Details of a multispectral imaging radiometer specially designed to retrieve fire characteristics from a nanosatellite platform are presented. The instrument consists of an assembly of three cameras providing co-registered midwave infrared, longwave infrared, and visible image data. Preliminary evaluation of the instrument budgets showed approximately a mass of 12 kg, an envelope of 220×240×200 mm3, and an average power consumption of 13 W. A method was devised to stagger two linear arrays of 512×3 VOx microbolometers in each infrared detector assembly. Investigation of the first completed detector assemblies showed an alignment accuracy better than 10% of pixel pitch and a response uniformity achieved across 92% of the pixels. Effects of the thermal environment seen by the pixels were evaluated to optimize the radiometric packaging design. It was found that the resulting thermal stability of the arrays, combined with the available electronic dynamic range, allows acquisition of targets with temperatures as high as 750 K with the desired accuracy and without saturation. The detector assemblies were able to withstand extreme environments with vibration up to 14 grms and temperatures from 218 to 333 K. Exposing the assembly’s window and bandpass filter to proton and Co-60 gamma radiation with successive dose of 10 krad and 100 Gy resulted in no adverse effect on their transmittance characteristics. Performance characteristics of the assembled midwave and longwave infrared telescopes were consistent with modeling predictions. Results of the point spread function measurement supported the conclusion that the lenses alignment had been achieved within mechanical tolerances for both telescopes.
We report on the design and instrumentation of an aircraft-certified far infrared radiometer (FIRR) and the resulting instrument characteristics. FIRR was designed to perform unattended airborne measurements of ice clouds in the arctic in support of a microsatellite payload study. It provides radiometrically calibrated data in nine spectral channels in the range of 8-50 μm with the use of a rotating wheel of bandpass filters and reference blackbodies. Measurements in this spectral range are enabled with the use of a far infrared detector based on microbolometers of 104-μm pitch. The microbolometers have a new design because of the large structure and are coated with gold black to maintain uniform responsivity over the working spectral range. The vacuum sealed detector package is placed at the focal plane of a reflective telescope based on a Schwarschild configuration with two on-axis spherical mirrors. The telescope field-of-view is of ~6° and illuminates an area of ~2.1-mm diameter at the focal plane. In operation, FIRR was used as a nonimaging radiometer and exhibited a noise equivalent radiance in the range of 10-20 mW/m2-sr. The dynamic range and the detector vacuum integrity of FIRR were found to be suited for the conditions of the airborne experiments.
Wavefront sensing is one of the key elements of an Adaptive Optics System. Although Shack-Hartmann WFS are the
most commonly used whether for astronomical or biomedical applications, the high-sensitivity and large dynamic-range
of the Pyramid-WFS (P-WFS) technology is promising and needs to be further investigated for proper justification in
future Extremely Large Telescopes (ELT) applications. At INO, center for applied research in optics and technology
transfer in Quebec City, Canada, we have recently set to develop a Pyramid wavefront sensor (P-WFS), an option for
which no other research group in Canada had any experience. A first version had been built and tested in 2013 in
collaboration with NRC-HIA Victoria. Here we present a second iteration of demonstrator with an extended spectral
range, fast modulation capability and low-noise, fast-acquisition EMCCD sensor. The system has been designed with
compactness and robustness in mind to allow on-sky testing at Mont Mégantic facility, in parallel with a Shack-
Hartmann sensor so as to compare both options.
Lens positioning accuracy and manufacturing cost are two main concerns for optomechanical engineers looking for solutions to reduce costs while meeting stringent optical and environmental requirements. Minimizing optical component positioning errors generally translates into significant cost increases. To maximize the precision-to-cost ratio, there are significant advantages in having both an accurate optomechanical tolerance calculation method and an effective technique to mount and align lenses. This paper presents a tool that has been developed at INO to easily perform complex optomechanical statistical tolerancing using Monte Carlo simulation to reduce manufacturing and alignment costs. This tolerancing method provides a more realistic prediction of optical component errors compared to the classical worst case and root sum square calculations. In addition, precision alignment using elastomeric lens mounting is presented. Thermal stability and often overlooked factors for effective alignments are discussed. Results of tests performed on real optical assemblies are presented for tolerancing, thermal stability and alignment performance. The use of these methods can considerably reduce cost while efficiently ensuring compliance with requirements.
