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.
After the more than six decades since D. D. Maksutov introduced his aplanatic, catadioptric camera with the meniscus corrector plate and spherical-mirror objective, the system and its variants are well familiar within the design community. But it would have achieved limited enthusiasm had it not been for the singlet corrector's propensity for achromatization, since no balancing aberration is available in the objective. However, the refractor-objective apochromat possesses balancing, chromatic residuals that may be coupled with a Maksutov-like corrector plate to provide improved correction for chromatic aberration. The combination may be further optimized to generate a chromatic error of longitudinal aberration that is only a small fraction of that of the progenitor refractor objective alone. Examples are provided with one and two corrector elements. Caveats are discussed.
Wide field-of-view, high-resolution near-infrared cameras on 4-m class telescopes have been identified by the astronomical community as critical instrumentation needs in the era of 8-m and larger telescopes. Acting as survey instruments, they will provide the input source discoveries for large-telescope follow-up observations. The NOAO Extremely Wide Field Infrared Mosaic (NEWFIRM) imaging instrument will serve this need within the US system of facilities. NEWFIRM is being designed for the National Optical Astronomy Observatory (NOAO) 4-m telescopes (Mayall at KPNO and Blanco at CTIO). NEWFIRM covers a 28 x 28 arcmin field of view over the 1-2.4 μm wavelength range with a 4k x 4k pixel detector mosaic assembled from 2k x 2k modules. Pixel scale is 0.4 arcsec/pixel. Data pipelining and archiving are integral elements of the instrument system. We present the science drivers for NEWFIRM, and describe its optical, mechanical, electronic, and software components. By the time this paper is presented, NEWFIRM will be in the preliminary design stage, with first light expected on the Mayall telescope in 2005.
The National Optical Astronomy Observatory is developing a new, wide-field, imaging spectrograph for use on its existing 4-meter telescopes. This Next Generation Optical Spectrograph (NGOS) will utilize volume-phase holographic grating technology and will have a mosaiced detector array to image the spectra over a field of view that will be something like 10.5 by 42 arc-minutes on the sky. The overall efficiency of the spectrograph should be quite high allowing it to outperform the current RC spectrograph by factors of 10 to 20 and the Hydra multi-fiber instrument by a facto of fiber to ten per object. The operational range of the instrument will allow observations within the optical and near-IR regions. Spectral resolutions will go from R equals 1000 to at least R equals 5000 with 1.4 arc-second slits. The large size of this instrument, with a beam diameter of 200 mm and an overall length of nearly 3 meters, presents a significant challenge in mounting it at the Cassegrain location of the telescope. Design trades and options that allow it to fit are discussed.
A unit device for supporting a telescope primary mirror in its cell is described. It replaces the traditional roller- ball or oil-bellows support unit. The device utilizes the levitating field from opposing magnets to support the primary's weight above the cell's surface. This frees the bearings of the device so that the primary may expand or contract smoothly, unimpaired with `sticky', loaded bearings. The mechanics of the device restrain the opposing magnets from drifting inappropriately and work to isolate the primary from undesirable bending moments. Supplying the near-cell magnet, which may advance toward the near-primary magnet, with the standard counterweight and fulcrum commonly seen behind the cell assures the primary/device, weight/force balance remains for any orientation. Design, forces, and ongoing research for levitated support is discussed. A prototype is under construction.
An interferometric test is described that allows the testing of a convex surface from its reflective side with conjugate off-axis light beams. The apertures of the auxiliary optics are smaller than that of the surface under test. A method of centering the convex surface in the testing beams is presented.
The classic Schmidt design has its aspheric corrector plate designed for and located at the center of curvature of its companion spherically concave primary mirror. So configured, it is well corrected for objects at infinity imaged at focus. Any closer object, however, is imaged with increasing spherical aberration as its distance from the corrector plate is diminished. Shifting the unchanged corrector plate toward its primary mirror increases the range of object distances for which the system retains excellent stigmatic correction. At the corrector plate's near-optimum location, this range may extend from a few of the primary mirror's radii in front of the corrector plate to infinity. Coma increases, but likely will remain tolerably small except for very fast systems.
Utilizing the Herschel condition, a method is presented for null testing most refractor doublet objectives, including the Baker air-spaced design with equal internal radii, and Maksutov corrector plates using auxiliary spherical mirrors. A special Baker air-spaced objective is offered that utilizes a spherical mirror in null testing that is generated by silvering the front face of the doublet's crown lens or immersing it into mercury. The classical Schmidt corrector is found to deny null testing using the technique.
The traditional qualitative Ronchi test has undergone various manifestations with efforts to produce quantifiable results. Usual schemes call for the portage of the Ronchi grating via a carriage along machined ways aligned nearly parallel with the axis of the reflecting concave surface under test. The distance the grating is moved provides the variable in the data for making the test. A new test geometry has the grating carried in a bearing that is clamped in a gauged position, and the angle of the turned grating is read on a scale to provide the data variable. Aside from an increase in accuracy, this testing geometry allows, with the reduction mathematics, a number of entry-data options. Full-surface evaluation and the testing of astigmatic surfaces are possible. Adding an accurate vernier to the angular scale on the bearing allows finding the radius of curvature for the surface under test to a fineness unmatched by the standard mechanical spherometer.
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