A set of Risley prisms has been designed that will allow an infrared optical and radio frequency (RF) beam to be simultaneously steered through the same aperture (patent pending). Risley prisms have the advantage of allowing such beams to be steered at the skin of a vehicle such as an aircraft. By providing a mechanism for both of these bands to be steered simultaneously, substantial system size, weight, and power reductions result, making integration of these two bands onto any platform more attainable. The prisms are made of single-crystal silicon, and have special coatings applied to them that reduce surface reflections for optical signals. Additionally, an impedance matching dielectric is applied to the prisms that allow efficient transmission of RF signals. The prisms are mounted in direct drive rotary motors with position feedback resolution of 6 microradians. A set of these prisms can steer beams within a conical field of regard of 120°, has a weight of about 6kg, and a size of about 175mm x 175mm x 100mm. We present some preliminary characteristics of this device, including C-band (~1550nm) optical and Ka RF (38GHz) throughput of the beams through the AR-coated and impedance-matched silicon, and an overview of Risley prism beam steering methodology.
We develop the Pediatric Vision Screener (PVS) to automatically detect ocular misalignment (strabismus) and defocus in human subjects. The PVS utilizes binocular retinal birefringence scanning to determine when both eyes are aligned, with a theoretical accuracy of <1 deg. The device employs an autoconjugate, bull's-eye detector-based system to detect focus. The focus and alignment pathways are separated by both wavelength and data acquisition timing. Binocular focus and alignment are detected in rapid alternating sequence, measuring both parameters in both eyes in <0.5 sec. In this work, the theory and design of the PVS are described in detail. With objective, automated measurement of both alignment and focus, the PVS represents a new approach to screening children for treatable eye disease such as amblyopia.
A nulling Phase Mask has been designed that is achromatic over a relatively wide wavelength range. The mask is made using a combination of Fused Silica and Silicon, resulting in a device that matches the needed dispersion to achieve a pi phase shift over a range of wavelengths. Used in a stellar coronagraph, this device could possibly produce on-axis nulls in excess of 60dB with a bandwidth of over 132nm, or 50dB nulls with a bandwidth of over 230nm. The design integrates a simple method of tuning the depth of the null to match observing conditions and bandwidth requirements, while also moving the center of the nulling band slightly. Such tuning capability allows for a dramatic simplification of the manufacturing tolerances, thereby substantially reducing the likely cost to manufacture. Tuning is accomplished by placing the mask in a chamber with a gas that has an accurately controllable pressure. This mask works in a similar manner whether the geometry is of the four-quadrant type or half-half type, but does not apply to circular phase masks.
Challenges in fabrication and testing have historically limited the choice of surfaces available for the design of reflective optical instruments. Spherical and conic mirrors are common, but, for future science instruments, more degrees of freedom will be necessary to meet performance and packaging requirements. These instruments will be composed of surfaces of revolution located far off-axis with large spherical departure, and some designs will even require asymmetric surface profiles. We describe the design and diamond machining of seven aluminum mirrors: three rotationally symmetric, off-axis conic sections, one off-axis biconic, and three flat mirror designs. These mirrors are for the Infrared Multi-Object Spectrometer instrument, a facility instrument for the Kitt Peak National Observatory’s Mayall Telescope (3.8 m) and a pathfinder for the future Next Generation Space Telescope multi-object spectrograph. The symmetric mirrors include convex and concave prolate and oblate ellipsoids, and range in aperture from 92 x 77 mm to 284 x 264 mm and in f-number from 0.9 to 2.4. The biconic mirror is concave and has a 94 x 76 mm aperture, (formula available in paper) and is decentered by -2 mm in x and 227 mm in y. The mirrors have an aspect ratio of approximately 6:1. The fabrication tolerances for surface error are < 63.3 nm RMS figure error and < 10 nm RMS microroughness. The mirrors are attached to the instrument bench using semi-kinematic, integral flexure mounts and optomechanically aligned to the instrument coordinate system using fiducial marks and datum surfaces. We also describe in-process profilometry and optical testing.