INO has designed, assembled and tested the Raven Multi-Object Adaptive Optics demonstrator calibration unit. This
sub-system consists in a telescope simulator that will allow aligning Raven's components during its integration, testing
its Adaptive Optics performances in the laboratory and at the telescope, and calibrating the Adaptive Optics system by
building the interaction matrix and measuring the non-common path aberrations. The system is presented with the
challenges that needed to be overcome during the design and integration phases. The system test results are also
presented and compared to the model predictions.
The Laser Tomographic Adaptive Optics system for Giant Magellan Telescope (GMT) uses a single conjugated
deformable mirror, the segmented Adaptive Secondary Mirror (ASM), to correct atmospheric wavefront aberrations with
the help of a constellation of six laser beacons equally spaced on the sky. We will present different approaches for the
design of the Laser Guide Star (LGS) Wave Front Sensor (WFS) system for GMT to cover all sodium emission altitudes
and telescope elevations, from 80 km to 200 km range and how the preliminary design was derived from these
approaches. The designed LGS WFS system includes a defocus-compensation mechanism working with a simple
zooming optics to achieve the pupil image with constant pupil size, nearly constant wavefront correction, as well as pupil
distortion correction. Either a trombone-mirror structure or a direct LGS-WFS translation is used for the defocus
compensation, when conjugating the LGS altitudes in the sky. In the designs, a zooming collimator images the ASM in
the GMT at the exit pupil of the LGS WFS system, where the designed lenslet-array is tailored for the selected CCD
format for the required plate scale on the sky. Additionally, we have proposed a novel and simple solution for pupilimage
segmentation when working with smaller CCD arrays. This novel solution consists of a single multi-aperture
blaze grating for pupil segmentation in the system.
As scientific quests and engineering applications reach down to a nanometer scale, there is a strong need to fabricate
three-dimensional nanostructures with regularity and controllability in their pattern, size, and shape, including chirped
pitch. Interference lithography is considered to be the most efficient way to make submicron-scale periodic patterns over
a large area with superior control of pattern regularity. In order to make a chirped nano-patterning with interference
lithography technology, we propose a zoom-chirped interferometer as a novel manufacturing tool with more flexibility
for 3-D nano-patterning fabrication. The optical concept of this novel zoom-chirped interferometer is based on the
Fresnel division-of-wavefront (DOW) interference combined with a pair of half-cylindrical zoom lenses, which are
located in front of a Fresnel 90°-mirror, to generate a cylindrical wavefront interfering with a plane wavefront in the
system. The collimated beam will be used as the incident beam for the interferometer. The interference patterning
between the cylindrical and plane wavefront is fully controlled, and the variation of the required chirped rate can be
adjusted by moving one half-cylindrical lens in the system. This novel zoom-chirped interferometer is more flexible and
stable than conventional interference-lithography systems for micro and nano-patterning fabrication.
