WFIRST is one of NASA’s Decadal Survey Missions and is currently in Phase-A development. The optical design of the WFIRST Integral Field Channel (IFC), one of three main optical channels of WFIRST, is presented, and the evolution of the IFC channel since the Mission Concept Review (MCR, end of Pre-Phase A) is discussed. The IFC has two subchannels: Supernova (IFC-S) and Galaxy (IFC-G) channels, with Fields of View of 3”x4.5” and 4.2”x9” respectively, and ~R 100 spectral analysis over waveband 0.42–2.0 μm. The Phase-A IFC optical design meets image quality requirements over the field of view (FOV) while balancing cost and volume constraints.
The NASA Goddard Space Flight Center (GSFC) and its partners have broad experience in the alignment of flight optical instruments and spacecraft structures. Over decades, GSFC developed alignment capabilities and techniques for a variety of optical and aerospace applications. In this paper, we provide an overview of a subset of the capabilities and techniques used on several recent projects in a “toolbox” format. We discuss a range of applications, from small-scale optical alignment of sensors to mirror and bench examples that make use of various large-volume metrology techniques. We also discuss instruments and analytical tools.
The sole instrument on NASA’s ICESat-2 spacecraft shown in Figure 1 will be the Advanced Topographic Laser Altimeter System (ATLAS)1. The ATLAS is a Light Detection and Ranging (LIDAR) instrument; it measures the time of flight of the six transmitted laser beams to the Earth and back to determine altitude for geospatial mapping of global ice. The ATLAS laser beam is split into 6 main beams by a Diffractive Optical Element (DOE) that are reflected off of the earth and imaged by an 800 mm diameter Receiver Telescope Assembly (RTA). The RTA is composed of a 2-mirror telescope and Aft Optics Assembly (AOA) that collects and focuses the light from the 6 probe beams into 6 science fibers. Each fiber optic has a field of view on the earth that subtends 83 micro Radians. The light collected by each fiber is detected by a photomultiplier and timing related to a master clock to determine time of flight and therefore distance. The collection of the light from the 6 laser spots projected to the ground allows for dense cross track sampling to provide for slope measurements of ice fields. NASA LIDAR instruments typically utilize telescopes that are not diffraction limited since they function as a light collector rather than imaging function. The more challenging requirements of the ATLAS instrument require better performance of the telescope at the ¼ wave level to provide for improved sampling and signal to noise. NASA Goddard Space Flight Center (GSFC) contracted the build of the telescope to General Dynamics (GD). GD fabricated and tested the flight and flight spare telescope and then integrated the government supplied AOA for testing of the RTA before and after vibration qualification. The RTA was then delivered to GSFC for independent verification and testing over expected thermal vacuum conditions. The testing at GSFC included a measurement of the RTA wavefront error and encircled energy in several orientations to determine the expected zero gravity figure, encircled energy, back focal length and plate scale. In addition, the science fibers had to be aligned to within 10 micro Radians of the projected laser spots to provide adequate margin for operations on-orbit. This paper summarizes the independent testing and alignment of the fibers performed at the GSFC.
We present the design, integration, and test of the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES) integral field spectrograph (IFS). The PISCES design meets the science requirements for the Wide-Field InfraRed Survey Telescope (WFIRST) Coronagraph Instrument (CGI). PISCES was integrated and tested in the integral field spectroscopy laboratory at NASA Goddard. In June 2016, PISCES was delivered to the Jet Propulsion Laboratory (JPL) where it was integrated with the Shaped Pupil Coronagraph (SPC) High Contrast Imaging Testbed (HCIT). The SPC/PISCES configuration will demonstrate high contrast integral field spectroscopy as part of the WFIRST CGI technology development program.
The James Webb Space Telescope (JWST) relies on several innovations to complete its five year mission. One vital
technology is microshutters, the programmable field selectors that enable the Near Infrared Spectrometer (NIRSpec) to
perform multi-object spectroscopy. Mission success depends on acquiring spectra from large numbers of galaxies by
positioning shutter slits over faint targets. Precise selection of faint targets requires field selectors that are both high in
contrast and stable in position. We have developed test facilities to evaluate microshutter contrast and alignment stability
at their 35K operating temperature. These facilities used a novel application of image registration algorithms to obtain
non-contact, sub-micron measurements in cryogenic conditions. The cryogenic motion of the shutters was successfully
characterized. Optical results also demonstrated that shutter contrast far exceeds the NIRSpec requirements. Our test
program has concluded with the delivery of a flight-qualified field selection subsystem to the NIRSpec bench.
