We compare the optical performance, alignment sensitivity, and thermal stability of a Non-Uniform Rational B-Spline (NURBS) freeform telescope design to two more conventional design forms with the goal of facilitating acceptance of this new optical surface for aerospace applications. We present the designs of three three-mirror anastigmat (TMA) wide field (4°) telescopes with identical first order optical design parameters. These TMAs consist of a conventional design using off-axis aspheric mirrors, a freeform design using off-axis Zernike polynomial surfaces, and a freeform design using NURBS surfaces. Of the three, the NURBS design gives the best image quality and lowest geometrical design residual. The three designs have similar misalignment sensitivities and sensitivity to thermal soaks, countering a common misconception that freeform designs are more sensitive to misalignment than conventional designs.
An off-axis, three-mirror anastigmat was optically aligned for minimum wavefront error using three different analysis methods to improve alignment efficiency. The three methods involved CODE V Automatic Design (AUT), CODE V Alignment Optimization (ALI), as well as a Zernike Sensitivity Analysis (SENS). Not all methods converged on the same solution during alignment, but all tools were used in unison to optimize the optical alignment process. During initial optical alignment, the AUT tool better estimated the proper magnitude of the required alignment. As alignment progressively became finer, the ALI and SENS tools both produced superior, more in-family alignment solutions. Conclusively, depending on the coarseness of the optical alignment required, all alignment strategies have their merits, but most importantly each tool provides a check against other alignment solutions. Therefore, all tools aid in directing the optical alignment towards a global minimum.
The Transiting Exoplanet Survey Satellite (TESS) will carry four visible waveband seven-element refractive f/1.4 lenses, each with a 34 degree diagonal field of view. This paper describes the tolerancing, assembly and alignment methods developed during the build of the TESS Risk Reduction Unit optical system. Lens assembly tolerances were derived from a sensitivity analysis using an image quality metric customized for mission performance. The optomechanical design consists of a two-stage lens housing that provides access for active alignment of each lens using a Trioptics OptiCentric measurement system. Thermal stresses and alignment shifts are mitigated by mounting the optics with cast RTV silicone spacers into individually aligned bezels, and custom fixtures were developed to aid in RTV bonding with reduced alignment error. The lens assembly was tested interferometrically over the field of view at room temperature and results were used to successfully predict lens performance and compensator adjustments and detector shim thickness for the -75C operational temperature and pressure.
The designs of two imaging freeform systems using nonuniform rational basis-spline (NURBS) optical surfaces are described. The first system, a 10 deg×9 degf/2 three-mirror anastigmat has four times higher spatial resolution over the image plane compared with the equivalent conventional rotational aspheric design, and 2.5 times higher resolution compared with a 10th-order XY polynomial freeform design. The mirrors for the NURBS freeform design have more than twice the asphericity than the conventional rotational and XY polynomial designs. In the second system, a Ritchey–Chretien telescope followed by a two-mirror NURBS freeform corrector is compared to a four-mirror Korsch telescope, for imaging to a visible-infrared imaging spectrometer. The freeform corrector design had 70% smaller spot sizes over the field and eliminated the large tertiary required in Korsch type design. Both of these NURBS freeform designs are possible due to a custom optical design code for fast accurate NURBS optimization, which now has parallel raytracing for thousands of NURBS grid points.
The Transiting Exoplanet Survey Satellite, a NASA Explorer-class mission in development, will discover planets around
nearby stars, most notably Earth-like planets with potential for follow up characterization. The all-sky survey requires a
suite of four wide field-of-view cameras with sensitivity across a broad spectrum. Deep depletion CCDs with a silicon
layer of 100 μm thickness serve as the camera detectors, providing enhanced performance in the red wavelengths for
sensitivity to cooler stars. The performance of the camera is critical for the mission objectives, with both the optical
system and the CCD detectors contributing to the realized image quality. Expectations for image quality are studied
using a combination of optical ray tracing in Zemax and simulations in Matlab to account for the interaction of the
incoming photons with the 100 μm silicon layer. The simulations include a probabilistic model to determine the depth of
travel in the silicon before the photons are converted to photo-electrons, and a Monte Carlo approach to charge diffusion.