In space infrared (IR) optics, to achieve better observation of ground target, a common aperture all-reflective telescope,
working at fast focal-ratio with multi-channel to cover different waveband and wide field-of-view (FOV), is a most
wanted optical system. The remarkable imaging properties of the fast focal-ratio, flat-field, anastigmatic, rotationally
symmetric Schwarzschild telescope have been well recognized historically, but suffer strong central obscuration and
limited FOV in the conventional axis-symmetric design. Our solution is to use an eccentric system evolved from the
Schwarzschild axially symmetric system, adding a tertiary off-axis mirror, to optimize the off-axis performance with the
appropriate system parameters and specs, as required by most space IR optical systems. The optical design system
consists of three powered mirrors, in which the primary (M1) is convex and secondary (M2) is either convex or concave,
with a tertiary (M3) always in concave shape respectively. Both secondary and tertiary mirrors have their size larger than
that of the primary. The entrance pupil of the system is projected behind M1. Dichroic filters can be used after the
tertiary mirror to achieve separation of multi-spectral channels. In the designs the mirrors with optimized aspherical
shapes, which are all in even-asphere warped up to 10th asphericities, are used for achieving the final image quality. The
final corrected wavefront in the system can result in the good optical performance with an encircled energy of better than
80% per pixel for all channels, working at F/1.66 to correct a wide FOV up to 27.70 (H) x 48.70 (V). The design is
scalable for different image scales, as usually required for different optical systems targeting different applications. The
broad spectral range from mid-wave infrared (MWIR) up to Far IR can be fully covered by this design. Multiple focalplane-
arrays (FPAs) can be used with respect to different spectral channels in the optical systems.
As a ground-layer adaptive optics (GLAO) system can correct the wavefront errors caused by turbulence close to the
Canada-France-Hawaii telescope (CFHT), an intensive study is in progress to determine the feasibility and the
pertinence of equipping the CFHT of such a GLAO system. More specifically, the study concerns the implementation
of GLAO capabilities using a deformable mirror inserted into the optical path of an optical relay. The studied system
called IMAKA would be used both for the dynamic correction of the wavefront errors caused by air turbulence and the
increases of the telescope effective field of view. The objective pursued by IMAKA is to achieve a PSF with Full Width
at Half Maximum of less than 0.15" over a 1-degree field of view for extended wavebands within the spectral waveband
of 470 nm - 900 nm. This paper presents the main results of a study conducted by INO about the optical design
challenges of the IMAKA system. INO has proposed 4 different approaches for the realization of the system and made
preliminary optical designs for each of them. The science camera and deformable mirror in the proposed designs are
located below the Cassegrain environment for three of the proposed configurations and between the primary mirror and
the top ring for the fourth design. In all the proposed configurations, the effective focal length of the telescope with the
added correction relay is about 20.63 m for a working focal ratio of about 5.74. The design configurations included in
this paper have achieved nearly diffraction limited performances with a deformable mirror having a diameter inferior to
0.5 m and flat or mild curvature nominal shape. Based on our preliminary optical design and performance analysis with
the 4 optical design approaches, it seems possible to achieve most of the IMAKA requirements.
The success of the high resolution nightglow studies conducted with the Keck telescopes on Mauna Kea and the Very
Large Telescopes in Chile led to the design of the Compact Echelle Spectrograph for Aeronomical Research (CESAR).
This is an echelle spectrograph with grating post-dispersion that will be dedicated to nightglow studies at high spectral
resolution (R ~ 20000) between 300-1000 nm, and that will be easily deployable at different sites. The development of
CESAR is conducted by SRI International, and INO is involved in the optical design and integration of the spectrograph
camera, whose all-spherical form is based on the camera of the HIRES spectrograph at the Keck I telescope. The
detailed optical design is used to calculate the position of the spectral elements on the detector, predict their image
quality, and estimate the level of stray light. This paper presents the methodology used in these analyses.
The Platform for the Observation of the Earth and for in-orbit Technology Experiments (POETE) mission concept has
been developed to help overcome the scientific and socio-economic issues associated with forest fires. The proposed
mission is based on a series of two highly autonomous and agile microsatellites, allowing for 3 to 7 visits per day. Each
satellite payload includs a VIS-NIR instrument and a MWIR-TIR instrument. The two instruments combined provide for
6 spectral channels spanning from the visible to the thermal infrared for fire monitoring, retrieval of quantitative fire
parameters (such as effective fire temperature, area and radiative energy release), and land surface temperature
measurement. The MWIR-TIR instrument concept is a pushbroom scanner filter radiometer with on-board radiometric
calibration capabilities. Its all-reflective three-mirror input optics delivers a 400-m GSD at an altitude of 700 km,
relaying the scene signal to detectors based on INO's microbolometer technology for detection in four spectral channels
centered at 3.8 μm, 8.8 μm, 10.5 μm and 12.0 μm. This paper presents an overview of the key mission requirements and
derived sensor level requirements. A description of the conceptual design of the MWIR-TIR payload of POETE is given
along with estimates of key performance parameters.