Image registration, or alignment of two or more images covering the same scenes or objects, is of great interest in many
disciplines such as remote sensing, medical imaging, astronomy, and computer vision. In this paper, we introduce a
new application of image registration algorithms. We demonstrate how through a wavelet based image registration
algorithm, engineers can evaluate stability of Micro-Electro-Mechanical Systems (MEMS). In particular, we applied
image registration algorithms to assess alignment stability of the MicroShutters Subsystem (MSS) of the Near Infrared
Spectrograph (NIRSpec) instrument of the James Webb Space Telescope (JWST). This work introduces a new
methodology for evaluating stability of MEMS devices to engineers as well as a new application of image registration
algorithms to computer scientists.
Microshutter arrays are one of the novel technologies developed for the James Webb Space Telescope (JWST).
It will allow Near Infrared Spectrometer (NIRSpec) to acquire spectra of hundreds of objects simultaneously
therefore increasing its efficiency tremendously. We have developed these programmable arrays that are based
on Micro-Electro Mechanical Structures (MEMS) technology. The arrays are 2D addressable masks that can
operate in cryogenic environment of JWST. Since the primary JWST science requires acquisition of spectra
of extremely faint objects, it is important to provide very high contrast of the open to closed shutters. This
high contrast is necessary to eliminate any possible contamination and confusion in the acquired spectra by
unwanted objects. We have developed and built a test system for the microshutter array functional and optical
characterization. This system is capable of measuring the contrast of the mciroshutter array both in visible and
infrared light of the NIRSpec wavelength range while the arrays are in their working cryogenic environment. We
have measured contrast ratio of several microshutter arrays and demonstrated that they satisfy and in many
cases far exceed the NIRSpec contrast requirement value of 2000.
The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.
This report describes the facility and experimental methods at the Goddard Space Flight Center Optics Branch for the measurement of the surface figure of cryogenically-cooled spherical mirrors using standard phase-shifting interferometry, with a standard uncertainty below 2nm rms. Two developmental silicon carbide mirrors were tested: both were spheres with radius of curvature of 600 mm, and clear apertures of 150 mm. The mirrors were cooled within a cryostat, and the surface figure error measured through a fused-silica window. The GSFC team developed methods to measure the change in surface figure with temperature (the cryo-change) with a combined standard uncertainty below 1 nm rms. This paper will present the measurement facility, methods, and uncertainty analysis.
This report describes the equipment, experimental methods, and first results at a new facility at the Goddard Space Flight Center Optics Branch for interferometric measurement of cryogenically-cooled spherical mirrors. A mirror is cooled to 80 K and 20 K within a cryostat; and its surface figure error is measured through a fused-silica window using standard phase-shifting interferometry. The first mirror tested was a concave spherical silicon foam-core mirror with a clear aperture of 120 mm. The optic surface was measured at room temperature outside the dewar using standard "absolute" techniques; and then the change in surface figure error within the dewar from room temperature to 80 K was measured, and the two measurements added to create a representation of the two-dimensional surface figure error at 80 K, with a combined standard uncertainty of 3.4 nm rms. The facility and techniques will be used to measure the surface figure error at 20K of prototype lightweight silicon carbide and Cesic mirrors developed by Galileo Avionica (Italy) for the European Space Agency (ESA).
The Infrared Multi-Object Spectrometer (IRMOS) is a principle investigator class instrument for the Kitt Peak National Observatory 4 and 2.1 m telescopes. IRMOS is a near-IR (0.8 - 2.5 μm) spectrometer with low- to mid-resolving power (R = 300 - 3000). IRMOS produces simultaneous spectra of ~100 objects in its 2.8 - 2.0 arc-min field of view (4 m telescope) using a commercial Micro Electro-Mechanical Systems (MEMS) micro-mirror array (MMA) from Texas Instruments. The IRMOS optical design consists of two imaging subsystems. The focal reducer images the focal plane of the telescope onto the MMA field stop, and the spectrograph images the MMA onto the detector. We describe ambient breadboard subsystem alignment and imaging performance of each stage independently, and ambient imaging performance of the fully assembled instrument. Interferometric measurements of subsystem wavefront error serve as a qualitative alignment guide, and are accomplished using a commercial, modified Twyman-Green laser unequal path interferometer. Image testing provides verification of the optomechanical alignment method and a measurement of near-angle scattered light due to mirror small-scale surface error. Image testing is performed at multiple field points. A mercury-argon pencil lamp provides a spectral line at 546.1 nm, a blackbody source provides a line at 1550 nm, and a CCD camera and IR camera are used as detectors. We use commercial optical modeling software to predict the point-spread function and its effect on instrument slit transmission and resolution. Our breadboard and instrument level test results validate this prediction. We conclude with an instrument performance prediction for cryogenic operation and first light in late 2003.