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.
The Infrared Multi-Object Spectrometer (IRMOS) is an innovative near-IR instrument approaching completion. IRMOS will provide R~300, 1000, and 3000 spectroscopy in the J, H, and K bands plus R~1000 in Z together with imaging in all bands. Using a Texas Instruments 848x600 element DMD as a micro mirror array to synthesize slits in an imaging spectrometer obtaining up to 100 simultaneous spectra will be possible. Designed for the KPNO 4 and 2.2 meter telescopes, IRMOS will provide 3x2 and 6x4 arc minute fields of view on these telescopes. IRMOS is constructed mainly of 6061 Aluminum using diamond machined optics which has permitted a complex, compact, all reflective optical design. We describe the design and status of IRMOS, summarize its expected performance, and discuss several interesting aspects of its development and the use of TI DMD devices. IRMOS is a joint project of the Space Telescope Science Institute, the NASA Next Generation Space Telescope Project, and the Kitt Peak National Observatory.
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 design of a detector array intended for use as a Shack-Hartman wavefront sensor is presented. The chip will output voltages that represent local tilts and currents that represent inferred piston information by processing the optical signals from a lenslet array. Each pixel in the array contains four phototransistors, arranged in a square, along with transistors needed to perform algorithms necessary to output signals. Each pixel has four built-in amplifiers to output a voltage for each of the four phototransistors. By combining these voltages off-chip, tilt information is obtained, and allows for positive and negative tilts to be output from a single-supply chip. All pixels contain a self-biasing circuit, allowing tilt information to be consistent across all pixels for apertures that are not uniformly illuminated. Algorithms for inferring piston information are generated by using CMOS transistors in current mode. The piston information is output as a current with a magnitude that is proportional to the actual piston value. The pixels are selected by means of bit-parallel row and column select. The outputs consist of four voltages, four currents to monitor the phototransistor behavior, and one current representing piston. There are inputs that allow the user to set a global piston offset, and overall bias for overriding the amplification of the tilt voltages. Simulations of this design suggest refresh rates in excess of 2kHz are easily attainable. The design of this wavefront sensor is optimized for low light levels and high refresh rates. Prototype detectors will be fabricated using a 0.5micron CMOS process. This detector could greatly simplify the process of wavefront reconstruction, and could even allow for direct hardware control of deformable mirrors in adaptive optics systems with an order of magnitude more channels than are currently used.
A wavefront sensing detector array is presented with capabilities suited towards high-order adaptive optics systems. The phase of the wavefront is sensed by modulating and synchronously sampling the fringes of a white light interferogram imaged onto the array. The nature of the modulation is characterized by a voltage signal, which is delivered as an input to the array. As a fringe moves across an individual detector, the null is sensed, triggering a sampling of the modulation signal. The sampled signal represents the phase of the wavefront and is held until the next null is sensed. The signal makes it possible to directly send commands to a deformable mirror to correct the phasefront. This chip is optically mapped such that each pixel in the array corresponds to an actuator on the deformable mirror. The array outputs both the photodetector current and the sampled modulation signal voltage from individual pixels by means of bit parallel row and column inputs. A prototype of this array has been fabricated in a 0.5μm CMOS process with 21 × 21 detectors, suitable for (circular) deformable mirrors with up to 349 actuators. Experimental results suggest refresh rates in excess of 3kHz are attainable. This wavefront sensor could greatly simplify the process of controlling a deformable mirror for many applications, thereby increasing refresh rates and improving sensitivity.