A unique collimated display concept has been developed by INO and its partners for wide angle immersive display
applications. The concept involves the reflection of scanned beams inside a reflective dome having a symmetry of
revolution and an elliptical profile. The axis of revolution of the reflective dome coincides with the axis of rotation of
the scanning mirror. The nominal position of the observer's eyes is also located on the reflective dome's axis of
revolution. The scanning mirror is centered close to one of the foci of the ellipsoidal reflective dome while the eyes of
the observer are located close to the other ellipsoid focus point. The system projects only one line at the time and the full
image is constructed by rotating the image line around the observer by means of the scanning mirror.
Light is generated by a linear array of individually addressable light elements such as a linear array of deformable micro-mirrors
illuminated by a laser line or an array of LED. The beams of light produced by the linear source are conditioned using specialized optical elements and introduced into the system from the outside by transmission through the dome shell.
A null-lens based on a Computer Generated Hologram (CGH) is designed to test the primary off-axis aspherical mirror of the GAIA space telescope. This custom-designed and fabricated CGH includes five zones (null CGH, alignment CGH, and beam-projection CGH) on the same substrate. The optical test configuration is simple and the designed five-zones CGHs can simultaneously provide the aberrated wavefront correction for null tests, CGH alignment with a commercial interferometer, pre-positioning of the mirror under test in a cryogenic chamber, and isolation of the diffraction orders in the test setup. Positioning of the five zones with respect to each other is extremely critical for the success of this custom-made CGH null testing. For this reason, the fringes of all five zones were manufactured on a single photolithographic mask. With INO's special micro-fabrication processes, including its photolithographic, etching, and coating technologies, this 4.5-inch in diameter CGH was successfully made. The RMS wavefront error is estimated at 7.33 nm for the fabricated null CGH.
The advent of wide-field imagers on large telescopes (Megacam at CFHT, Suprime-Cam at Subaru, and others)
with degree-wide fields of view is largely motivated by a renewed interest in our own solar system, in the history
of the Milky Way and its neighbors, and in the large-scale structure of the Universe. Smaller, university-based
telescopes can of course also benefit from wide-field imagery. We present in this paper the design and first
results of Panoramix-II, the new wide-field imager of the Mont Megantic Observatory (OMM). This instrument
is conceptually a focal reducer designed to image and correct the F/8 cassegrain focal plane of the telescope onto
a pair of 2KX4K EEV detectors. The camera is optimized for the SLOAN g' (410-550 nm), r' (550-690 nm), i'
(690-850 nm) and z' (850-950 nm) wave bands. The sky will be imaged onto the focal plane at an image scale
of 0.52 arcsecond per 13.5 μm pixel. The design image quality is 1.00 arcsecond 50% diffraction encircled
energy over the central 35 arcmin field and no images worse than 1.25 arcsecond over the 49 arcminute diameter
camera field. The optical design distortion at the corners is less than 1%. The Panoramix-II camera has a filter
wheel at the internal stop. Panoramix-II can also support the FaNTOmM photon-counting camera used in
conjunction with a Fabry-Perot interferometer to provide spectrometric data.
This paper reports on a novel pair of microlens arrays (MLA's) for efficient coupling of the high aspect ratio optical beam emitted by high-power laser diode linear arrays (also referred to as laser diode bars) into the core of multimode optical fibers. These novel MLA's overcome the limitations observed when using high fill factors laser diode bars. The MLA designs are described. Results from modelling work show good coupling performances for laser diode bars with fill factors up to 75%. The technique for fabricating the complex surface profiles of the MLA's is discussed. Masters are first fabricated and MLA's are then replicated, so that volume production at low cost can be envisioned. The fabricated MLA's have been used for reshaping and fiber coupling the output of a 10-mm laser diode bar. An efficiency of 74% has been obtained when coupling into an optical fiber having a core diameter of 400 μm and a numerical aperture of 0.22.
