John G. Hagopian

Cassini infrared Fourier spectroscopic investigation

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 Saturns 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.

Fourier-transform optical microsystems.

The design, fabrication, and initial characterization of a miniature single-pass Fourier-transform spectrometer (FTS) that has an optical bench that measures 1 cm x 5 cm x 10 cm is presented. The FTS is predicated on the classic Michelson interferometer design with a moving mirror. Precision translation of the mirror is accomplished by microfabrication of dovetailed bearing surfaces along single-crystal planes in silicon. Although it is miniaturized, the FTS maintains a relatively high spectral resolution, 0.1 cm-1, with adequate optical throughput.

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in stray light when applied to an entire optical train. Development of this technology will provide numerous benefits including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology extend to space science where observations of extremely dim objects require suppression of stray light.

Design and fabrication of diamond-machined aspheric mirrors for ground-based near-IR astronomy

Challenges in fabrication and testing have historically limited the choice of surfaces available for the design of reflective optical instruments. Spherical and conic mirrors are common, but, for future science instruments, more degrees of freedom will be necessary to meet performance and packaging requirements. These instruments will be composed of surfaces of revolution located far off-axis with large spherical departure, and some designs will even require asymmetric surface profiles. We describe the design and diamond machining of seven aluminum mirrors: three rotationally symmetric, off-axis conic sections, one off-axis biconic, and three flat mirror designs. These mirrors are for the Infrared Multi-Object Spectrometer instrument, a facility instrument for the Kitt Peak National Observatory’s Mayall Telescope (3.8 m) and a pathfinder for the future Next Generation Space Telescope multi-object spectrograph. The symmetric mirrors include convex and concave prolate and oblate ellipsoids, and range in aperture from 92 x 77 mm to 284 x 264 mm and in f-number from 0.9 to 2.4. The biconic mirror is concave and has a 94 x 76 mm aperture, (formula available in paper) and is decentered by -2 mm in x and 227 mm in y. The mirrors have an aspect ratio of approximately 6:1. The fabrication tolerances for surface error are < 63.3 nm RMS figure error and < 10 nm RMS microroughness. The mirrors are attached to the instrument bench using semi-kinematic, integral flexure mounts and optomechanically aligned to the instrument coordinate system using fiducial marks and datum surfaces. We also describe in-process profilometry and optical testing.

Hemispherical reflectance and emittance properties of carbon nanotubes coatings at infrared wavelengths

Recent visible wavelength observations of Multiwalled Carbon Nanotubes (MWCNT) coatings have revealed that they represent the blackest materials known in nature with a Total Hemispherical Reflectance (THR) of less than 0.25%. This makes them exceptionally good as absorbers, with the potential to provide order-ofmagnitude improvement in stray-light suppression over current black surface treatments when used in an optical system. Here we extend the characterization of this class of materials into the infrared spectral region to further evaluate their potential for use on instrument baffles for stray-light suppression and to manage spacecraft thermal properties through radiant heat transfer process. These characterizations will include the wavelength-dependent Total Hemispherical Reflectance (THR) properties in the mid- and far-infrared spectral regions (2-110 μm). Determination of the temperature-dependent emittance will be investigated in the temperature range of 40 to 300 K. These results will be compared with other more conventional black coatings such as Acktar Fractal Black or Z306 coatings among others.

Comparison of Stress Relief Procedures for Cryogenic Aluminum Mirrors

The Infrared Multi-Object Spectrograph is a facility instrument for the KPNO Mayall Telescope. IRMOS is a low- to mid-resolution, near-IR (0.8-2.5 um) spectrograph that produces simultaneous spectra of ~100 objects in its 2.8 × 2.0 arcmin field of view. The instrument operating temperature is ~80 K and the design is athermal. The bench and mirrors are machined from Al 6061-T651. In spite of its baseline mechanical stress relief, Al 6061-T651 harbors residual stress, which, unless relieved during fabrication, may distort mirror figure to unacceptable levels at the operating temperature (~80 K). Other cryogenic, astronomy instruments using Al mirrors have employed a variety of heat treatment formulae, with mixed results. We present the results of a test program designed to empirically determine the best stress relief procedure for the IRMOS mirrors. Identical test mirrors are processed with six different stress relief formulae from the literature and institutional heritage. After figuring via diamond turning, the mirrors are tested for figure error at room temperature and at ~80 K for three thermal cycles. The heat treatment procedure for the mirrors that yielded the least and most repeatable change in figure error is applied to the IRMOS mirror blanks. We correlate the results of our optical testing with heat treatment and metallographic data.

