Optical Telescope Assembly Research Articles

Laser scanner module with large sending aperture and inherent high angular position accuracy for three-dimensional light detecting and ranging

One design of the state-of-the-art laser scanner systems in automotive applications is based on oscillating mirror modules. The requirement of a large mirror surface for eye-safe transmission beams and long measurement distances is a major drawback for fast and reproducible scanning. Tolerances of angular positioning, position sensing, and vibrational perturbations limit the position accuracy of such a mirror and, thus, the accuracy of the transmission spot position in the field of view (FoV). Our approach for a scanner module with maximum transmission beam diameter combines a microlens array with an objective lens for generating one optical telescope assembly for each angular scan position exclusively. Aperture stops define the beam positions in the FoV and avoid positioning errors caused by angle deviations of the scanner mirror. This increases the reliability of the angular position accuracy of the scanner module significantly. To minimize the shadings between adjacent scan spots in the target distance, created by beam cutoffs at the aperture stop of the objective lens, an array of optimized microwedge prisms is provided in combination with the microlens array. Therefore, we can increase the throughput of transmission power into the FoV and improve the measurement distance, especially at large scan angles.

Optical design for CETUS: a wide-field 1.5-m aperture UV payload being studied for a NASA probe class mission study

As part of a study funded by NASA headquarters, we are developing a probe-class mission concept called the Cosmic Evolution through UV Spectroscopy (CETUS). CETUS includes a 1.5-m aperture diameter telescope with a large field of view (FOV). CETUS includes three scientific instruments: a far ultraviolet (FUV) and near ultraviolet (NUV) imaging camera (CAM); a NUV multiobject spectrograph (MOS); and a dual-channel point/slit spectrograph (PSS) in the Lyman ultraviolet (LUV), FUV, and NUV spectral regions. The large FOV three-mirror anastigmatic (TMA) optical telescope assembly (OTA) simultaneously feeds the three separate scientific instruments. That is, the instruments view separate portions of the TMA image plane, enabling parallel operation by the three instruments. The field viewed by the MOS, whose design is based on an Offner-type spectrographic configuration to provide wide FOV correction, is actively configured to select and isolate numerous field sources using a next-generation micro-shutter array. The two-channel CAM design is also based on an Offner-like configuration. The PSS performs high spectral resolution spectroscopy on unresolved objects over the NUV region with spectral resolving power, R ∼ 40,000, in an echelle mode. The PSS also performs long-slit imaging spectroscopy at R ∼ 20,000 in the LUV and FUV spectral regions with two aberration-corrected, blazed, holographic gratings used in a Rowland-like configuration. The optical system also includes two fine guidance sensors, and wavefront sensors that sample numerous locations over the full OTA FOV. In-flight wavelength calibration is performed by a wavelength calibration system, and flat-fielding is also performed, both using in-flight calibration sources. We describe the current optical design of CETUS and the major trade studies leading to the design.

EXPERIMENTAL INVESTIGATION OF LABORATORY PROTOTYPE OF SPACECRAFT OPTICAL TELESCOPE ASSEMBLY IN LABORATORY CONDITIONS

In this paper the specifications of precision system of ensuring thermal regime of spacecraft optical telescope assembly designed for Earth remote sensing are considered. The laboratory model of thermal control system is designed for ex- perimental investigations in the vacuum chamber. Experimental investigations of laboratory model of thermal regime under conditions of simulating space enviroment (low space temperature, direct and reflected from the Earth solar flux, Earth heat flux) is carried out. High temperature stability and low power consumption of the system is shown.

Update to single-variable parametric cost models for space telescopes

Parametric cost models are an important tool routinely used to plan missions, compare concepts, and justify technology investments. In 2010, the article, “Single-variable parametric cost models for space telescopes,” was published [H. P. Stahl et al., Opt. Eng. 49(7), 073006 (2010)]. That paper presented new single-variable cost models for space telescope optical telescope assembly. These models were created by applying standard statistical methods to data collected from 30 different space telescope missions. The results were compared with previously published models. A postpublication independent review of that paper’s database identified several inconsistencies. To correct these inconsistencies, a two-year effort was undertaken to reconcile our database with source documents. This paper updates and revises the findings of our 2010 paper. As a result of the review, some telescopes’ data were removed, some were revised, and data for a few new telescopes were added to the database. As a consequence, there have been changes to the 2010 published results. But our two most important findings remain unchanged: aperture diameter is the primary cost driver for large space telescopes, and it costs more per kilogram to build a low-areal-density low-stiffness telescope than a more massive high-stiffness telescope. One significant difference is that we now report telescope cost to vary linearly from 5% to 30% of total mission cost, instead of the previously reported average of 20%. To fully understand the content of this update, the authors recommend that one also read the 2010 paper.

