Engineering topics
Publications and source records attributed to Gatewood, G. D..
The Astrometric Imaging Telescope will detect extra-solar planetary systems with imaging and astrometry. The optical system contains a high-efficiency coronagraph and scatter-compensated mirrors to detect Jupiter-size planets around nearby stars.
The Astrometric Imaging Telescope (AIT) is a proposed spaceborne observatory whose primary goal is the detection and study of extra-solar planetary systems.
The intellectual issues that must be confronted in any attempt to discover planetary systems are examined along with the implications that these issues have for the performance of instruments built for that task. The progress that potentially can be made with precise astrometric measurements is examined. Particular attention is given to the Astrometric Telescope Facility, based on the Multichannel Astrometric Photometer.
A new photoelectric astrometric system, the Multichannel Astrometric Photometer (MAP), was developed, and it was used in a regular observing program designed to detect extrasolar planetary systems. The 76-cm refractor of the Allegheny Observatory was fully renovated to make it suitable as the observing instrument for the MAP. The MAP itself was designed, constructed and tested in actual operation. A data analysis system was developed and tested.
The Astrometric Telescope Facility (ATF) will be operated in the NASA Space Station in the 1990s, furnishing long term, highly accurate relative astrometry of nearby stars in order to detect gravitational perturbations by companion stars with masses as small as that of Neptune. An accuracy of 10 microarcsec is required; this is 100 times better than ground observatory performance. In the Gatewood et al. (1980) astrometric technique used, the relative positions of star images in the telescope focal plane are indicated by the relative phases of the modulations of star brightnesses introduced by translating a Ronchi ruling across the focal plane at uniform speed. Space Station vibration damping, fine guiding accuracy, optical configuration, Ronchi ruling metric accuracy, and the choice of detectors, are discussed.
As a follow-up to the development of astrometric detectors of sufficient accuracy to detect extrasolar planetary systems, the Allegheny Observatory is modernizing their 30-inch Thaw Refractor. Changes include a new objective lens and the transition of the observational bandpass from blue light to red. The new system will have sufficient accuracy to detect Jovian planets orbiting any of several dozen of the sun's nearest stellar neighbors.
A total of 371 photographic plates were made of the binary Mu Cassiopeia (Mu Cas) region with a 76 cm refractor. An astrometric orbit of nearly 22.09 yr was calculated, accompanied by an eccentricity of close to 0.58 and a semimajor axis close to 0.19 arcsec. The absolute magnitude was approximately 5.83 and the radial velocity 164 km/sec.
Astrometric studies have been completed for four central stars Wolf 294, 39 Com, Case 2, and Vega - and selected reference stars in each of those regions. The parallax of Wolf 294 was measured as 0.1806 + or - 0.0033 arcsec. 39 Com is a visual/astrometric binary with an estimated period of 125 yr and a parallax of 0.0297 + or - 0.0044 arcsec. C 2 was measured as a white dwarf from a Warner and Swasey Observatory spectral survey; its white dwarf status is confirmed by its parallax of 0.0186 + or - 0.0109 arcsec. Vega's parallax was measured as 0.1243 + or - 0.0049 arcsec; a small apparent nonrandom distribution in the residuals is most likely an error from the neutral density filter used for magnitude reduction of the central star.
Astrometric data are perturbed by turbulent density fluctuations in the atmosphere over the frequency range from 0.0001 to 10 Hz by amounts that would limit the accuracy of solar-diameter measures significantly. Power spectra of the perturbations are compared with meteorological data to argue that thermal turbulence is dominant above 0.001 Hz and that mechanical turbulence (weather) is important below that frequency. Noise power in astrometry should be comparable under night or day conditions, but site location may be important for the strength of slowly passing waves.
The flying spot digitizer can measure the coordinates of star images on photographic plates in one pass with an accuracy of 4.8 microns r.m.s. error. A field of the size 50 x 100 square millimeters is measured in a few seconds. Suggestions are made as to how a better accuracy can be achieved through the way in which the measurements are arranged and by repetition of the measurements. It is suggested that the speed of photographic astrometric measurements can be improved by about two orders of magnitude, at no loss of accuracy, by proper use of the FSD.
The construction of a catalog of precise star positions in the region of Praesepe is proposed. A list of the 408 stars to be included is given. The program stars were chosen on the basis of their position, magnitude and color, and the availability of accurate early epoch positions. The present status of the project is noted.