As part of its technology demonstration, the Nancy Grace Roman Space Telescope (RST) Coronagraph Instrument (CGI) will demonstrate point source spectroscopy and polarization measurements of disks. The specific implementation of spectroscopy is a zero-deviation Amici prism and a slit to be placed on the planet after high contrast has been achieved by CGI. The polarization optics are a set of Wollaston prisms so that orthogonal polarization states can be measured simultaneously. The CGI spectral characterization mode, being designed and built and Goddard Space Flight Center (GSFC), has a spectral resolution of R50 at its central wavelength and is designed to accommodate a 15% bandpass spanning 610785nm. In order to recover Stokes information, there are two sets of Wollaston prisms clocked 45 degrees with respect to one another with each measurement taken in series. The Wollaston design and optical elements are a contribution by the Japanese Aerospace Exploration Agency (JAXA), with final alignment and testing being done at GSFC. The spectroscopy mode is designed to target Methane absorption features around 730nm, keeping the spectral resolution as low as possible to improve the signal-to-noise ratio and hence reduce detection time. We highlight the requirements for these modes and address the challenge of on-orbit spectral calibration for a deployable slit in the presence of pointing drifts. Of unique interest is how the observatory error budget couples into good stellar spectrum calibration and subsequent speckle subtraction. We also provide further detail on the optomechanical design, its modeled performance, and operations concept. These performance metrics are simulated to demonstrate how a slit located at an arbitrary field point is homed onto the planet and converted to a calibrated spectrum.
The LRCTF (Laser Ranging Characterization and Test Facility) is a unique facility built at NASA GSFC to provide thermal-optical testing of the next generation GPS LRA (Global Positioning Satellite’s Laser Retroreflector Array) laser ranging target. The 400mm diameter target is an array consisting of 48 total internal reflection retroreflectors and has an optical cross section requirement of 100 MSM (million square meters). To verify that the array meets this requirement during on-orbit conditions, the LRCTF is equipped with a 400mm test beam, a data product output consisting of full aperture FFDPs (Far Field Diffraction Patterns) and a thermal chamber. The FFDPs are used to calculate the OCS. This paper will describe the facility design, alignment approach, and verification process.
We have superpolished a diamond-turned aluminum mandrel to an axial roughness of 0.34 nm rms. The mandrel is made to the Astro-E secondary mirror design for the 81st shell. Precision metrology at 100 mm to submicron scales has established the power spectral density of the mandrel and ultralightweight gold coated replicated segments. Predicted image quality of a set of optimally aligned replicated segments of this and a matching primary is substantially improved as compared to the flight mirrors for Astro-E. This approach using metal mandrels, superpolishing, and replicated ultralightweight foil mirrors, may represent a cost-effective approach to meeting the 15 arcsec half-energy width and weight requirements for the Constellation-X mission. Descriptions of the polishing apparatus, the precision metrology instruments, and the surface data analysis are presented. The general methods describe dare applicable to precision optics for both normal incidence and grazing incidence optics.
The Composite Infrared Spectrometer (CIRS) instrument on the Cassini Mission launched in October of 1997. The CIRS instrument contains a mid-infrared (MIR) and a far-infrared interferometer and operates at 170 Kelvin. The MIR is a Michelson Fourier transform spectrometer utilizing a 76 mm (3 inch) diameter potassium bromide beamsplitter and compensator pair. The potassium bromide elements were tested to verify effects of cooldown and vibration prior to integration into the instrument. The instrument was then aligned to ambient temperatures, tested cryogenically and re-verified after vibration. The stringent design optical figure requirements for the beamsplitter and compensator included fabrication errors, mounting stress and vibration load effects. This paper describes the challenges encountered in mounting the elements to minimize distortion and to survive vibration.
The Cassini Composite InfraRed Spectrometer half-mirror diameter beryllium flight telescope's optical performance was tested at the instrument operating temperature of 170 Kelvin. The telescope components were designed at Goddard Space Flight Center (GSFC) but fabricated out-of-house and then assembled, aligned, and tested upon receipt at GSFC. A 24-inch aperture cryogenic test facility utilizing a 1024 X 1024 CCD array was developed at GSFC specifically for this test. The telescope's image quality (measured as encircled energy), boresight stability and focus stability were measured. The gold coated beryllium design exceeded the cold image performance requirement of 80% encircled energy within a 460 micron diameter cycle.
The composite infrared spectrometer (CIRS) of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The instrument is aligned at ambient temperature, but operates at 170 Kelvin and has challenging boresight and interferometric alignment tolerances. This paper describes how the aluminium mirrors were aligned to the CIRS optics module to tolerances of .5 milliradians in biaxial tilt and 100 microns in decenter and how the instrument boresight was aligned.
The composite infrared spectrometer (CIRS) of the CAssini mission to Saturn has two interferometers covering the far- IR (FIR) and mid-IR (MIR) wavelength region. The FIR is a polarizing interferometer utilizing dihedral retroreflectors and a polarizing beamsplitter. As such, it is sensitive to extremely small alignment change of the dihedrals and beamsplitter elements. The alignment stability required of the beamsplitter through all cryogenic cycling, handling, test, and launch-induced disturbances is better than 10 arc seconds. The mount is also required to induce minimal distortion to the 1.5-micron-thick mylar polarizing element ont he FIR channel and the potassium bromide beamsplitter/compensator elements on the MIR channel. It is also required to provide biaxial tilt adjustment at the arc second level and translation adjustment of the beamsplitter elements to the few micron level, and must be locked without changing the alignment of the element. This may be the first mount to have achieved these requirements on a cryogenic instrument.
