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.