Search

The Venusian atmosphere creates inhospitable temperature and pressure conditions for the surface of Venus, Earth's twin planet, making in-situ measurements of any appreciable length difficult, expensive, and risky to obtain. Yet, because of the apparent youthfulness of Venus' surface features, long-duration seismic observations are in high demand in order to determine and understand the dynamic processes taking place in lieu of plate tectonics. The Venus Airglow Measurements and Orbiter for Seismicity (VAMOS) mission concept would make use of the dense Venusian atmosphere as a medium to conduct seismic vibrations from the surface to the ionosphere. Here, the resulting atmospheric gravity waves and acoustic waves can be observed in the form of perturbations in airglow emissions, the basic principles for which have been demonstrated at Earth following a tsunami and at Venus with the European Venus Express's Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument. In addition, these observations would enable VAMOS to determine the crustal structure and ionospheric variability of Venus without approaching the surface or atmosphere themselves. Equipped with an instrument of modest size and mass, the baseline VAMOS spacecraft is designed to fit within a SmallSat form factor and travel to Venus predominantly under its own power. VAMOS would enter into an orbit uniquely suited for the long-duration, full-disk staring observations required for seismic readings. VAMOS' journey would be enabled by modern solar electric propulsion technology and SmallSat avionics, which allow the spacecraft to reach Venus and autonomously filter observation data on board to detect Venus-quake events. Currently, trade studies are being conducted to determine mission architecture robustness to launch and rideshare opportunities. Key spacecraft challenges for VAMOS, just as with many SmallSat-based mission concepts, include thermal and power management, onboard processing capabilities, telecommunications throughput, and propulsion technology. The VAMOS mission concept is being studied at JPL as part of the NASA Planetary Science Deep Space SmallSat Studies (PSDS3) program, which will not only produce a viable and exciting mission concept for a Venus SmallSat, but will have the opportunity to examine many issues facing the development of SmallSats for planetary exploration. These include SmallSat solar electric propulsion, autonomy, telecommunications, and resource management that can be applied to various inner solar system mission architectures.

The author has open-sourced a program for optical modeling of astronomical instrumentation. The code allows for optical systems to be described in a programming language. An optical prescription may contain coordinate systems and transformations, arbitrary polynomial aspheric surfaces and complex volumes. Rather than using a plethora of rays to evaluate performance, all the derivatives along a ray are computed by automatic differentiation. By adaptively controlling the patches around each ray, the system can be modeled to a guaranteed known precision. The code currently consists of less than 10,000 lines of C++/stdlib code.

The Thirty Meter Telescope (TMT) is unbaffled and has stability requirements tighter than the previous generation of 10- m class telescopes, leading to tougher requirements on atmospheric dispersion correctors (ADCs). Since instruments are internally baffled, ADCs may no longer shift the position of the telescope exit pupil. Designs that control pupil position are explored.

The Inamori-Magellan Areal Camera and Spectrograph (IMACS) is a wide-field, multipurpose imaging spectrograph on the Magellan-Baade telescope at Las Campanas Observatory. IMACS has two channels—f/2 and f/4, each with an 8K × 8K pixel mosaic of CCD detectors, that service the widest range of capabilities of any major spectrograph. These include wide-field imaging at two scales, 0.20'' pixel-1 and 0.11'' pixel-1, single-object and multislit spectroscopy, integral-field spectroscopy with two 5'' × 7'' areas sampled at 0.20'' pixel-1 (Durham IFU), a multiobject echelle (MOE) capable of N ~ 10 simultaneous full-wavelength R ≈ 20,000 spectra, the Maryland-Magellan Tunable Filter (MMTF), and an image-slicing reformatter for dense-pack multislit work (GISMO). Spectral resolutions of 8 < R < 5000 are available through a combination of prisms, grisms, and gratings, and most modes are instantly available in any given IMACS configuration. IMACS has a spectroscopic efficiency over 50% in f/2 multislit mode (instrument only) and, by the AΩ figure of merit (telescope primary surface area times instrument field of view ), IMACS scores 5.7 m2 deg2, compared with 3.1 for VIMOS on VLT3 and with 2.0 for DEIMOS on Keck2. IMACS is the most versatile, and—for wide-field optical spectroscopy—the most powerful spectrograph on the planet.

NASA's Orbiting Carbon Observatory (OCO) was designed to make measurements of carbon dioxide concentrations from space with the precision and accuracy required to identify sources and sinks on regions scales (~1,000 km). Unfortunately, OCO was lost due to a failure of the launch vehicle. Since then, work has started on OCO-2, planned for launch in early 2013. This paper will document the OCO instrument performance and discuss the changes planned for the OCO-2 instrument.