Commentary on multivariable parametric cost model for ground optical telescope assembly

In 2005, Stahl et al. published a multivariable parametric cost model for ground telescopes that included primary mirror diameter, diffraction-limited wavelength, and year of development. The model also included a factor for primary mirror segmentation and/or duplication. While the original multivariable model is still relevant, we better explain the rationale behind the model and present a model framework that may better account for prescription similarities. Also, we correct the single-variable diameter model presented in the 2005 Stahl paper with the addition of a leading multiplier.

High-Contrast Imaging Performance of a Tunable Filter for Space-Based Applications. II. Detection and Characterization Capabilities

ABSTRACTThe scanning capability of a tunable filter represents an attractive option for performing high-contrast observations through spectral differential imaging (SDI), a speckle-attenuation technique widely used by current ground-based, high-contrast imaging instruments. The performance of such a tunable filter is illustrated through the Tunable Filter Imager (TFI), which used to be part of the science instrument complement of the James Webb Space Telescope (JWST). TFI features a low-order Fabry-Perot etalon enabling imaging spectroscopy at an average resolution of 100 in the 1.5 to 5 μm range. TFI also includes a high-contrast imaging mode featuring a Lyot coronagraph aided by SDI. TFI’s on-sky performance is determined by performing an end-to-end Fresnel propagation of the telescope and instrument using the measured wavefront error maps of TFI’s optical elements and the theoretical wavefront error maps of the optical telescope assembly. Using this simulation, we determine that SDI offers an improvement in contrast ranging from a factor of ∼7 to ∼100, depending on the instrument’s configuration. We present the companion detection capability using both the coronagraphic and noncoronagraphic modes of TFI and demonstrate the characterization capability using the HR 8799 and Fomalhaut systems. The performance of roll subtraction is also determined and compared with that of SDI. We also present the SDI capability of the Near-Infrared Imager and Slitless Spectrograph, the science instrument module to replace TFI in the JWST Fine Guidance Sensor.

Single-variable parametric cost models for space telescopes

Parametric cost models are routinely used to plan missions, compare concepts, and justify technology investments. Unfortunately, there is no definitive space telescope cost model. For example, historical cost estimating relationships (CERs) based on primary mirror diameter vary by an order of magnitude. We present new single-variable cost models for space telescope optical telescope assembly (OTA). They are based on data collected from 30 different space telescope missions. Standard statistical methods are used to derive CERs for OTA cost versus aperture diameter and mass. The results are compared with previously published models

The Solar Optical Telescope for the Hinode Mission: An Overview

The Solar Optical Telescope (SOT) aboard the Hinode satellite (formerly called Solar-B) consists of the Optical Telescope Assembly (OTA) and the Focal Plane Package (FPP). The OTA is a 50-cm diffraction-limited Gregorian telescope, and the FPP includes the narrowband filtergraph (NFI) and the broadband filtergraph (BFI), plus the Stokes Spectro-Polarimeter (SP). The SOT provides unprecedented high-resolution photometric and vector magnetic images of the photosphere and chromosphere with a very stable point spread function and is equipped with an image-stabilization system with performance better than 0.01 arcsec rms. Together with the other two instruments on Hinode (the X-Ray Telescope (XRT) and the EUV Imaging Spectrometer (EIS)), the SOT is poised to address many fundamental questions about solar magnetohydrodynamics. This paper provides an overview; the details of the instrument are presented in a series of companion papers.

The Solar Optical Telescope of Solar-B (Hinode): The Optical Telescope Assembly

The Solar Optical Telescope (SOT) aboard the Solar-B satellite (Hinode) is designed to perform high-precision photometric and polarimetric observations of the Sun in visible light spectra (388 – 668 nm) with a spatial resolution of 0.2 – 0.3 arcsec. The SOT consists of two optically separable components: the Optical Telescope Assembly (OTA), consisting of a 50-cm aperture Gregorian with a collimating lens unit and an active tip-tilt mirror, and an accompanying Focal Plane Package (FPP), housing two filtergraphs and a spectro-polarimeter. The optomechanical and optothermal performance of the OTA is crucial to attain unprecedented high-quality solar observations. We describe in detail the instrument design and expected stable diffraction-limited on-orbit performance of the OTA, the largest state-of-the-art solar telescope yet flown in space.

