We are developing a stable and precise spectrograph for the Large Binocular Telescope (LBT) named “iLocater.” The instrument comprises three principal components: a cross-dispersed echelle spectrograph that operates in the YJ-bands (0.97-1.30 μm), a fiber-injection acquisition camera system, and a wavelength calibration unit. iLocater will deliver high spectral resolution (R~150,000-240,000) measurements that permit novel studies of stellar and substellar objects in the solar neighborhood including extrasolar planets. Unlike previous planet-finding instruments, which are seeing-limited, iLocater operates at the diffraction limit and uses single mode fibers to eliminate the effects of modal noise entirely. By receiving starlight from two 8.4m diameter telescopes that each use “extreme” adaptive optics (AO), iLocater shows promise to overcome the limitations that prevent existing instruments from generating sub-meter-per-second radial velocity (RV) precision. Although optimized for the characterization of low-mass planets using the Doppler technique, iLocater will also advance areas of research that involve crowded fields, line-blanketing, and weak absorption lines.
ATLAST is a particular realization of the Large Ultraviolet Optical Infrared telescope (LUVOIR), a ∼10-m diameter space telescope being defined for consideration in the 2020 Decadal Review of astronomy and astrophysics. ATLAST/LUVOIR is generally thought of as an ambient temperature (∼300 K) system, and little consideration has been given to using it at infrared wavelengths longward of ∼2 μm. We assess the scientific and technical benefits of operating such a telescope further into the infrared, with particular emphasis on the study of exoplanets, which is a major science theme for ATLAST/LUVOIR. For the study of exoplanet atmospheres, the capability to work at least out to 5.0 μm is highly desirable. Such an extension of the long wavelength limit of ATLAST would greatly increase its capabilities for studies of exoplanet atmospheres and provide powerful capabilities for the study of a wide range of astrophysical questions. We present a concept for a fiber-fed grating spectrometer, which would enable R=200 spectroscopy on ATLAST with minimal impact on the other focal planet instruments. We conclude that it is technically feasible and highly desirable scientifically to extend the wavelength range of ATLAST to at least 5 μm.
The Star Formation Camera (SFC) is a wide-field (~19'×~15', >280 arcmin2), high-resolution (18 mas pixels) UV/optical
dichroic camera designed for the Theia 4-m space-borne space telescope concept. SFC will deliver diffraction-limited
images at λ > 300 nm in both a blue (190-517nm) and a red (517-1075nm) channel simultaneously. The goal is to
conduct a comprehensive and systematic study of the astrophysical processes and environments relevant for the births
and life cycles of stars and their planetary systems, and to investigate the range of environments, feedback mechanisms,
and other factors that most affect the outcome of star and planet formation.
Techniques for passive remote sensing of aerosol optical and microphysical properties from space include visible, near- and shortwave-infrared imaging (e.g., from MODIS), multiangle intensity imaging (e.g., ATSR-2, AATSR, MISR), near-ultraviolet mapping (e.g., TOMS/OMI), and polarimetry (e.g., POLDER, APS). Each of these methods has unique strengths. In this paper, we present a concept for integrating these approaches into a unified sensor. Design goals include spectral coverage from the near-UV to the shortwave infrared; intensity and polarimetric imaging simultaneously at multiple view angles; global coverage within a few days; kilometer to sub-kilometer spatial resolution; and measurement of the degree of linear polarization (DOLP) for a subset of the spectral complement with an uncertainty of 0.5% or less. This high polarimetric accuracy is the most challenging aspect of the design, and is specified in order to achieve climate-quality uncertainties in optical depth, refractive index, and other microphysical properties. Based upon MISR heritage, a pushbroom multi-camera architecture is envisioned, using separate line arrays to collect imagery within each camera in the different spectral bands and in different polarization orientations. For the polarimetric data, accurate cross-calibration of the individual line arrays is essential. An electro-optic polarization "scrambler", activated periodically during calibration sequences, is proposed as a means of providing this cross-calibration. The enabling component is a rapid retardance modulator. Candidate technologies include liquid crystals, rotating waveplates, and photoelastic modulators (PEMs). The PEM, which uses a piezoelectric transducer to induce rapid time-varying stress birefringence in a glass bar, appears to be the most suitable approach. An alternative measurement approach, also making use of a PEM, involves synchronous demodulation of the oscillating signal to reconstruct the polarization state. The latter method is potentially more accurate, but requires a significantly more complex detector architecture.