The optical design for an Infrared Multiple Object Spectrometer (IRMOS) intended for Astronomical research is presented. To accomplish spectroscopy of multiple objects simultaneous, IRMOS utilizes a Micro- Mirror array (MMA) as an electronically controlled slit device. This approach makes object selection simple and offers great versatility for performing spectral analysis on many objects within a field location. Furthermore, it allows a field location to be imaged without spectra prior to object selection. The optical design of IRMOS has two distinct stages. The first stage reduces an f/15 incoming beam to f/4.5, with a tilted focal plane located at the MMA (the MMA removes some of the tilt of the focal plane, since the micro-mirrors tilt individually). The second stage consists of the spectrometer, capable of resolutions of 300, 1000, and 3000 in the astronomical J, H and K bands. This stage transforms the tilted focal plane into a collimated pupil on a grating, and then re-images onto a HAWAII detector. When used with the Kitt Peak National Observatory 4 meter telescope, a plate scale of approximately equals 0.2 arcseconds per pixel is realized at both the MMA and the detector. A total of 6 mirrors are used, two flat fold mirrors, two off-axis concave aspheres, one off-axis convex asphere, and one off-axis concave biconic mirror. The selection of a biconic surface in this design helped reduce the overall size of the instrument by reducing the size and number of necessary mirrors, simplifying alignment.
We simulate the actions of a coronagraph matched to diffraction-limited adaptive optics (AO) systems on the Calypso 1.2 m, Palomar Hale 5 m and Gemini 8.1 m telescopes, and identify useful parameter ranges for AO coronagraphy on these systems. We model the action of adaptive wavefront correction with a tapered, high-pass filter in spatial frequency rather than a hard low frequency cutoff, and estimate the minimum number of AO channels required to produce sufficient image quality for coronagraphic suppression within a few diffraction widths of a central bright object (as is relevant to e.g., brown dwarf searches near late-type dwarf stars). We explore the effect of varying the occulting image- plane stop size and shape, and examine the trade-off between throughput and suppression of the image halo and Airy rings. We discuss our simulations in the context of results from the 241-channel Palomar Hale AO coronagraph system, and suggest approaches for future AO coronagraphic instruments on large telescopes.
A design for a Deformable Mirror (DM) with closely spaced actuators is presented. The DM surface is made of a thin membrane type glass with a thickness of 300 microns. It is supported by a series of piezoelectric actuator tubes with a square grid spacing of 4 mm. A conventional epoxy bond is used to hold the actuators to the membrane, with a small steel ball interfacing between the two (at each actuator) for desirable deformation characteristics. The actuators are also bonded to a base structure made of commercially pure Titanium to help athermalize the overall design. The base structure is designed to protect the epoxy bonds from atmospheric moisture. With this design, an actuator could be capable of up to 7 microns of displacement with respect to neighboring actuators, allowing for considerable ability to correct wavefront error in a compact design. Such capability is required to achieve the goals of adaptive coronagraph systems on large, ground based telescopes. Low cost is achieved through the use of inexpensive actuators and a relatively simple fabrication process.
We demonstrate the feasibility of glass membrane deformable mirror (DM) support structures intended for very high order low-stroke adaptive optics systems. We investigated commercially available piezoelectric ceramics. Piezoelectric tubes were determined to offer the largest amount of stroke for a given amount of space on the mirror surface that each actuator controls. We estimated the minimum spacing and the maximum expected stroke of such actuators. We developed a quantitative understanding of the response of a membrane mirror surface by performing a Finite Element Analysis (FEA) study. The results of the FEA analysis were used to develop a design and fabrication process for membrane deformable mirrors of 200 - 500 micron thicknesses. Several different values for glass thickness and actuator spacing were analyzed to determine the best combination of actuator stoke and surface deformation quality. We considered two deformable mirror configurations. The first configuration uses a vacuum membrane attachment system where the actuator tubes' central holes connect to an evacuated plenum, and atmospheric pressure holds the membrane against the actuators. This configuration allows the membrane to be removed from the actuators, facilitating easy replacement of the glass. The other configuration uses precision bearing balls epoxied to the ends of the actuator tubes, with the glass membrane epoxied to the ends of the ball bearings. While this kind of DM is not serviceable, it allows actuator spacings of 4 mm, in addition to large stroke. Fabrication of a prototype of the latter kind of DM was started.