Photoelectric Observations of the Occultation of Stars by the Moon.

Abstract

view Abstract Citations (3) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric Observations of the Occultation of Stars by the Moon. Rakos, Karl D. Abstract The author has made observations of the change in intensity of the light from stars as they disappear behind the dark limb of the moon. Two different methods have given very encouraging results. In the first, the starlight after passing through a 10 sec of arc diaphragm in the focal plane of a 42-in. telescope, was chopped 6000 times per second by a rotating mirror. The corresponding pulses from a 13 stage EMI 6256 S/A photomultiplier were individually fed to a differential amplifier and integrator and displayed on an oscilloscope. The cathode ray tube face was photographed with a camera using a continuous film motion of 25 cm/sec at right angles to the time axis of the oscilloscope and with a sweep speed which was 2 msec/cm. In the second method the stellar intensity was recorded with a differential amplifier, oscilloscope, and camera but without chopping the light beam. The first method has the advantage that the telescope can be guided during the measurement; however, half the available light is wasted. The second method does not have this disadvantage, but demands better tracking and better control of the time constant of the electronic equipment. The present observations have shown that one can obtain a significant number of measures of good accuracy with a telescope of only moderate size. During a single year one might expect the following distribution of stars to be occulted by the moon: Magnitude limits No. of stars <5.0 19 5.0-7.5 40 7.~8.5 60 About 20% will be occulted twice or more per year. For almost all stars brighter than fifth magnitude the angular diameter can be determined with an accuracy of better than 5%. For stars between magnitudes 5 and 7.5 the accuracy is strongly dependent upon the spectral type. The stars fainter than 8th magnitude can be considered to be point sources but can still be successfully used to determine the statistical frequency of close double stars and for an investigation of the roughness of the lunar surface. The present measurements show that the moon's limb can be considered to be smooth that is, undulations between 3 and 150 ft very seldom appear. Successive occultations of the same star should provide important information regarding possible limb effects. The minimum apparent transition in brightness during an occultation lasts about 20 msec. For a telescope of moderate size, the observational accuracy for stars fainter than magnitude 8.5 is influenced by the statistical variation of the photo- current due to the relatively small number of photoelectrons per second. The sky brightness is unimportant for stars brighter than 6th magnitude; for stars fainter than the 7th magnitude, observations unaffected by sky brightness can be made up to 7 days after new moon. Atmospheric fluctuations did not, in general, influence the measurements at all since they are relatively slow. Only if the duration of the occultation is longer than 0.1 sec, which very seldom happens, would this be the case. Publication: The Astronomical Journal Pub Date: 1964 DOI: 10.1086/109407 Bibcode: 1964AJ.....69Q.556R full text sources ADS |