The Optical Telescope Assembly for the Terrestrial Planet Finder Coronagraph: Design and Analysis Results

The Terrestrial Planet Finder Coronagraph (TPFC) is a National Aeronautics and Space Administration (NASA) exploration mission to directly detect and characterize terrestrial exoplanets at visible and near-infrared wavelengths. The TPFC mission is currently in a “pre-formulation” stage where requirements and designs are traded. TPFC must distinguish a planet that is more than 10 orders of magnitude fainter than its parent star at a separation of 62 mas ( nm). Coronagraphic detection requires a large aperture telescope to resolve the exoplanet from its parent star, and great system (wavefront) stability during detection and characterization. This paper discusses the design considerations, trade studies and analysis leading to the current, “reference” design for the TPFC telescope. We present the salient features of the design and the most significant structural, thermal and optical analysis results. We also discuss the planned model validation and performance verification approach.

Multivariable parametric cost model for ground optical telescope assembly

A parametric cost model for ground-based telescopes is developed using multi-variable statistical analysis of both engineering and performance parameters. While diameter continues to be the dominant cost driver, diffraction limited wavelength is found to be a secondary driver. Other parameters such as radius of curvature were examined. The model includes an explicit factor for primary mirror segmentation and/or duplication (i.e. multi-telescope phased-array systems). Additionally, single variable models based on aperture diameter were derived.

Vibro Acoustic Response of the Satellite Telescope (1st Report, Lower Frequency Response Analysis)

The goal of this research is to analyze vibration response when the main mirror in a Satellite Solar-B optical telescope assembly (OTA) is subjected to acoustic load. First, we determined the maximum frequency for the frequency range in this analysis from the results of vibration tests and acoustic tests. Next, we derived a frequency response analysis method using BEM, and calculated the internal sound pressure in the OTA. We then considered this internal sound pressure as the input, conducted a Power Spectrum Density (PSD) analysis using an FEM model, and analyzed the vibration response of the OTA. Results of these studies indicate good correlation between the results from the proposed simulation and the results of the acoustic test.

Vibro Acoustic Response of the Satellite Telescope (2nd Report, The Higher Frequency Response Analysis)

The goal of this research is to analyze vibration response when the main mirror in a Satellite Solar-B optical telescope assembly (OTA) is subjected to an acoustic load. Specifically, in this second report, we focused on high frequency band vibration in which the satellite's local vibration mode is the main vibration. First, we studied methods of analyzing the response of the OTA structure and calculated the response using a Statistical Energy Analysis (SEA) method. Next, we measured the response multiple from the OTA structure to the main mirror and multiplied this by the previous response analysis results of the OTA structure to calculate the response of the main mirror. Regarding response RMS, the calculation results and the acoustic test results matched with an error of less than 3 dB. During these studies, we also confirmed significant vibration attenuation effects in such structures as the shield tubes that we designed.

Optical design procedure for duplicating wavefront errors of an optical instrument

A new optical design procedure for designing an optical device that will duplicate the wavefront errors of another optical instrument over the full field of view is described in this paper. We call this optical design procedure the inside-out method. It was devised to design an optical instrument that generated wavefronts simulating those of the Hubble Optical Telescope assembly (OTA). As is now common knowledge, during fabrication the OTA primary mirror was made using a defective null-test device; therefore the oTa wavefronts showed perceptible spherical aberration. With the inside-out method it was possible to design an instrument called Red-2 Stimulus that matched the OTA aberrated wavefronts to less than 0.0005? (638 nm) peak to valley on axis and 0.006 X maximum over the full field of view. The inside-out method is described, and two illustrative examples are given.

Complete Atlases of Recalibrated Archival HST FOS Spectra of Quasars and AGN

The ionization structures of AGNs are determined by the specific ionization mechanisms and the shapes of the ionizing continua. The technique of using emission-line intensity ratios to classify objects and investigate their ionization mechanisms is enhanced significantly by combining high-quality UV and optical spectrophotometry. The HST archive is an excellent source of such data.The FOS spectra are recalibrated using the latest ‘average inverse sensitivity’ (AIS) calibration. This new method for flux calibrating FOS data (1) normalizes count data from all apertures to the 4”.3 aperture to account for changes of aperture throughput as a function of the Optical Telescope Assembly focus; (2) corrects the data for time-dependent detector sensitivity degradation; and (3) scales the data to the white-dwarf reference scale. The AIS calibration method has been developed over a period of several years, with improvements applied progressively based on observed deficiencies and/or discrepancies of the existing calibrations. Indeed, the final AIS calibration incorporates corrections in the wavelength overlap regions of adjacent gratings derived based on inconsistencies discovered as a result of this study of over 1000 FOS spectra.