The control of optical distortion is useful for the design of a variety of optical system. The most popular is the F-theta lens used in laser scanning system to produce a constant scan velocity across the image plane. Many authors have designed during the last 20 years distortion control corrector. Today, many challenging digital imaging system can use distortion the enhanced their imaging capability. A well know example is a reversed telephoto type, if the barrel distortion is increased instead of being corrected; the result is a so-called Fish-eye lens. However, if we control the barrel distortion instead of only increasing it, the resulting system can have enhanced imaging capability. This paper will present some lens design and real system examples that clearly demonstrate how the distortion control can improve the system performances such as resolution. We present innovative optical system which increases the resolution in the field of view of interest to meet the needs of specific applications. One critical issue when we designed using distortion is the optimization management. Like most challenging lens design, the automatic optimization is less reliable. Proper management keeps the lens design within the correct range, which is critical for optimal performance (size, cost, manufacturability). Many lens design presented tailor a custom merit function and approach.
With the upcoming of the next-generation large-capacity CDs and DVDs using a blue-violet laser, the track's pitch will continue to decrease. The conventional methods for pregrooves will have a technique challenge to meet tiny-pitch specs at low cost. In order to get an equispaced fringe pattern with concentric-ring structure by using optical interference lithography, a novel optical-interference method is proposed with the use of a plane wave plus a conical wave along the same optical-axis (Z) propagation. Based on the optical interference analysis with the two waves, plane and conical waves, the resulting intensity varies consinusoidally with the phase difference, as modulated by the optical path difference between the plane and conical waves. The constructive and destructive interference will occur in the interference light field, only depending on if in phase or out of phase for the two waves. The concentric-ring structure of equispaced fringe pattern can be produced, and the fringe period depends on the wavelength of the light beams and conical constant of the conical wavefront as well. The novel optical interference system has been modeled and simulated with a kind of Mach-Zender interferometer arrangement by commercial optical code. The optical simulation results are proved to have a good agreement with that as given by the theoretical analysis.
It is well known that a fish-eye lens produces a circular image of the scene with a particular distortion profile. When using a fish-eye lens with a standard sensor (e.g. 1/3", 1/4",.), only a part of the rectangular detector area is used, leaving many pixels unused. We proposed a new approach to get enhanced resolution for panoramic imaging. In this paper, various arrangements of innovative 180-degree anamorphic wide-angle lens design are considered. Their performances as well as lens manufacturability are also discussed. The concept of the design is to use anamorphic optics to produce elliptical image that maximize pixel resolution in both axis. Furthermore, a non-linear distortion profile is also introduced to enhance spatial resolution for specific field angle. Typical applications such as panoramic photography, video conferencing, and homeland/transportation security are also presented.
This paper describes the fabrication of very high-sag (up to 42 μm) microlenses by direct laser writing and their integration onto a simple microoptical bench processed by conventional microfabrication technologies pertaining to MOEMS. At the heart of such a work is INO's laser writer. It is based on a He-Cd laser operating at 442 nm whose intensity can be modulated up to 1024 levels, and on a 40 nm accuracy X-Y translation stage. Laser writing into thick photoresist layers introduces however particular problems in terms of the roughness achievable. Simulations show that the writing beam diameter, the line-to-line spacing and the translation stage accuracy contribute to some unavoidable residual roughness. By applying optimized laser writing parameters, arrays of 1 x 5 aspherical microlenses were fabricated in a thick positive photoresist, along with alignment marks concurrently generated for on-chip alignment purposes. The microlenses were successfully integrated with a microoptical bench by first generating a UV-transparent mold from the photoresist laser written master. The microlenses imprinted in the mold were then replicated in a layer of hybrid glass material cast on the microoptical bench by UV-embossing with a modified MA6 mask aligner. The uniformity of focal lengths was approximately 3% as determined from best fits of profilometric traces. The replication with alignment of this array in a hybrid glass material was demonstrated on a 12 mm x 12 mm microoptical bench chip. An alignment accuracy of less than 5 μm was obtained. The replication error was less than about 4%. The measured surface roughness was 50-60 nm RMS, in good agreement with simulation results.