The charge diffusion model varies with the remaining depth for the photo-electron to traverse and the strength of the
intermediate electric field. The simulations are compared with laboratory measurements acquired by an engineering unit
camera with the TESS optical design and deep depletion CCDs. In this paper we describe the performance simulations
and the corresponding measurements taken with the engineering unit camera, and discuss where the models agree well in
predicted trends and where there are differences compared to observations.
The optical design of the wide field of view refractive camera with a 34 degree diagonal field for the TESS payload is described. This fast f/1.4 cryogenic camera, operating at -75°C, has no vignetting for maximum light gathering within the size and weight constraints. Four of these cameras capture full frames of star images for photometric searches of planet crossings. The optical design evolution, from the initial Petzval design, takes advantage of Forbes aspheres to develop a hybrid design form. This maximizes the correction from the two aspherics resulting in a reduction of average spot size by sixty percent in the final design. An external long wavelength pass filter has been replaced by an internal filter coating on a lens to save weight, and has been fabricated to meet the specifications. The stray light requirements are met by an extended lens hood baffle design, giving the necessary off-axis attenuation.
An interferometer for measuring dynamic properties of the in vivo tear film on the human cornea has been developed. The system is a near-infrared instantaneous phase-shifting Twyman-Green interferometer. The laser source is a 785 nm solid-state laser, and the system has been carefully designed and calibrated to ensure that the system operates at eye-safe levels. Measurements are made over a 6 mm diameter on the cornea. Successive frames of interferometric height measurements are combined to produce movies showing both the quantitative and qualitative changes in the topography of the tear film surface and structure. To date, measurement periods of up to 120 s at 28.6 frames per second have been obtained. Several human subjects have been examined using this system, demonstrating a surface height resolution of 25 nm and spatial resolution of 6 μm. Examples of features that have been observed in these preliminary studies of the tear film include postblink disruption, evolution, and stabilization of the tear film; tear film artifacts generated by blinking; tear film evaporation and breakup; and the propagation of foreign objects in the tear film. This paper discusses the interferometer design and presents results from in vivo measurements.
A long-term research program has been in place at the College of Optical Sciences to apply interferometry to ophthalmic applications. These unique systems have been developed in response to industrial need. The first system is a transmission Mach-Zehnder interferometer used to measure the transmitted wavefront of a contact lens while it is submersed in saline. This interferometer allows the refractive power distribution of the lens to be measured. A second system makes use of a low-coherence interferometer to measure the index of refraction of contact lens materials. This task is complicated by the fact that the material is only available in very thin, flexible samples, and because the sample must remain hydrated in saline during the measurement. A third system also makes use of low-coherence interferometry to characterize the surface profile of both surfaces of a contact lens. Combined with index information, a complete model of the contact lens can be produced. Two additional interferometers examine the dynamics of fluid layers on the surface of a contact lens (in vitro) and of the tear film on the surface of the cornea (in vivo). Both systems are instantaneous phase shifting Twyman-Green interferometers. The evolution and changes to the fluid surface is measured at video rates with sub-wavelength precision. This paper tells the story of this research program.
An interferometer for measuring dynamic properties of the in vivo tear film on the human cornea has been developed. The
system is a near-infrared instantaneous phase-shifting Twyman-Green interferometer. The laser source is a 785 nm solidstate
laser; the system has been carefully designed and calibrated to ensure that the system operates at eye safe levels.
Measurements are made over a 6 mm diameter on the cornea. Successive frames of interferometric height measurements
are combined to produce movies showing both the quantitative and qualitative changes in the topography of the tear film
surface and structure. To date, measurement periods of up to 120 seconds at 28.6 frames per second have been obtained.
Several human subjects have been examined using this system, demonstrating a surface height resolution of 25 nm and
spatial resolution of 6 μm. Examples of features that have been observed in these in preliminary studies of the tear film
include: post-blink disruption, evolution, and stabilization of the tear film; tear film artifacts generated by blinking; tear
film evaporation and break-up; and the propagation of foreign objects in the tear film. This paper discusses the
interferometer design and presents results from in vivo measurements.
