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.
During cryogenic vacuum testing of the James Webb Space Telescope (JWST) Integrated Science Instrument Module
(ISIM), the global alignment of the ISIM with respect to the designed interface of the JWST optical telescope element
(OTE) will be measured through a series of optical characterization tests. These tests will determine the locations and
orientations of the JWST science instrument projected focal surfaces and entrance pupils with respect to their
corresponding OTE optical interfaces. Thermal, finite element and optical modeling will then be used to predict the on-orbit
optical performance of the observatory. If any optical performance non-compliances are identified, the ISIM will
be adjusted to improve its performance. If this becomes necessary, ISIM has a variety of adjustments that can be made.
The lengths of the six kinematic mount struts that attach the ISIM to the OTE can be modified and five science
instrument focus positions and two pupil positions can be individually adjusted as well. In order to understand how to
manipulate the ISIM’s degrees of freedom properly and to prepare for the ISIM flight model testing, we have completed
a series of optical-mechanical analyses to develop and identify the best approaches for bringing a non-compliant ISIM
Element back into compliance. During this work several unknown misalignment scenarios were produced and the
simulated optical performance metrics were input into various mathematical modeling and optimization tools to
determine how the ISIM degrees of freedom should be adjusted to provide the best overall optical performance.
For many optical systems the properties and alignment of the internal apertures and pupils are not critical or controlled
with high precision during optical system design, fabrication or assembly. In wide angle imaging systems, for instance,
the entrance pupil position and orientation is typically unconstrained and varies over the system's field of view in order
to optimize image quality. Aperture tolerances usually do not receive the same amount of scrutiny as optical surface
aberrations or throughput characteristics because performance degradation is typically graceful with misalignment,
generally only causing a slight reduction in system sensitivity due to vignetting. But for a large deployable space-based
observatory like the James Webb Space Telescope (JWST), we have found that pupil alignment is a key parameter. For
in addition to vignetting, JWST pupil errors cause uncertainty in the wavefront sensing process that is used to construct
the observatory on-orbit. Furthermore they also open stray light paths that degrade the science return from some of the
telescope's instrument channels. In response to these consequences, we have developed several pupil measurement
techniques for the cryogenic vacuum test where JWST science instrument pupil alignment is verified. These approaches
use pupil alignment references within the JWST science instruments; pupil imaging lenses in three science instrument
channels; and unique pupil characterization features in the optical test equipment. This will allow us to verify and
crosscheck the lateral pupil alignment of the JWST science instruments to approximately 1-2% of their pupil diameters.
A large depth-of-field Particle Image Velocimeter (PIV) has been developed at NASA GSFC to characterize dynamic
dust environments on planetary surfaces. This instrument detects and senses lofted dust particles. To characterize a
dynamic planetary dust environment, the instrument would have to operate for at least several minutes during an
observation period, easily producing more than a terabyte of data per observation. Given current technology, this
amount of data would be very difficult to store onboard a spacecraft and downlink to Earth. We have been developing
an autonomous image analysis algorithm architecture for the PIV instrument to greatly reduce the amount of data that
it has to store and downlink. The algorithm analyzes PIV images and reduces the image information down to only the
particle measurement data we are interested in receiving on the ground - typically reducing the amount of data to be
handled by more than two orders of magnitude. We give a general description of the PIV algorithms and describe in
detail the algorithm for estimating the direction and velocity of the traveling particles, which was done by taking
advantage of the optical properties of moving dust particles along with image processing techniques.
The Mid Infrared Instrument (MIRI), one of the four instruments on the Integrated Science Instrument Module (ISIM) of
the James Webb Space Telescope (JWST), supports all of the science objectives of the observatory. MIRI optical
alignment is an important step in the verification process, directly affecting mission success. The MIRI optical alignment
is verified on the ground at the integrated ISIM level using an element in the MIRI Filter Wheel, the pupil alignment
reference (PAR), developed by NASA GSFC and provided to MIRI. It is a ~2.3g aluminum piece that has a flat,
specularly reflective, 3mm diameter surface in its center, with laser-etched fiducials within its aperture. The PAR is
illuminated via an optical stimulus (ground support equipment) and imaged using a pupil imaging camera, during the
ISIM test program in order to determine absolute and relative changes in the alignment that impact pupil shear and roll.
Here we describe the MIRI PAR; its physical properties and challenges during its design, manufacturing, and testing.
The James Webb Space Telescope (JWST) is an infrared space telescope scheduled for launch in 2013. JWST has a 6.5 meter diameter deployable and segmented primary mirror, a deployable secondary mirror, and a deployable sun-shade. The optical train of JWST consists of the Optical Telescope Element (OTE), and the Integrated Science Instrument Module (ISIM), which contains four science instruments. When the four science instruments are integrated to ISIM at NASA Goddard Space Flight Center, the structure becomes the ISIM Element. The ISIM Element is assembled at ambient cleanroom conditions using theodolite, photogrammetry, and laser tracker metrology, but it operates at cryogenic temperature, and temperature-induced mechanical and alignment changes are measured using photogrammetry. The OTE simulator (OSIM) is a high-fidelity, cryogenic, telescope simulator that features a ~1.5 meter diameter powered mirror. OSIM is used to test the optical performance of the science instruments in the ISIM Element, including focus, pupil shear, and wavefront error. OSIM is aligned to the flight coordinate system in six degrees of freedom via OSIM-internal cryogenic mechanisms and feedback from alignment sensors. We highlight optical metrology methods, introduce the ISIM and the Science Instruments, describe the ambient alignment and test plan, the cryogenic test plan, and verification of optical performance of the ISIM Element in cryo-vacuum environment.