A fast (F/1.55) and wide-angle refractive camera objective has been developed to work with a large-size CCD on the focal plane of the spectrograph of Mont Megantique telescope for large spectral coverage from UV to VIS and up to NIR. The novel camera objective has been designed and optimized with three different glass combinations in order to have higher throughputs for large spectral coverage, especially in UV region. The tolerance analysis of the camera objective is given in the paper. The final opto-mechanical design of the objective is discussed with the consideration of the manufacturing tolerance in the optical and mechanical parts, as given in this paper. The used components have been minimized to reduce the light inherent lost. The optical testing results displayed the good optical performance of the camera with the required resolution for the whole FOV, as predicted by the optical simulation and computation. The broadband AR coating, enhanced on UV region, have been used on each surface of the lenses in the optical system.
Test and characterization of the optical performance for the novel and advanced sub-optical-systems, as designed for large astronomical optical telescopes, often need to be carried out with the simulated telescope wavefront in the labs before working with the telescopes. Computer generated hologram (CGH) is a simple solution for monochromatic wavefront generation. Severely off-axis aberrated wavefront coming from the one-mirror or two-mirrors telescope can be generated by CGH with symmetric and un-symmetric terms of the Zernike polynomials. The chrome mask and/or the binary CGHs have been made as the CFH telescope simulator. The unused diffraction orders of the CGH can be completely eliminated by adding the carrier frequency on the phase function of the designed CGHs. The test results that are given with the fabricated CGH simulator have a good agreement with that of the simulation. The aberrated wavefront of the simulated telescope can be also generated by the ferrofluid deformable mirror (FDM) at low cost. A 100-mm FDM is designed and built with 271 actuators for performance evaluation. The revolution symmetric aberrated wavefront has been experimentally generated and proven by the FDM prototyping. The simulation and test results for the prototyping FDM are both given in this paper.
A wavefront-generated simulator for Canada-France-Hawaii (CFH) 3.6-m telescope to test a novel wide-field IR camera (WIRCAM) of the telescope is presented. We investigate a Computer-Generated Hologram (CGH) wavefront generator to demonstrate off-axis aberrated wavefront generation with and with no rotational symmetry modes of Zernike polynomial as telescope wavefront simulator. A CGH simulator system to generate field optical performance of CFH telescope, while working on a F/3.8 prima focus with an field-of-view (FOV) of ±15 arcmin, is designed and computed for WIRCAM tests in optical labs for system performance evaluation. The CGH simulator is modeled as a diffractive surface defined by a phase function, and the optical phase function is specified by a Zernike polynomial phase equation. As the CGH simulator has to work in the first diffraction order, a carrier frequency must be added to the phase function in such a way to ensure the separation of the diffraction orders for eliminating unwanted light in the WIRCAM tests. Finally the nominal-design performance of the CGH simulator is compared with the tolerance analysis predictions as well as the system misalignment sensitivity.
