Some of the techniques used for wavefront sensing that do not require coherent light sources are reviewed and a new arrangement developed by the authors is described. The latter ws produced as part of a study looking at absolute measurement of sphericity. It uses a measurement technique that basically determines the transverse ray-aberration (in effect the wavefront slope) associated with different parts of the pupil of the test piece and from this computes the wavefront distortion. The pixels of a CCD camera define the separate areas of the wavefront and in conjunction with scanning gratings, the signal from each pixel measures the associated wavefront slope. The wavefront distortion is computed from the slope values.
The paper describes the results that have been obtained so far using the current breadboard arrangement as well as some of the advantages and disadvantages of the method compared to other techniques.
This paper describes the initial flight experience of the Compact High Resolution Imaging Spectrometer (CHRIS) developed at Sira Electro-Optics Ltd. The imaging spectrometer is flying on PROBA, a small agile satellite, which was launched in October 2001. The main purpose of the instrument is to provide images of land areas. The platform provides pointing in both across-track and along-track directions, for target acquisition and multi-angle observations, particularly for measurement of the Bi-directional Reflectance Distribution Function (BRDF) properties of selected targets. The platform also provides pitch motion compensation during imaging in order to increase the integration time of the instrument, increasing the number of spectral bands that can be read and enhancing radiometric resolution. The instrument covers a spectral range from 400 nm to 1050 nm, at ≤11 nm resolution. The spatial sampling interval at perigee is approximately 17 m. In this mode it is possible to read out 19 spectral bands. The locations and widths of the spectral bands are programmable. Selectable on-chip integration can increase the number of bands to 63 for a spatial sampling interval of 34 m. The swath width imaged is 13 km at perigee.
The CHRIS instrument is a space-based imaging spectrometer that will provide 10 nm spectral resolution over the spectral range from 415 nm to 1050 nm. The nominal spatial sampling interval will be 25 m, however, larger sampling intervals are possible. Band selection, spectral bandwidths and the spatial sampling interval will be programmable. The instrument is planned to be launched on an agile small satellite of the 100 kg class. This satellite will operate in a sun-synchronous, high inclination orbit at approximately 830 km. At this altitude the instrument can provide 19 spectral bands with a spatial sampling interval of 25 m at nadir. The field of view of CHRIS is 18.6 km. Attitude control of the platform will allow access to non-nadir targets, multi-angle observations of selected targets and improved radiometric resolution. This paper describes the optical design of the instrument, including the telescope, spectrometer detector and in-flight calibration hardware, as well as critical alignment procedures, with emphasis on spectrometer assembly and stray light control. Results of performance and calibration measurements are presented.
The paper describes some measurements carried out in order to obtain reliable data for designing a passively athermalized MWIR telescope for observation of earth from a satellite and for finally evaluating the change of focus with temperature for the completed telescope. The telescope was a catadioptric system and the main body was a carbon composite structure, while the lens elements in the system were of either germanium or silicon. Reliable data was of course available for the coefficient of thermal expansion (CTE) of the metal components used in the structure, as well as for Ge and Si. However, the CTE of a carbon structure will to some extent depend on the exact form of construction that is used and therefore needs to be measured, or at least checked, on the actual structure. A survey of the published dn/dT data for Ge and Si as well as that provided by the material suppliers showed variations and uncertainties that were unacceptable.
The satisfactory performance of a helmet mounted display (HMD) depends very much on satisfying a number of critical criteria relating in particular to the interface between the user and the optical system. The paper describes a test facility that enables these and other optical and electro- optical parameters of an HMD system to be evaluated. A particular feature of the equipment is that it can be used for testing display systems mounted on, or forming an integral part of, the complete helmet. This is made possible by the design of a compact probe that simulates the relevant features of the users eyes and can be placed inside a helmet at the eye positions.
There are several problems is using MTF to measure the performance of visual afocal sights. Earlier research by one of the authors has shown that a good correlation exists between a subjective assessment of performance and the Strehl intensity ratio of visual sights. The paper outlines some of the advantages in using this as a criterion of image quality and describes equipment for measuring the line Strehl ratio of binoculars both on and off axis.
A wavefront sensor design is presented. The wavefront sensor will be applied typically to adaptive correction of a ground based astronomical telescope. It comprises a pair of crossed biprisms to act as star image splitters in the telescope image plane, splitting the incident wave into four beams. The biprisms are followed by a lens which forms four pupil images. These exhibit intensity map differences related to the phase gradient of the incident wave, providing that the point spread function overlaps the biprism crossing. This allows standard wavefront reconstruction algorithms to be used to reconstruct the phase map over the aperture. The results from a computer diffraction simulation of this system are presented to show the useful range of the sensor. In addition, a design for an experimental prototype system is presented.
A portable collimator and MRTD target system has been developed which provides a rapid go/no-go means of checking the performance of FLIR's installed on aircraft, tanks and other vehicles. The system is also suited to production testing.
Two MRTD collimator systems have been developed for measuring the performance
of FLIR's installed on aircraft, tanks and other vehicles. One of these is a
general purpose equipment for performing a full assessment of an imager, whilst
the other i s a hand portable test set for doing go/no-go checks on performance.
The latter is also very suitable for production testing..
Two methods of making an objective measurement of MRTD have been under
development. The techniques and some results obtained using two different types
of thermal imager, are described and discussed with particular consideration
given to the effect of the MTF of the eye. Conclusions are that objective
measurement techniques can now seriously be considered as an alternative to
subjective measurements for production testing.
Two methods of making an objective measurement of MRTD have been under development. The techniques and some results obtained using different types of thermal imager, are described and discussed. Conclusions are that objective measurement techniques can now usefully replace subjective measurements as a production test.