground-based telescope that can survey the entire
visible sky every three nights to construct a detailed map of the universe while searching for faint and moving objects
(www.lsst.org). Stray light control is important for optimum sensitivity over decades of stellar magnitude. A critical /
illuminated object study of the baseline design identified several stray light mechanisms that required unique baffling
approaches. Point source transmittance (PST) calculations over multiple azimuth angles quantify the stray light
background levels and provide an indication of baffle effectiveness. Baffle design trades and their effect on stray light
levels are discussed.
The Photovoltaic Cavity Converter (PVCC) under development is a novel approach to convert highly concentrated solar radiation into electricity via a photon entrapment process and subsequent spectral stripping. Equipped with a multi-bandgap, single junction cell system PVCC circumvents most of the present limitations of the four (or more)-junction cell systems with vertical architecture. Our previous studies have shown that the PVCC concept has the potential to reach a collective conversion efficiency of 50% in the near term. Based on our past experiences regarding the cavity geometry and the light injection method we have developed a second generation design for the PVCC that overcomes the limitations of the first generation prototype.
A uniquely packaged three-mirror anastigmat (TMA) is discussed. The telescope has 5 major parts, including the optics. This compact TMA exhibits diffraction-limited performance in the MWIR and LWIR wavelength regions and operates at fast F-numbers and relatively wide fields of view. Optical surfaces and critical mechanical interfaces are single-point diamond-turned, allowing for drop-together assembly and alignment. This paper discusses the evolution of the optical/mechanical design, developed under the auspices of the Standard Missile Company and NAVSEA PMS422.
KEYWORDS: Sensors, Avalanche photodetectors, Information operations, LIDAR, Receivers, Signal to noise ratio, Telescopes, Signal detection, Analog electronics, Optical design
NASA Langley has an active water vapor differential absorption lidar program taking measurements from both C-130 and ER-2 aircraft. A research effort has started to increase the signal-to-noise ratio in the DIAL receiver by 1) evaluating new very low noise avalanche photo didoes (APD), 2) designing an optics system that will focus the return light signal to the APD efficiently and 3) constructing a 10-MHz waveform digitizer board that will be small enough to be placed at the APD and telescope. With these advances we anticipate improving the signal-to-noise ratio by a factor of ten over the current receiver system.
Richard Couch, Carroll Rowland, K. Ellis, Michael Blythe, Curtis Regan, Michael Koch, Charles Antill, Wayne Kitchen, John Cox, Joseph DeLorme, Sharon Crockett, Rubin Remus, Joseph Casas, William Hunt
KEYWORDS: LIDAR, Electronics, Space telescopes, Telescopes, Sensors, Space operations, Aerospace engineering, Signal processing, Transmitters, Control systems
The Lidar In-Space Technology Experiment (LITE) is being developed by NASA/Langley Research Center for flight on the Space Shuttle. The system will detect stratospheric and tropospheric aerosols, probe the planetary boundary layer, measure cloud top heights, and measure atmospheric temperature and density in the range of 10 to 40 km. The system consists of a nominal 1 m diameter telescope receiver, a three-color neodymium: YAG laser transmitter, and the system electronics. The instrument makes extensive use of Space Shuttle resources for electrical power,
thermal control, and command and data handling. The instrument will fly on the Space Shuttle in mid-1993. This paper presents the engineering aspects of the design, fabrication, integration, and operation of the instrument. A companion paper by members of the LITE Science Steering Group that details the science aspects of LITE is in preparation and will be published at a later time.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.