All-reflective optics is conventionally required for extended spectral region observations in astronomical spectrograph. But the spatial resolution is usually not good enough while the large-size CCD will be used for observation in all-reflective optics. In this paper, all-refractive optics has been investigated to design a fast (F/1.55) and wide angle camera objective for large spectral coverage, from UV to VIS and up to NIR, when a large-size CCD is used on the focal plane of the spectrograph of Mont Megantique telescope. The case of achromatic and apochromatic condition has been investigated for axial and lateral color controls. The new proposed solutions have been optimized from two to three different glass combinations in order to have higher throughputs for large spectral coverage, especially in UV region. The used components have been minimized to reduce the light inherent lost. The monochromatic aberrations have been corrected and controlled by using optimized lens bending and shapes to make the camera have the CCD pixel resolution. Ray tracing results displayed the good optical performance of the camera to cover from 350 nm to 1000 nm spectral region with high resolution. The broadband AR coating, enhanced on UV region, will be used on each surface of the lenses in the camera. Final throughputs for the designed camera has been estimated and given in the paper.
We present three designs and tolerances of wide-field imagers (30'30 arc-minutes or larger) for astronomical surveying. Two infrared cameras (CPAPIR and PANORAMIX II) were designed for the 0.8-2.4 μm band and a third one (WIRCAM) for the visible and near-infrared band extending from 410 nm to 950 nm. The cameras are installed on the telescopes of the Canada-France-Hawaii (Hawaii, USA) and Mont Megantic Observatories (Quebec, Canada). The three cameras are compact, use only spherical refractive components and have an internal pupil accessible for insertion of filtering components. A Lyot stop must be used in the infrared camera for background rejection. For PANORAMIX II, a set of filters is used at the internal pupil. Correction of the large off-axis aberrations generated by the telescopes, wide spectral coverage, material choices, cryogenic temperature and alignment were the main design challenges. Also, tolerancing was particularly critical for the infrared cameras because they are cryogenically cooled, thus forbidding adjustment of internal components. The cameras’ theoretical performances are presented in terms of point-spread function, encircled energy and distortion.
In this paper we discuss the results given by the nominal design, tolerance analysis, and tests of aspheric CGH null. The optical testing configuration has been used contained a Zygo interferometer, a self-alignment CGH null and a convex aspheric surface under test with revolution symmetry. The CGH null design has been carried out while taking into consideration the parameters of the test surface such as focal ratio, axial coordinates of the test surface normal as function of radial height, as well as the diffractive technology limits. The worst case generated by Monte Carlo analysis of the tolerance has been used to determine the photolithography tolerances. Finally the CGH null testing results are compared with the nominal design and tolerance analysis predictions.
Correlation between computational models and corresponding experimental data of stray light produced by unwanted diffraction orders of diffractive lenses is described. It is shown that the attractive scalar model under Fresnel approximation does not agree with experimental data. The validity of Fresnel approximation for multi-order focusing optics is derived to ex[plain the limitation of the scalar approach. Geometrical models with both coherent and incoherent summation of each diffraction orders coupled with efficiency estimation is used with success. The model takes into account the local diffraction efficiency and also of the number of diffraction orders supported locally by the structure. This geometrical optics model can be used in standard optical design software. Therefore it may be recommended as an additional tool for stray light analysis at the design stage of hybrid refractive/diffractive optical systems. In a second part, the impact of unwanted diffraction orders when diffractive optics is used in visible imaging system is discussed. Several experimental observations about the use of plastic/diffractive lens in relation to different fabrication process used to build the lens are also discussed. From those observations, we recommend an approach to reduce the stray light produced by the diffractive lens. In conclusion, a serious stray light analysis is mandatory in the design of visible imaging systems using diffractive lenses.
We review the status of the liquid mirror project. Interferometric tests of a f/1.2 2.5-m diameter liquid mirror carried out with a scatterplate interferometer show Strehl ratios of order 0.6, close to the value of 0.8 usually taken to signify that diffraction limit has been reached. The mirror is certainly better than implied by the data because the interferograms were taken with 1/500 second exposures and the wavefronts therefore include the effects of seeing in the testing tower. Correctable small variations of the rotational velocity account for another substantial fraction of the deviations from a parabola. We have videotaped hours of interferogram and PSF observations that show that those we analyze are representative.
The TEMOS concept has been developed by the LOOM in order to evaluate several aspects of the new technologies that will be certainly used in the construction of future giant telescopes.
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