Improvement in the fabrication of large solid-state focal plane array photonic sensors has emphasized the need for telescopes with flat and wide angular fields of high resolution and with large spectral passbands. To increase the angular field of Cassegrain-like telescopes, various designs of correctors have been incorporated in the optical train. This paper compares specific designs based on the Dall-Kirkham format of prolate ellipsoid primary and spherical secondary mirrors, in which excellent correction is achieved over a significant field angle. The attraction of the Dall-Kirkham format is the relative ease of fabrication to a very high accuracy by simple null tests of the concave prolate-ellipsoid primary by specific separation of light source and knife-edge, and of the convex secondary by interference-matching with a previously null-tested concave spherical master. Null tests of the more common Ritchie-Chretien design require the additional fabrication of two "Hindle spheres" or other auxiliary optics such as holograms for testing of the hyperboloidal primary and for the hyperboloidal secondary. If the Dall-Kirkham format is modified to include variants in which the "Cassegrain" image is imperfect or even afocal, it is possible to design simple corrector relays with exceptional final imaging characteristics. Examples are presented that provide distortionless, diffraction-limited, flat-field imaging over usefully wide fields and large passbands at speeds of f/5 - f/4, and having an unusually compact layout.
An ultra-fast focal reducer design originally intended for use with spherical primary mirrors (published in Optical Engineering, February, 2003), has been adapted for use as a retro-fit for existing telescopes of ~2m pupil diameter that have aspheric mirrors. The focal reducer has only one aspheric surface, located at the aperture stop, and can provide speeds up to f/0.75 while maintaining a wide field with pixel-limited resolution and a broad spectral passband. Examples have been computed for Ritchie-Chretien, Dall-Kirkham and Cassegrain systems, and for use at the prime focus of mirrors with non-zero conic constants.
A four-mirror optic has been derived from a previous three- mirror system, with characteristics that have significant advantages for imaging in the EUV. The compact format enables the total length to be less than the entrance pupil diameter; the focal surface is flat and with rear access; the aperture stop is located coplanar with the secondary, thus ensuring that bignetting is zero and that the pupil is identical at all field angles. Stray light baffling is 100% when the optic is fully enclosed. The aberration correction is good enough for diffraction- limited imaging over most of a 1 degree(s) field angle at wavelengths down to 50nm for a typical 1m entrance pupil diameter. The speed of the 1m pupil diameter example system is f/2.4, but trade-off can be made with other characteristics to achieve faster or slower systems. An f/1.2 system has been designed as an exercise. For all derived systems, scaling is possible to much larger pupil diameters by sacrificing field angle. Realization of the most complex design would require significantly advanced optical fabrication technology. All four surfaces are aspheric, the convex secondary being the only pure hyperboloid. The primary and tertiary can be fabricated on a single substrate. The quaternary mirror is in the form of a zone plate. The performance data for this design show that 90% enclosed energy does not exceed 1 micrometers diameter at all field angles up to 0.4 degree(s) off-axis. Simplified designs are possible for imager devices with larger pixels. For a given pupil diameter, [mass x volume to orbit]-related costs would be unusually favorable.
Design variants of a recently developed optical imaging system have been computed for the thermal infrared spectral bands, which offer some advantages for long-range surveillance and astronomy. Only the spherical primary mirror has the full pupil diameter, all other components being sub-diameter, so scaling is possible up to relatively large pupils. Low-cost fabrication is enabled by the prevalence of spherical optical surfaces. Both MWIR and LWIR spectral transmissions are enabled by the choice of corrector materials, the examples given employing germanium and sapphire for 3.5 - 5.5 micrometers and germanium and zinc selenide for 3.5 - 5.5 micrometers and 8 - 12 micrometers passbands. Diffraction at these wavelengths is the main contributor to resolution constraints, so high numerical aperture values are preferred to enable a better match of blur spot diameter to generally available pixel dimensions. The systems described can routinely be designed to have speeds of f/0.8 or faster, while maintaining diffraction-limited performance over useful angular fields. Because the new design system employs a relayed catadioptric, it is possible to make the aperture stop of the system coincident with the window of the detector cryostat, enabling precise radiometric geometry. The central obscuration provides a convenient location for a calibration source, and both this and a mask for secondary spider supports can be included within the detector cold screen structure. Dual-band operation could be enabled by inclusion of a spectral beam splitter prior to a dual relay/imager system.
If gossamer primary mirrors were to be constructed in a spherical form, it would be possible to arrange a simple null- test in situ. However, spherical mirrors would require correction of the large amount of spherical aberration created in pupils that generally will be greater than 2 m diameter. The design requirement is for diffraction-limited performance over a useful angular field. The otherwise excellent wide- field design solutions of the classical Schmidt and Maksutov are inapplicable in gossamer structures because of the mass and size penalty of large refractive components. However, it is possible for this mode of correction to be achieved near the prime focus by means of pupil transfer optics that minify the large entrance pupil down to more acceptable dimensions. A problem with these solutions is constraint of field coverage due to pupil aberrations created by the large spherical aberration of the primary mirror. This leads the designer towards slower primaries and the penalty of larger, heavier structures. A solution is presented here for spherical primaries with speeds up to f/4. This is based on the 'KiwiStar' principle presented here in 1997, in which a large spherical catoptric is combined by pupil-transfer with a smaller spherical catadioptric to give well corrected wide field images of high speed. This system is well suited to correction at the prime focus of large spherical mirrors, and has only one relatively small weak aspheric surface to provide zonal correction, all other surfaces being spherical. An example is presented of a 4 m diameter, f/2.5 system that is diffraction-limited over the whole of a 0.25 degree field (43 mm diameter), for a bandpass of 486 - 850 nm.
