To meet ambitious science goals and leverage NASA investments for the James Webb Space Telescope, some proposed mission concepts include large aperture telescopes with segmented primary mirrors. Aberration control at the segment level becomes critical for these architectures because rigid body motion of the individual mirrors overtakes full aperture aberrations as the driver for wavefront stability. Perturbations at the segment level cannot be effectively sensed by existing full pupil low-order wavefront sensors because the errors are discontinuous and low-order techniques applied to individual segments would have insufficient photon flux. Thus, an additional “mid-order” control loop is required. We propose the use of capacitive edge sensors to locally sense the relative motion of segments in piston, tip and tilt, which then provide input to a compensation arm. Ball Aerospace has developed capacitive sensor technology with proven measurement precision of <12 pm RMS that can be adapted for measuring primary mirror segment motion. This performance approaches the 10 pm stability often stated as a requirement for direct imaging of earth-like exoplanets with a coronagraph. Using the geometry of the existing hardware as a baseline, the sensor gap and plate area can be scaled to accommodate mounting to mirror segments while maintaining or increasing sensitivity and multiple plates in different orientations can be used to sense individual degrees of freedom. This paper will present measured results from the Ball capacitive sensor and use those results to develop expected sensitives and a notional sensor head geometry for stabilizing a large, segmented primary mirror with edge sensors.
The James Webb Space Telescope is a large, deployable telescope that will operate at cryogenic temperatures at the Earth-Sun Lagrange 2 point. The Webb Optical Telescope Element (OTE) consists of 18 actively controlled Primary Mirror Segment Assemblies (PMSAs), an actively controlled Secondary Mirror Assembly (SMA), and an Aft-Optics Subsystem (AOS) that contains a fixed Tertiary Mirror and a Fine Steering Mirror. The OTE is combined with the Integrated Science Instrument Module (ISIM) to create the full optical train called OTIS (OTE and ISIM).
OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.
We propose an innovative low-cost mission capable of detecting potentially habitable planets around a sample of solartype stars near the sun. The finding of rocky planets in temperate orbits among our immediate stellar neighbors will be a signature discovery. Our mission will deliver relative measurements of stellar position and motion at sub-micro arcsecond precision. These data, in turn, will reveal the presence of orbiting exoplanets. For the case of our primary targets Alpha Centauri A and B, objects below one Earth mass will be accessible when the end-of-mission astrometric precision requirement of 0.4 micro arcsecond is achieved. TOLIMAN will directly reveal the presence of sub-earth mass planets and constrain it orbit and mass This paper describes the optical and mechanical architecture of the mission, and first order instrument design. We also explain the instrument stability requirements imposed by the diffractive pupil post-processing calibration limitations. Our design baseline is a stable two-mirror telescope that images the field directly on CCD camera minimizing the number of reflections and optical components.
A subset of the Wavefront Sensing and Controls (WFSC) operations for JWST were demonstrated during its recent cryo-vac testing using the flight telescope and instruments, and a functional simulation of the spacecraft and ground system. The demonstration had three goals: to confirm the operation of the flight data collection scripts, to check the WFSC optical components, and to verify the coordinates and influence functions that will be used for flight WFSC. In this paper, we present the results and lessons learned from this demonstration.
The James Webb Space Telescope is a large space-based astronomical telescope that will operate at cryogenic temperatures. Because of its size, the telescope must be stowed in an inoperable configuration for launch and remotely reconfigured in space to meet the operational requirements using active Wave Front Sensing and Control (WFSC). Predicting optical performance for the flight system relies on a sequence of incremental tests and analyses that has culminated with the cryogenic vacuum test of the integrated Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) referred to as OTIS. The interplay between the optical budgeting process, test verification results at incrementally increasing levels of integration, use of test validated models, and the WFSC process to produce the final optical performance predictions for final verification by analysis will be presented.
Future astronomical telescopes in space will have architectures with complex and demanding requirements in order to meet their science goals. The missions currently being studied by NASA for consideration in the next Decadal Survey range in wavelength from the X-ray to Far infrared; examining phenomenon from imaging exoplanets and characterizing their atmospheres to detecting gravitational waves. These missions have technical challenges that are near or beyond the state of the art from the telescope to the detectors. This paper describes some of these challenges and possible solutions. Promising measurements and future demonstrations are discussed that can enhance or enable these missions.