The Infrared Multi-Object Spectrometer (IRMOS) is a facility-class instrument for the Kitt Peak National Observatory 4 and 2.l meter telescopes. IRMOS is a near-IR (0.8-2.5 μm) spectrometer and operates at ~80 K. The 6061-T651 aluminum bench and mirrors constitute an athermal design. The instrument produces simultaneous spectra at low- to mid-resolving power (R = λ/Δλ = 300-3000) of ~100 objects in its 2.8×2.0 arcmin field.
We describe ambient and cryogenic optical testing of the IRMOS mirrors across a broad range in spatial frequency (figure error, mid-frequency error, and microroughness). The mirrors include three rotationally symmetric, off-axis conic sections, one off-axis biconic, and several flat fold mirrors. The symmetric mirrors include convex and concave prolate and oblate ellipsoids. They range in aperture from 94×86 mm to 286×269 mm and in f-number from 0.9 to 2.4. The biconic mirror is concave and has a 94×76 mm aperture, Rx=377 mm, kx=0.0778, Ry=407 mm, and ky=0.1265 and is decentered by -2 mm in X and 227 mm in Y. All of the mirrors have an aspect ratio of approximately 6:1. The surface error fabrication tolerances are < 10 nm RMS microroughness, best effort for mid-frequency error, and < 63.3 nm RMS figure error.
Ambient temperature (~293 K) testing is performed for each of the three surface error regimes, and figure testing is also performed at ~80 K. Operation of the ADE PhaseShift MicroXAM white light interferometer (micro-roughness) and the Bauer Model 200 profilometer (mid-frequency error) is described. Both the sag and conic values of the aspheric mirrors make these tests challenging. Figure testing is performed using a Zygo GPI interferometer, custom computer generated holograms (CGH), and optomechanical alignment fiducials.
Cryogenic CGH null testing is discussed in detail. We discuss complications such as the change in prescription with temperature and thermal gradients. Correction for the effect of the dewar window is also covered. We discuss the error budget for the optical test and alignment procedure. Data reduction is accomplished using commercial optical design and data analysis software packages. Results from CGH testing at cryogenic temperatures are encouraging thus far.
The Infrared Multi-Object Spectrometer (IRMOS) is a facility instrument for the Kitt Peak National Observatory 4 and 2.1 meter telescopes. IRMOS is a near-IR (0.8 - 2.5 μm) spectrometer with low- to mid-resolving power (R = 300 - 3000). The IRMOS spectrometer produces simultaneous spectra of ~100 objects in its 2.8 x 2.0 arcmin field of view using a commercial MEMS multi-mirror array device (MMA) from Texas Instruments. The IRMOS optical design consists of two imaging subsystems. The focal reducer images the focal plane of the telescope onto the MMA field stop, and the spectrograph images the MMA onto the detector. We describe the breadboard subsystem alignment method and imaging performance of the focal reducer. This testing provides verification of the optomechanical alignment method and a measurement of near-angle scattered light due to mirror small-scale surface error. Interferometric measurements of subsystem wavefront error serve to verify alignment and are accomplished using a commercial, modified Twyman-Green laser unequal path interferometer. Image testing is then performed for the central field point. A mercury-argon pencil lamp provides the spectral line at 546.1 nm, and a CCD camera is the detector. We use the Optical Surface Analysis Code to predict the point-spread function and its effect on instrument slit transmission, and our breadboard test results validate this prediction. Our results show that scattered light from the subsystem and encircled energy is slightly worse than expected. Finally, we perform component level image testing of the MMA, and our results show that scattered light from the MMA is of the same magnitude as that of the focal reducer.