If intended to operate outside the Earth's atmosphere, the imaging optics demanded of a designer become critical. A typical specification would be diffraction-limited imaging over a usefully wide field angle with a broad spectral bandpass, high transmittance, no distortion and no vignetting. When combined with demands for a light and compact structure, such a specification calls for some trade-offs if a real system is to be devised. This paper describes an exceptionally compact imaging system in which the only significant trade-off is the 21% transmittance loss caused by the central obstruction of a Cassegrain-like optic. The entire 1° field angle is diffraction limited for all pupil diameters up to 1m (which would give 0.2 arcsec resolution), while providing a flat, distortionless, unvignetted, f/2.4 image, bandpass-limited only by the reflective coatings of the totally catoptric design. For fields less than 1 degree(s), scaling is possible to very large pupil diameters while maintaining full-field diffraction-limited imaging. By employing a single-axis, double-pass, four-reflection format, compactness is such that the overall length is less than the pupil diameter. The primary and tertiary mirror surfaces are formed from a single substrate. The primary, secondary and tertiary mirrors are hyperboloids of increasing eccentricity, and the quaternary is a weak aspheric zonal corrector. The geometry is such that stray light exclusion is simply achieved without occluding the optical path. The rearward propagating final beam provides significant useful free surrounding volume for cryostat structures, filters and shutters. A further example is given of a scalable f/1.25 version of the system.
Several large telescopes are now being proposed that would benefit from the cost reduction due to the use of spherical primary mirror. However, structural cost constraints require compact formats that tend to impose very high speeds, e.g. f/1.5, which renders difficult the correction of the resulting very large spherical aberration. A technique is described here in which a spherical concentric Cassegrain-like primary-secondary combination is followed by a simple catadioptric focal modifier. The spherical primary is 9m diameter, f/1.5, and the final focus is f/5 with a sub-arcsecond resolution over a 5 arcminute angular field for a passband of 480-850nm. Primary- secondary separation is only 11m and central obscuration is only 11% of pupil area. The two relatively small corrector components provide the functions of concentric meniscus and zonal corrector plate and are made from the same single glass- BK7 is the example given, but silica or any other preferred glass is possible. The relatively small zonal corrector is the only aspheric surface in the entire system. A related system is described elsewhere in which a 30 arcminute angular field can be achieved with a similar resolution, but with more complex glass requirements. However, supply of such exotic glasses may be difficult in large diameters, and the system presented here may find a place in some specialized applications.
The cost of astrographs with pupil diameters greater than 1 or 2 meters can be somewhat reduced if the primary mirror is made spherical and the aberration corrected at or near the prime focus, thus avoiding the classical Schmidt or Maksutov full pupil diameter refractive components. Such a corrector solution is presented here for large spherical primaries with speeds up to f/4. This is based on the KiwiStar principle presented here in 1997, in which a large spherical-concentric catoptric is combined by pupil transfer with a smaller spherical-concentric catadioptic to give well-corrected wide-field images of high speed and broad passband. The designs studied at the time were limited to those with entrance pupils <2m diameter, but it has been found possible to extend the aberration correction capability to pupils of larger diameter. The only significant constraint found in the present study is that the spherical aberration of primaries faster than f/4 is difficult to control. The design presented here is a 4m entrance-pupil-diameter, f/1.4 optic of 0.2 arcsec rms resolution over the whole of a 0.5° field (50mm diameter), for a bandpass of 486-850nm. To fully sample this field would require 108 pixels of 5μm dimension. An extended design with an 8m diameter primary is also shown, that may be scalable to ~30m. To achieve this performance, the strictly concentric format of the original KiwiStar design has been relaxed, thus providing extra degrees of freedom to compensate the large pupil aberrations introduced by the spherical aberration of the primary. Nevertheless, the new design still has only one relatively small and weak aspheric surface to provide zonal correction, all other surfaces being spherical.
A new imaging system format is presented which permits near-zero values for all the classical Seidel aberrations at relative apertures faster than f/1, with field angles of up to 7°arc, ~106 resolved pixels, a 600nm bandwidth in the visible/NIR, and zero vignetting. Performance is limited only by high-order aberrations. The only full- aperture component is a spherical mirror; all other surfaces are spherical except for an optional small weak zonal corrector. The new design approach is suited to aperture diameters of 100mm to >2m when used with appropriate electronic detectors. Thermal infrared variants are possible. In general, at least one order improvement in data acquisition rate is possible compared to that of existing designs.
The manufacture of food products for human consumption is an operation requiring strict levels of quality assurance to ensure that no foreign material is entrained in the final product. Once the product has been packaged, the options for inspection are severely limited. The machine vision team at Industrial Research Ltd. has, on a number of occasions, undertaken the inspection of large quantities of such products using x-ray video imaging and visual inspection. To perform such an operation manually is at best tedious, but also entails a degree of concentration that is difficult to maintain over long periods of time. This paper discusses the development of a real-time system for automatically inspecting canned products. The system uses high speed vision hardware to inspect the contents of each can. The system is capable of automatically rejecting cans containing foreign material.
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