Final assembly and integration of the Orbiting Carbon Observatory instrument at the Jet Propulsion Laboratory in Pasadena, California is now complete. The instrument was shipped to Orbital Sciences Corporation in March of this year for integration with the spacecraft. This observatory will measure carbon dioxide and molecular oxygen absorption to retrieve the total column carbon dioxide from a low Earth orbit. An overview of the design-driving science requirements is presented. This paper then reviews some of the key challenges encountered in the development of the sensor. Diffraction grating technology, lens assembly performance assessment, optical bench design for manufacture, optical alignment and other issues specific to scene-coupled high-resolution grating spectrometers for this difficult science retrieval are discussed.

The Wide Field-of-view Imaging Spectrometer (WFIS), a high-performance pushbroom hyperspectral imager designed for atmospheric chemistry and aerosols measurement from an aircraft or satellite, underwent initial field testing in 2004. The results of initial field tests demonstrate the all-reflective instrument's imaging performance and the capabilities of data processing algorithms to render hyperspectral image cubes from the field scans. Further processing results in spectral and photographic imagery suitable for identification, analysis, and discrimination of subjects in the images. The field tests also reveal that the WFIS instrument is suited for other applications, including in situ imaging and geological remote sensing.

The Orbiting Carbon Observatory (OCO) will measure the distribution of total column carbon dioxide in the Earth's atmosphere from an Earth-orbiting satellite. Three high-resolution grating spectrometers measure two CO2 bands centered at 1.61 and 2.06 μm and the oxygen A-band centered at 0.76 μm in the near infrared region of the spectrum. This paper presents the optical design and highlights the critical optical requirements flowed down from the scientific requirements. These requirements necessitate a focal ratio of f/1.9, a spectral resolution of 20,000, and precedence-setting requirements for polarization stability and the instrument line shape function. The solution encompasses three grating spectrometers that are patterned after a simple refractive spectrometer approach consisting of an entrance slit, a two-element collimator, a planar reflection grating, and a two-element camera lens. Each spectrometer shares a common field of view through a single all-reflective telescope. The light is then re-collimated and passed through a relay system, separating the three bands before re-imaging the scene onto each of the spectrometer entrance slits using an all-reflective inverse Newtonian re-imager.

The optical performance of a large, optically fast, all-refracting spectrograph camera is extremely sensitive to potential temperature changes which might occur during an extended signle observation, over the duration of an observing run, and/or on seasonal time scales. A small temprature change, even at the level of a few degrees C, will lead to changs in the rerfractive indices of the glasses and the coupling medium, changes in the lens-element geometries and in the dimensions of the lens cell. These effects combine in a design-specific manner to cause potential changes of focus and magnification within the camera as well as inherent loss of image quality. We have used an optical design technique originally developed for the Smithsonian Astrophysical Observatory's BINOSPEC instrument in order to produce a construction optical design for the Carnegie IMACS Short camera. This design combines the above-mentioned temperature-dependent parameter variations in such a way that their net effect upon focus and magnification is passively reduced to negligible residuals, without the use of high-expansion plastics, "negative-c.t.e." mechanisms or active control within the lens cell. Simultaneously, the design is optimized for best inherent image quality at any temperature within the designated operating range. The optically-athermalized IMACS Short camera is under construction. We present its quantitative optical design together with our assessment of its expected performance over a (T = -4.0 to +20.0) C temperature range.

By adding a prism-cross-dispersed echellette grating as an optional module to the Inamori Magellan Areal Camera and Spectrograph (IMACS), complete spectra from 3400 to 11000a of 15 simultaneous objects may be achieved with a resolution of R = 21,000 for a projected 0.5-arcsec slit width and a 5.0-arcsec slit length. The additional cost of this module is on the order of $50,000. This echellette module (IMACS-E) is intended for studies of stellar abundances where the targets are sufficiently dense over the 15 arcmin IMACS field of view to take advantage of the multi-slit capability. Such applications include the study of Galactic bulge stars, stars in local group galaxies, stars in Galactic globular and open clusters, and the integrated light of extra-galactic globular cluster systems.

The Inamori-Magellan Areal Camera and Spectrograph is nearing completion. This reimaging spectrograph will have fields of view of 15 arcmin and 27 arcmin in its relecting grating and grism spectrographic modes, respectively, the largest such areas available on one of the new generation of large optical-IR ground-based telescopes. In addition to wide field imaging and a range of low- to medium-resolution spectroscopic modes, IMACS will have a 2 × 1000 fiber-fed integral field unit built by Durham University, an ecellette mode, and the potential for a full-field tunable filter. We review some of the planned science programs for IMACS, ranging from spectroscopy of stars in the Galactic halo and nearby dwarf spheroidal galaxies, the search for stars between galaxies, internal kinematics in normal galaxies and AGN, and the evolution of high redshift galaxies and galaxy clusters.

