Engineering topics
Publications and source records attributed to Howell, B. J..
All-reflective system unvignetted. Wide-angle telescope uses unobstructed reflecting elements to produce flat image. No refracting elements, no chromatic aberration, and telescope operates over spectral range from infrared to far ultraviolet. Telescope used with such image detectors as photographic firm, vidicons, and solid-state image arrays.
Described is an unobscured three mirror wide angle telescopic imaging system comprised of an input baffle which provides a 20 deg (Y axis) x 30 deg (X axis) field of view, a primary mirror having a convex spherical surface, a secondary mirror having a concave ellipsoidal reflecting surface, a tertiary mirror having a concave spherical reflecting surface. The mirrors comprise mirror elements which are offset segments of parent mirrors whose axes and vertices commonly lie on the system's optical axis. An iris diaphragm forming an aperture stop is located between the secondary and tertiary mirror with its center also being coincident with the optical axis and being further located at the beam waist of input light beams reflected from the primary and secondary mirror surfaces. At the system focus following the tertiary mirror is located a flat detector which may be, for example, a TV imaging tube or a photographic film. When desirable, a spectral transmission filter is placed in front of the detector in close proximity thereto.
The Diffuse Infrared Background Experiment (DIRBE) is a cryogenically-cooled 10-band photometer with a large field-of-view (0.886 x 0.886 sq deg) which scans by rotation of the Cosmic Background Explorer about a spin axis. In-orbit calibration requires that the DIRBE detect and measure with precision the signatures of compact sources as they transit the field-of-view. Analysis of the conceptual optical design revealed that response of the 10 bands would vary significantly as a function of source position in the field-of-view, caused by anamorphic pupil distortion and field separation. The optical design reported in this paper is the result of changes which greatly improve the response uniformity and radiometric accuracy of the DIRBE.
An all-reflective wide-angle flat-field telescope (WAFFT) designed and built at Goddard Space Flight Center demonstrates the markedly improved wide-angle imaging capability which can be achieved with a design based on a recently announced class of unobscured 3-mirror optical systems. Astronomy and earth observation missions in space dictate the necessity or preference for wide-angle all-reflective systems which can provide UV through IR wavelength coverage and tolerate the space environment. An initial prototype unit has been designed to meet imaging requirements suitable for monitoring the ultraviolet sky from space. The unobscured f/4, 36 mm efl system achieves a full 20 x 30 deg field of view with resolution over a flat focal surface that is well matched for use with advanced ultraviolet image array detectors. Aspects of the design and fabrication approach, which have especially important bearing on the system solution, are reviewed; and test results are compared with the analytic performance predictions. Other possible applications of the WAFFT class of imaging system are briefly discussed. The exceptional wide-angle, high quality resolution, and very wide spectral coverage of the WAFFT-type optical system could make it a very important tool for future space research.
Computer program GENOPTICS is generalized aid for analysis and evaluation of optical systems that employ lenses, mirrors, diffraction gratings, and other geometrical optical surfaces. It can exactly trace up to 800 rays through as many as 40 surfaces. Results can be used to compute third order aberation coefficients including spheric contributions.
Accuracy of a radiometer is adversely affected by scene polarization if the receiving optical system is sensitive to polarization. It is therefore necessary to specify and measure the sensitivity of the system to polarized light. The Mueller-Stokes matrix of an instrument may be determined experimentally and used to predict the effects of the instrument on any beam. The specification of a maximum polarization sensitivity stated in terms of the degree of polarization produced in an unpolarized beam can be experimentally verified even though an unpolarized beam is not available in the laboratory for direct measurement.
Program prints and plots results of computations such as ray traces, radial energy distributions, and designs of two-mirror telescopes.
Diffraction theory, developed in vectorial form and coded into ray tracing routines, permits tracing rays of any wavelength through surfaces that are plane, spherical, conical, or aspheric polynomial. Ruled diffraction gratings may run in either X-direction or Y-direction, where Z is optical axis.
Optical system designed by third order aberration theory is significantly improved by placing it into a ray deviation design program composed of two distinct computer programs. Tests were conducted on telescope systems, an ultraviolet relay lens, and a four lens corrector system.