The composite infrared spectrometer (CIRS) instrument is scheduled to fly on NASA's Cassini Orbiter to Saturn in 1997. CIRS consists of two Fourier transform spectrometers, the mid-IR (MIR) and the far-IR (FIR), which measure a spectral range from 7 to 1000 microns. The optical alignment of CIRS begins with alignment of the optical subsystems which are then integrated and aligned to each other. These subsystems include the 0.5 meter Cassegrain telescope, collimating optics aft of the telescope, the moving mirror scanning mechanism, MIR and FIR interferometers, and the MIR and FIR focal planes. This paper discusses the alignment verification test developed to verify the opto-mechanical alignment of the collimating optics and the fixed mirrors of the interferometers. The verification test utilized the ZYGO Mark IVxp interferometer to test the wavefront of the aligned subsystem. The test set-up, requirements and results are presented.
Virgil Kunde, Peter Ade, Richard Barney, D. Bergman, Jean-Francois Bonnal, R. Borelli, David Boyd, John Brasunas, Gregory Brown, Simon Calcutt, F. Carroll, R. Courtin, Jacky Cretolle, Julie Crooke, Martin Davis, S. Edberg, Rainer Fettig, M. Flasar, David Glenar, S. Graham, John Hagopian, Claef Hakun, Patricia Hayes, L. Herath, Linda Spilker, Donald Jennings, Gabriel Karpati, C. Kellebenz, Brook Lakew, J. Lindsay, J. Lohr, James Lyons, Robert Martineau, Anthony Martino, Mack Matsumura, J. McCloskey, T. Melak, Guy Michel, Armando Morrell, C. Mosier, LaTunia Pack, M. Plants, D. Robinson, Louis Rodriguez, Paul Romani, Bill Schaefer, Stephen Schmidt, Carlos Trujillo, Tim Vellacott, K. Wagner, D. Yun
The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm-1 with a spectral resolution up to 0.5 cm-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm-1 in two spectral bandpasses: 600-1100 cm- 1100-1400 cm(superscript -1 with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.
The purpose of this project is to investigate the feasibility of using composite materials to reduce the weight of high quality optical mirrors for space flight applications. The approach that is used to fabricate test optics uses the same optical quality glass material (Zerodur) as the mold used to form the composite. The mold is the actual material that is ground into the finished polished surface, and it is bonded onto the composite during the normal curing of the composite. No secondary bonding is necessary. The mold is now ground away leaving a thin layer of Zerodur that becomes the polished surface. The composite material used in this experiment is P75/ERL1962 laid up in a quasi-isotropic laminate. Its properties will be tailored such that its coefficient of thermal expansion is close to that of the mold in the plane of the laminate. One flat optic has been made and tested. A second flat optic is close to completion, but it has not been tested. The first optic was tested in a cryogenic vacuum chamber using a Zygo interferometer to measure the distortion of the surface of the optic while the chamber was cooled down to 200 K. The measured change in flatness was 0.4 lambda using a test wavelength of 633 nm. The average fiber volume was calculated to be 56.5% with a change in fiber volume from one side to the other of 2.5%.
The Cassini mission to Saturn will contain the CIRS instrument which is currently being developed and assembled at the Goddard Space Flight Center. The CIRS instrument contains two science interferometers that operate in the mid and far infrared regions of the spectrum and one reference interferometer which operates in the visible. The heart of each of the interferometers is a series of hollow glass retroreflectors (cube corners) and hollow dihedrals. The hollow retroreflectors are constructed of individual facets of zerodur glass which are bonded 90 degrees to each other to sub arc-second accuracies. They are then coated with a reflective overcoat to meet the wavelength requirements. The effort at Goddard resulted in the development of retroreflectors that not only performed well at ambient temperatures, but also retained a wavefront error of approximately 2 waves p-v with a maximum beam deviation of 15 arc seconds at a temperature of 170 degrees kelvin or below. Also developed at GSFC is a successful means of mounting the retroreflectors on a fixed zerodur mount to allow cooling them down to these temperatures without introducing any added stresses that are not already present in the unmounted retroreflectors.
The Composite Infrared Spectrometer (CIRS) instrument is scheduled to fly on NASA's Cassini mission to Saturn. CIRS operates at 170 Kelvin and utilizes two Michelson interferometers to measure the infrared spectrum between 7.1 and 1000 microns. The Mid-InfraRed interferometer (MIR) is a classical Michelson design operating in the 7.1 to 16.7 micron band. The Far-InfraRed interferometer (FIR) is a polarizing Michelson design measuring the 16.7 to 1000 micron band. Both the MIR and FIR use retroreflector elements rather than flat mirrors. The MIR requires hollow cube corner style retroreflectors while the FIR polarizing nature requires roof-top mirror style retroreflectors. Initial testing of available technology indicated that interferometric quality retroreflectors do exist in ambient temperatures. Tests were performed using commercially available mounted and unmounted cube corners and commercial cube corners mounted to GSFC designed mounts to characterize their cryogenic, interferometric performance. The Goddard Space Flight Center's ambient and cryogenic test and results are presented here.
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