Field correcting lenses have been used to increase the fields of view of large telescopes since at least Ross's work in 1935. In recent years, due to the advent of electronic image sensors, there has been movement toward combining field correction with focal length reduction in so-called field compressor/corrector or telecompressor lenses. Such lenses are now commonly employed in both professional and amateur astronomy. This paper demonstrates that field compressor/corrector lenses can be of great utility in a wider context - the design of optical instruments. In the finite conjugate and relay applications prevalent in optical instrument design, standard achromats are often employed because there is currently no better alternative short of custom lenses. I show that there exists a particularly attractive family of cemented doublet compressor/correctors that can be combined with standard achromats to greatly improve the imaging performance obtainable in these applications. I propose that these field compressing/correcting doublets should be made available from stock as standard optical components. I introduce a novel viewpoint for analyzing the imaging capability of a lens that makes it simple to visualize the performance obtainable with a lens when it is employed over a wide range of fields and focal ratios. Using the new viewpoint, I demonstrate the performance improvement obtainable when employing the new compressor/correctors. I also show that the new lenses are flexible, that is, that one may obtain their advantages while employing them with a range of achromats or over a range of focal lengths, and that the alignment tolerances between the achromat and the field compressor/corrector are modest.
One of the limits to the sensitivity of differential absorption lidar (DIAL) when using a topographic reflector is the spectral variation of the reflectance of the topography (differential albedo). This is especially a problem when DIAL is attempted from a mobile platform that views changing background scenes. Recent advances in technology allow one to generate laser radiation that is tunable over broad spectral ranges. We show that the differential albedo problem can be largely corrected by judicious selection of a set of at least three lidar wavelengths to detect and measure each single species of interest. We show that the degree of correction which can be obtained depends on the joint spectral properties of the reflectance of the background and of the species absorption coefficient. We show that use of a simple polynomial model for the background reflectance provides detection sensitivities at the part per million-meter level for hydrocarbon species in the 3 micron region. We propose that the multi-wavelength technique can also be used to determine changes in background absorption when that background absorption is not small.
A new system for making dimensional measurements through a rigid borescope is described. Unlike commercially available systems, the random error in the measurement is proportional to the range to the object, rather than to the square of the range. The system requires only the addition of a video cursor generator, a computer, software, and a precision translator to any standard, substantially side-looking borescope. the system is implemented so that the user can effectively interpolate between camera pixels along the axis to which the measurement is most sensitive to pointing errors. Under ideal conditions, the relative positions of object points can be located in three dimensions to within 0.1 percent of the range, and the precision to which the 3D distance between points can be determined is several times better than that. Measurements of distances under field realistic conditions are demonstrated to have a precision of 0.4 percent of the range. In all cases, the systematic error in the measurement is demonstrated to be consistent with the level of precision.