Very high contrast imagery, required for exoplanet image acquisition, imposes significantly different criteria upon telescope architecture than do the requirements imposed upon most spaceborne telescopes. For the Eclipse Mission, the fundamental figure-of-merit is a stellar contrast, or brightness reduction ratio, reaching a factor of 10-9 or better at star-planet distances as close as the 4th Airy ring. Factors necessary to achieve such contrast ratios are both irrelevant and largely ignored in contemporary telescope design. Although contemporary telescoeps now meet Hubble Space Telescope performance at substantially lower mass and cost than HST, control of mid-spatial-frequency (MSF) errors, crucial to coronagraphy, has not been emphasized. Accordingly, roughness at MSF has advanced little since HST. Fortunately, HST primary mirror smoothness would nearly satisfy Eclipse requirements, although other aspects of HST are undesirable for stellar coronagraphy. Conversely, the narrow field required for Eclipse eases other drivers of traditional telescope design. A systematic approach to telescope definition, with primary and sub-tier figures-of-merit, will be discussed in the context of the Eclipse Mission.
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.
An 85 cm aperture beryllium mirror was fabricated as part of the IR Telescope Technology Testbed (ITTT), a facility to which the SIRTF flight telescope will be traceable. The ITTT was developed to demonstrate that diffraction-limited performance at a wavelength of 6.5 micrometers is attainable from an ultra-lightweight meter-class beryllium telescope operating at a temperature of 5.5K. Cryo-null figuring was employed to meet the requirements for the shape of the primary mirror at its operating temperature over an aperture of 79cm. The results of this process will be presented, including the repeatability of the surface through cryogenic temperature cycling. Modeling of system performance using the residual figure error will be described. Image-based methods were used to characterize a turned up edge that is too steep to be measured with an interferometer.
This paper describes the process by which Hughes Danbury Optical Systems successfully manufactured a beryllium secondary mirror for the NASA Jet Propulsion Laboratory's IR Telescope Technology Testbed. The secondary was fabricated and tested using conventional methods. Fiducialization was sued to calibrate and remove all systematic errors from the Hindle test. Additionally, surface roughness and scatter were fully characterized.
The Infrared Technology Testbed Telescope (1T1T) is a demonstration telescope meeting the needs of the SIRTF mission. It is a Ritchey-Cretien form designed for diffraction limited performance at 6.5 pm, at 5.5 K with an 85 cm. clear aperture. The mirror and system focal ratios are f/1.2 and f/12 respectively. This paper describes the design and fabrication of the efficient, ultra-lightweight, all-beryllium telescope. The design incorporates a central metering tower and single arch primary mirror to achieve a total telescope mass of less than 30 kg. Cryogenic testing of the primary mirror demonstrates the stability of the I-70-H (special) Be and the fabrication process. No thermal hysteresis was observed after repeated cycling to 5 K, and cryo-null figuring was utilized to overcome the small thermal instability observed at that temperature.
The SIRTF Telescope Test Facility (STTF) consists of an optical dewar for testing mirrors of up to 1m diameter and f < 6 at temperatures from 300K to 5K and a phase shift interferometer for optical characterization. The STTF was brought on-line in early 1995. The STTF was initially used to cool a 50cm diameter beryllium mirror that had been previously tested at NASA Ames Research Center. The initial tests validated the performance of the STTF by proving that the STTF could cool a mirror to 5K and achieve high quality optical data on the mirror, consistent with the previous results achieved at NASA Ames. The STTF has also been used to provide cryogenic optical testing of the ultra- lightweight 85cm diameter beryllium primary mirror assembly for the Infrared Telescope Technology Testbed (ITTT). Currently the facility is preparing for testing the complete ITTT. Also, the long wavelength photon background in the facility will be measured and characterized in 1996.
In this paper we describe the key features of the SIRTF Telescope Test Facility developed at the Jet Propulsion Laboratory. Information on the cryogenic performance including details of the test cycle time and cryogen hold time are included. Emphasis is on the operation of the facility. Data are presented on the cryogenic optical testing of the ultra-lightweight 85 cm diameter beryllium primary mirror assembly for the infrared telescope technology testbed.
OCA Applied Optics has devised and optimized methods for designing, building and testing precision mirror systems whose performance is not compromised by large changes in environmental temperature. A key to our approach is the use of a single material for the construction of all telesope components to minimize the differences in the contraction and expansion of the components with changes in the operating temperature. Specifically, we designed and built a simple, on-axis, monometallic telescope suitable for cryogenic testing in order to investigate and optimize methods for accurately testing the optical performance at cryogenic temperatures. We then designed and built a sophisticated, off-axis monometallic telescope representative of the current technology in advanced spectrometer instruments for deep-space applications. The novel design of this telescope facilitated assembly, alignment, and testing. We characterized the performance of the instrument in both laboratory and cryogenic environmental conditions. The results of these tests show that the instrument focus position and image quality showed negligible change at cryogenic temperatures, compared to a room temperature environment. This research has already contributed to improved performance and reduced cost for advanced reflective optical instruments for several space applications.
The alignment and performance of the optical system for the Pressure Modulator Infrared Radiometer (PMIRR) are described. This limb and nadir scanning instrument will be used for remote sounding of the Martian atmosphere and will be launched on Mars Observer in 1992. The instrument has nine channels distributed over the wavelength range 0.3 to 50 microns and has two pressure modulator cells for water vapor and carbon dioxide.