New approach to cryogenic optical testing the James Webb Space Telescope

The James Webb Space Telescope is a 6.5 meter segmented cryogenic telescope scheduled to be launched in 2013. A key development challenge has been the cost and complexity of cryogenic optical testing of the telescope and observatory. A new approach to cryogenic optical testing the telescope and observatory has been developed that eliminates the need for a complex and expensive cryogenic optical test tower and which also allows all critical test equipment to be external to the chamber and accessible during testing. This paper summarizes the motivation for this change, the conceptual design of it, and status of implementing it.

Alignment and cryogenic testing of the Cassini Composite InfraRed Spectrometer (CIRS) far-infrared (FIR) focal plane

The CIRS instrument to be flown on the Cassini mission to Saturn is a cryogenic spectrometer with far-IR (FIR) and mid-IR (MIR) channels. The CIRS FIR focal plane consists of focussing optics, an output polarizer/analyzer which splits the output radiation according to polarization. The reflected and transmitted components are imaged by concentrating cones onto gold black foil thermopiles. The focal plane covers the spectral range of 10-600 cm(-1). The geometric field-of-view requirement is 4.3 mrad. This paper details the assembly, alignment, characterization, cryogenic testing, and flight qualification of the CIRS FIR focal plane.

Cryogenic pupil alignment test architecture for the James Webb Space Telescope integrated science instrument module

The James Webb Space Telescope (JWST) is a space-based, infrared observatory designed to study the early stages of galaxy formation in the Universe. It is currently scheduled to be launched in 2013 and will go into orbit about the second Lagrange point of the Sun-Earth system and passively cooled to 30-50 K to enable astronomical observations from 0.6 to 28 μm. The JWST observatory consists of three primary elements: the spacecraft, the optical telescope element (OTE) and the integrated science instrument module (ISIM). The ISIM Element primarily consists of a mechanical metering structure, three science instruments and a fine guidance sensor with significant scientific capability. One of the critical opto-mechanical alignments for mission success is the co-registration of the OTE exit pupil with the entrance pupils of the ISIM instruments. To verify that the ISIM Element will be properly aligned with the nominal OTE exit pupil when the two elements come together, we have developed a cryogenic pupil measurement test architecture to measure three of the most critical pupil degrees-of-freedom during optical testing of the ISIM Element. The pupil measurement scheme makes use of: specularly reflective pupil alignment references located inside of the JWST instruments; ground support equipment that contains a pupil imaging module; an OTE simulator; and pupil viewing channels in two of the JWST flight instruments. Current modeling and analysis activities indicate this measurement approach will be able to verify pupil shear to an accuracy of 0.5-1%.

Optical Alignment and Test of the James Webb Space Telescope Integrated Science Instrument Module

The James Webb Space Telescope (JWST) is a 6.6 m diameter, segmented, deployable telescope for cryogenic IR space astronomy (~40 K). The JWST observatory architecture includes the optical telescope element (OTE) and the integrated science instrument module (ISIM) element that contains four science instruments (SI) including a guider. The SIs and Guider are mounted to a composite metering structure with outer dimensions of 2.1 times 2.2 times 1.9 m. The SI and guider units are integrated to the ISIM structure and optically tested at NASA/Goddard Space Flight Center as an instrument suite using an OTE SIMulator (OSIM). OSIM is a high-fidelity, cryogenic JWST telescope simulator that features a 1.5 m diameter powered mirror. The SIs are aligned to the structures coordinate system under ambient, clean room conditions using laser tracker and theodolite metrology. Temperature-induced mechanical SI alignment and structural changes are measured using a photogrammetric measurement system at ambient and cryogenic temperatures. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors in six degrees of freedom. SI performance, including focus, pupil shear and wavefront error, is evaluated at the operating temperature using OSIM. We describe the ambient and cryogenic optical alignment, test and verification plan for the ISIM element.