All Cassegrain spectrographs suffer from gravitationally- induced flexure to some degree. While such flexure can be minimized via careful attention to mechanical design and fabrication, further performance improvements can be achieved if the spectrograph has been designed to minimize hysteresis and has active compensation for any residual flexure. The Echellette Spectrograph and Imager (ESI), built at UCO/Lick Observatory for use at Cassegrain focus on Keck II, compensates for such residual flexure via its collimator mirror. The collimator is driven by three actuators that provide control of collimator focus, tip, and tilt. The ESI control software utilizes a mathematical model of gravitationally-induced flexure to periodically compute and apply flexure corrections by commanding the corresponding tip and tilt motions to the collimator. In addition, the ESI control software provides an optional, manual, closed-loop method for adjusting the collimator position to compensate for any non-repeatable errors. Such errors may result from mechanical hysteresis or from thermally-induced structural deformations of the instrument and are thus not accounted for by the gravitational flexure model. This method relies on measuring the centroid position of fiducial spots within each echellete image. The collimator is adjusted so that the positions of these spots match those in a reference image. These spots are produced by a small round hole in the slit mask located near one end of the slit. We discuss the design and calibration of this flexure compensation system and report on its performance ont he telescope.

The Echelle Spectrograph and Imager (ESI) is a multipurpose instrument which has been delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. ESI saw first light on August 29, 1999. The optical performance of the instrument has been measured using artificial calibration sources and starlight. Measurements of the average image FWHM in echelle mode are 22 microns (0.22 arcseconds), 16 to 18 microns (0.16 to 0.18 arcseconds) in broad band imaging mode, and comparable in the low-dispersion prismatic mode (LDP). Images on the sky, under best seeing conditions show FWHM sizes of 34 microns (0.34 arcseconds). Maximum efficiencies are measured to be 30% for echelle and anticipated to be greater than 38% for low dispersion prismatic mode including atmospheric, telescope and detector losses. In this paper we describe the instrument and its specifications. We discuss the testing that led to the above conclusions., Comment: 9 pages, 5 figures, replaced figures with gifs

The Echellette Spectrograph and Imager (ESI), currently being delivered for use at the Cassegrain focus of the Keck II telescope employs an all-spherical, 308 mm focal length f/1.07 Epps camera. The camera consists of 10 lens elements in 5 groups: an oil-coupled doublet; a singlet, an oil- coupled triplet; a grease-coupled triplet; and a field flattener, which also serves as the vacuum-dewar window. A sensitivity analysis suggested that mechanical manufacturing tolerances of order +/- 25 microns were appropriate. In this paper we discuss the sensitivity analysis, the assembly and the testing of this camera.

Once the optical design for a spectrograph is finalized, a number of tasks remain for the optical designer which largely simplify the engineering, fabrication, and assembly of the instrument. Such tasks include sensitivity analysis for alignment tolerances, flexure tolerances and flexure compensation, distributions of radiation incidence angles for coating design, and thermal analysis for thermal compensation. For the Keck ESI instrument, the entire spectrograph and guiding system optical designs were directly translated into a 3-dimensional AutoCADr file, complete with clear apertures, actual traced rays, beam envelopes, stray light, and footprints of the beam paths at the optical surfaces. The mechanical engineers could then design the spectrograph structure in 3-dimensions around the existing optical layout.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The ESI (echellette spectrograph and imager) is a multi-mode Cassegrain spectrograph currently funded and under construction at UCO/Lick Observatory for the Keck II telescope. The ESI instrument has three modes. The 170.0-mm collimated beam can be sent directly into the camera for imaging, through a prism disperser, or to an echellette grating with prism cross-dispersion. An all-refracting Epps camera and a single 2 K by 4 K detector are used for all three modes. The direct-imaging mode has a 2.0 multiplied by 8.0- arcmin field of view with 0.15-arcsec pixels. Filters may be placed either near the focal surface of the telescope or in the parallel beam, and the option of a future upgrade including a Fabry-Perot at the pupil image is available. The low-dispersion prism-only mode has a dispersion of 50 to 300 km/sec/pix, depending on wavelength, and this mode can be used with a 8.0-arcmin long slit or in a multi-slit mode with user- made slit-masks. The high-dispersion echellette mode gives the entire spectrum from 0.39 to 1.09 microns with a 20.0-arcsec slit length in a single exposure, with a dispersion of 9.6 to 12.8 km/sec/pix.