Deflectometry has been proven as a high precision and high dynamic range surface metrology technique. We report on the use of deflectometry to diagnose mount-induced optical surface deformations. A surrogate mirror from the OLI-2 earthobserving satellite mission is tested with deflectometry in a non-null configuration using only a CCD camera and an LCD computer monitor. Moments are mechanically induced at each flight-like mirror mount and the deformed surface is measured. Systematic errors in the surface measurements are significantly reduced by maintaining a consistent measurement geometry and evaluating moment-induced deformations differentially. The surface deformation modes from orthogonal moments at each mirror mount are compared to FEA predictions. The agility of this metrology sets the groundwork for in situ measurements of flight aspheric mirror surface deformations during component integration and prior to system testing.
The Hobby-Eberly Telescope (HET) Wide Field Corrector (WFC) is a four-mirror optical system which corrects for aberrations from the 10-m segmented spherical primary mirror. The WFC mirror alignments must meet particularly tight tolerances for the system to meet performance requirements. The system uses 1-m class highly aspheric mirrors, which precludes conventional alignment methods. For the WFC system alignment a “center reference fixture” has been used as the reference for each mirror’s vertex and optical axis. The center reference fixtures have both a CGH and sphere mounted retroreflector (SMR) nests. The CGH is aligned to the mirror’s optical axis to provide a reference for mirror decenter and tilt. The vertex of each mirror is registered to the SMR nests on the center reference fixtures using a laser tracker. The spacing between the mirror vertices is measured during the system alignment using these SMR nest locations to determine the vertex locations. In this paper we present the procedures and results from creating and characterizing these center reference fixtures. As a verification of our alignment methods we also present results from their application in the WFC system alignment are also presented.
A procedure that uses computer-generated holograms (CGHs) to align an optical system’s meters in length with low uncertainty and real-time feedback is presented. The CGHs create simultaneous three-dimensional optical references, which are decoupled from the surfaces of the optics allowing efficient and accurate alignment even for systems that are not well corrected. The CGHs are Fresnel zone plates, where the zero-order reflection sets tilt and the first-diffracted order sets centration. The flexibility of the CGH design can be used to accommodate a wide variety of optical systems and to maximize the sensitivity to misalignments. An error analysis is performed to identify the main sources of uncertainty in the alignment of the CGHs and to calculate the magnitudes in terms of general parameters, so that the total uncertainty for any specific system may be estimated. A system consisting of two CGHs spaced 1 m apart is aligned multiple times and re-measured with an independent test to quantify the alignment uncertainty of the procedure. The calculated and measured alignment uncertainties are consistent with less than 3 μrad of tilt uncertainty and 1.5 μm of centration uncertainty (1σ ).
We characterize the precision of a low uncertainty alignment procedure that uses computer generated holograms as
center references to align optics in tilt and centration. This procedure was developed for the alignment of the Wide Field
Corrector for the Hobby Eberly Telescope, which uses center references to provide the data for the system alignment.
From previous experiments, we determined that using an alignment telescope or similar instrument would not achieve
the required alignment uncertainty. We developed a new procedure that utilizes computer generated holograms to create
multiple simultaneous images to perform the alignment. The center references are phase etched Fresnel zone plates that
act like thin lenses. We use zero order reflections to measure tilt and first order imaging from the zone plates to measure
centration. We performed multiple alignments with a prototype system consisting of two center references spaced one
meter apart to characterize this method's performance. We scale the uncertainties for the prototype experiment to
determine the expected alignment errors in the Wide Field Corrector.
We characterize the precision of five approaches used to align a series of targets over a distance of two meters. For
many applications, an alignment telescope provides the necessary precision for positioning targets. However, for
systems with tight tolerances, we must have a measure of the uncertainties in the alignment telescope to determine if it
can truly meet the system requirements. We develop a procedure to measure the precision of each alignment approach
and compare their performances. We use a telescope constructed from off-the-shelf optics and mechanics to determine
if we can obtain alignment precision comparable to an alignment telescope of superior optical quality.
We present a study of the imaging of the interference of spatial-helical modes of single photons. This work
includes a mathematical treatment that accounts for the direction of propagation and spatial mode degrees of
freedom in the situation where light travels through an interferometer that prepares the light in distinct spatial
modes and makes them interfere. We present results of the interference at the single photon level of the spatialhelical
modes with topological charge 1 and 0. The results are consistent with the expectation that each photon
carries the entire spatial mode information.