Optical design of zero-power Hubble Space Telescope wave-front correctors for null testing

The optical design of the second-generation wide-field/planetary-camera instrument for the Hubble Space Telescope has been modified to compensate for the spherical aberration of the optical telescope assembly (OTA) by introduction of undercorrected spherical aberration into the wave front. This instrument can be tested in a simple manner to ensure that its aberration contribution has the proper sign and magnitude. We present designs for a near-zero power doublet lens that can be used to generate a spherically aberrated wave front that is similar to the OTA wave front. When this lens is used in combination with the instrument, a near-perfect or nulled wave front should be produced, resulting in a high-quality point image on axis. We also present lens designs for a similar test that can be performed on the OTA simulators now being built to verify the other second-generation instruments.

Astrometry with the HST Fine Guidance Sensors

This paper presents an overview of the current status of astrometry with the Fine Guidance Sensors (FGSs) on the Hubble Space Telescope. The FGSs have two astrometric modes of operation, one for positional (POS) and one for transfer function (TRANS) astrometry. The positional mode is intended for parallax and angular velocity work. Owing to optical aberrations in the Optical Telescope Assembly (OTA) (i.e., the main telescope optics) beyond the spherical aberration, and instabilities within the combined OTA/FGS optical system, it is undergoing additional feasibility review. Preliminary engineering tests showed changes in stellar pair positions of ∼ 100 milli-arc seconds which are far too large to allow for the expected ∼ 1 mas calibration. Alternate calibration procedures and data reduction algorithms are currently being developed to maximize the scientific potential of this mode which can be fulfilled at the ∼ 3 mas level.The other operational mode-primarily intended for work on double stars but also usable for angular diameter determination-has proven more stable. Even the deleterious effects of the thermally induced solar array jitter have mostly been overcome. A large collection of reduction and analysis software is available to support this measurement technique (Lattanzi, Bucciarelli, Holfeltz, and Taff 1992) and its calibration is proceeding apace. Interesting scientific work from the FGSs on bright Hyades binaries and the highly eccentric double star ADS 11300 has already been published (Franz et al. 1991, 1992).Finally, a new FGS Instrument Handbook (Taff 1992a) has been prepared which should minimize the complications of dealing with the General Observer Proposal Forms.

The astrometric calibration of the Hubble Space Telescope Fine Guidance Sensors

Three main calibrations required for the Optical Telescope Assembly and a Fine Guidance Sensor are discussed, and attention is given to the constraints of in-flight metrology, the complexity of the optical components, the demands of a milliarcsecond precision, and the absence of ideal measurement standards. The three Fine Guidance Systems, each composed of an optical system, detector system, and control computer are described. Astrometric modes are covered, with focus placed on positional astrometry, angular diameter and color index measurements, and double star astrometry. Inflight calibration with the help of either a minor planet or artificial satellite is considered in the framework of the plate scale calibration. As far as the optical field angle distortion calibration is concerned, it is pointed out that with a milliarcsecond level contour map of the distortions, high-precision relative positional astrometry can be carried out successfully.

Evaluating Clean Room Products for Aerospace Applications

Large aerospace assemblies, such as the Optical Telescope Assembly (OTA) for the Hubble Space Telescope built by Perkin-Elmer, present a unique challenge to anyone evaluating clean room products for use on these programs. Very stringent requirements for molecular and particulate contamination control are often not taken into account by the manufacturers of clean room products. The realization that extractables of plastic products pose a serious contamination threat to optical assemblies is gaining wider recognition, but even so-called "solvent compatible" products may not meet the requirements of all sensitive payloads. A process is described for evaluating, selecting and monitoring products in the most widespread use—garments, gloves, and wipers. Notwithstanding well-trained, conscientious personnel, items such as garments, gloves and wipers can be used in such a variety of ways that their tolerance for misuse without untoward consequences must be considered. The final selection process must therefore consider both compatibility and durability and, as was discovered, a unique choice is not always possible for all applications.

Status of the Hubble Space Telescope observatory

The Hubble Space Telescope activity by NASA and ESA began in 1971 and is now nearing completion. The Scientific Instruments are complete and their performance confirmed. The Optical Telescope Assembly integration is proceeding well, with the crucial Fine Guidance Sensors expected to perform as designed. Telescope delivery to the spacecraft contractor is expected in December 1984. Completion of the Verification and Assembly Program will then be followed by launch in the summer of 1986.