The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of the contact lens. Under normal conditions, this fluid layer is less than 10 μm thick. The fluid layer thickness and topography change over time and are affected by the material properties of the contact lens and may affect vision quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on mechanical substrates has been developed by use of a phase-shifting Twyman-Green interferometer. This interferometer continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid layer. Movies showing this fluid layer behavior can be generated. Quantitative analysis beyond typical contact angle or visual inspection methods is provided. Different fluid and contact lens material combinations have been evaluated, and variations in fluid layer properties have been observed. This paper discusses the interferometer design and analysis methods used. Example measurement results of different contact lens are presented.
Ocular interferometry has potential value in a variety of ocular measurement applications, including measuring ocular thicknesses, topography of ocular surfaces or the wavefront of the eye. Of particular interest is using interferometry for characterizing corneal shape and irregular corneal features, making this technology attractive due to its inherent accuracy and spatial resolution. A particular challenge of designing an ocular interferometer is determining safe laser exposure levels to the eye, including both the retina and anterior segment. Described here are the laser exposure standards relevant in the interferometer design and the corresponding calculations and results. The results of this work can be used to aid in the design of similar laser-based systems for ocular evaluation.
The anterior refracting surface of the eye is the thin tear film that forms on the surface of the cornea. Following a blink, the tear film quickly smoothes and starts to become irregular after 10 seconds. This irregularity can affect comfort and vision quality. An in vivo method of characterizing dynamic tear films has been designed based upon a near-infrared phase-shifting interferometer. This interferometer continuously measures light reflected from the tear film, allowing sub-micron analysis of the dynamic surface topography. Movies showing the tear film behavior can be generated along with quantitative metrics describing changes in the tear film surface. This tear film measurement allows analysis beyond capabilities of typical fluorescein visual inspection or corneal topography and provides better sensitivity and resolution than shearing interferometry methods. The interferometer design is capable of identifying features in the tear film much less than a micron in height with a spatial resolution of about ten microns over a 6 mm diameter.
This paper presents the design of the tear film interferometer along with the considerations that must be taken when designing an interferometer for on-eye diagnostics. Discussions include eye movement, design of null optics for a range of ocular geometries, and laser emission limits for on-eye interferometry.
The anterior refracting surface of the eye when wearing a contact lens is the thin fluid layer that forms on the surface of
the contact lens. Under normal conditions, this fluid layer is less than 10 microns thick. The fluid layer thickness and
topography change over time and are affected by the material properties of the contact lens, and may affect vision
quality and comfort. An in vitro method of characterizing dynamic fluid layers applied to contact lenses mounted on
mechanical substrates has been developed using a phase-shifting Twyman-Green interferometer. This interferometer
continuously measures light reflected from the surface of the fluid layer, allowing precision analysis of the dynamic fluid
layer. Movies showing this fluid layer behavior can be generated. The fluid behavior on the contact lens surface is
measured, allowing quantitative analysis beyond what typical contact angle or visual inspection methods provide.
The interferometer system has measured the formation and break up of fluid layers. Different fluid and contact lens
material combinations have been used, and significant fluid layer properties have been observed in some cases. The
interferometer is capable of identifying features in the fluid layer less than a micron in depth with a spatial resolution of
about ten microns. An area on the contact lens approximately 6 mm wide can be measured with the system.
This paper will discuss the interferometer design and analysis methods used. Measurement results of different material
and fluid combinations are presented.
This past spring a new for-credit course on illumination engineering was offered at the College of Optical Sciences at
The University of Arizona. This course was project based such that the students could take a concept to conclusion. The
main goal of the course was to learn how to use optical design and analysis software while applying principles of optics
to the design of their optical systems. Projects included source modeling, displays, daylighting, light pollution, faceted
reflectors, and stray light analysis. In conjunction with the course was a weekly lecture that provided information about
various aspects of the field of illumination, including units, étendue, optimization, solid-state lighting, tolerancing, litappearance
modeling, and fabrication of optics. These lectures harped on the important points of conservation of
étendue, source modeling and tolerancing, and that no optic can be made perfectly. Based on student reviews, future
versions of this course will include more hands-on demos of illumination components and assignments.
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