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%.
The James Webb Space Telescope Observatory will consist of three flight elements: (1) the Optical Telescope Element
(OTE), (2) the Integrated Science Instrument Module Element (ISIM), and (3) the Spacecraft Element. The ISIM
element consists of a composite bench structure that uses kinematic mounts to interface to each of the optical benches of
the three science instruments and the guider. The ISIM is also kinematically mounted to the telescope primary mirror
structure. An enclosure surrounds the ISIM structure, isolates the ISIM region thermally from the other thermal regions
of the Observatory, and serves as a radiator for the science instruments and guider. Cryogenic optical testing of the ISIM
Structure and the Science Instruments will be conducted at Goddard Space Flight Center using an optical telescope
simulator that is being developed by a team from Ball Aerospace and Goddard Space Flight Center, and other local
contractors. This simulator will be used to verify the performance of the ISIM element before delivery to the Northup
Grumman team for integration with the OTE. In this paper, we describe the O
OTE Sim TE Simulator (OSIM) and provide a brief
overview of the optical test program.
ulator
KEYWORDS: James Webb Space Telescope, Imaging systems, Systems modeling, Diffraction, Data modeling, Process control, Mirrors, Monochromatic aberrations, Radio propagation, Fourier transforms
Pupil imaging performance is analyzed from the perspective of physical optics. A multi-plane diffraction model is constructed by propagating the scalar electromagnetic field, surface by surface, along the optical path comprising the pupil imaging optical system. Modeling results are compared with pupil images collected in the laboratory. The experimental setup, although generic for pupil imaging systems in general, has application to the James Webb Space Telescope (JWST) optical system characterization where the pupil images are used as a constraint to the wavefront sensing and control process. Practical design considerations follow from the diffraction modeling which are discussed in the context of the JWST Observatory.
KEYWORDS: James Webb Space Telescope, Point spread functions, Space telescopes, Telescopes, Sensors, Code v, Linear filtering, Mirrors, Cameras, Wavefronts
The James Webb Space Telescope (JWST) is a segmented deployable telescope that will require on-orbit alignment
using the Near Infrared Camera as a wavefront sensor. The telescope will be aligned by adjusting seven degrees of
freedom on each of 18 primary mirror segments and five degrees of freedom on the secondary mirror to optimize the
performance of the telescope and camera at a wavelength of 2 microns. With the completion of these adjustments, the
telescope focus is set and the optical performance of each of the other science instruments should then be optimal
without making further telescope focus adjustments for each individual instrument. This alignment approach requires
confocality of the instruments after integration and alignment to the composite metering structure, which will be verified
during instrument level testing at Goddard Space Flight Center with a telescope optical simulator. In this paper, we
present the results from a study of several analytical approaches to determine the focus for each instrument. The goal of
the study is to compare the accuracies obtained for each method, and to select the most feasible for use during optical
testing.
The James Webb Space Telescope (JWST) is a space-based, infrared observatory designed to study the early stages of galaxy formation in the Universe. The telescope will be launched into orbit about the second Lagrange point and passively cooled to 30-50 K to enable astronomical observations from 0.6 to 28 μm. A group from the NASA Goddard Space Flight Center and the Northrop Grumman Space Technology prime contractor team has developed an optical and mechanical layout for the science instruments within the JWST field of view that satisfies the mission requirements. Four instruments required accommodation within the telescope’s field of view: a Near-Infrared Camera (NIRCam), a Near-Infrared Spectrometer (NIRSpec), a Mid-Infrared Instrument (MIRI) and a Fine Guidance Sensor (FGS) with a tunable filter module. The size and position of each instrument’s field of view allocation were developed through an iterative, concurrent engineering process involving key observatory stakeholders. While some of the system design considerations were those typically encountered during the development of an infrared observatory, others were unique to the deployable and controllable nature of JWST. This paper describes the optical and mechanical issues considered during the field of view layout development, as well as the supporting modeling and analysis activities.
The James Webb Space Telescope (JWST) Observatory, the follow-on mission to the Hubble Space Telescope and to the Spitzer Space Facility, will yield astounding breakthroughs in the realms of infrared space science. The science instrument suite for this Observatory will consist of a Near-Infrared Camera, a Near-Infrared Spectrograph, a Mid-Infrared Instrument with imager, coronagraph and integral field spectroscopy modes, and a Fine Guider System Instrument with both a Guider module and a Tunable Filter Module. In this paper we present an overview of the optical designs of the telescope and instruments.
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