PHOTOMETRIC OBSERVATIONS OF THE LUNAR ECLIPSE OF NOVEMBER 17-18, 1956

Transcription

1 PHOTOMETRIC OBSERVATIONS OF THE LUNAR ECLIPSE OF NOVEMBER 17-18, 1956 Elmo C. Bruner, Jr. Michelson Laboratory, U.S. Naval Ordnance Test Station, China Lake, California During the November 1956 lunar eclipse, a series of photo- metric observations of the brightness of the lunar disk were made from Cactus Peak, California. The measurements were made in two narrow wavelength intervals : one in the green centered about, X 5458 and the other in the red centered about X The ap- paratus used telescopes from one of the early photoelectric pho- tometers used at Cactus Peak for night-sky observations. 1 2 mounting was assembled by E. V. Ashburn for use in auroral photometry (Plates I and II). It was equipped with two identical f/3.5 telescopes of 2.6-inch diameter. A circular field stop on each telescope restricted the field of view to about I o. Interference filters were used in front of the telescope objec- tives to pass the desired wavelength intervals. Filters used were as follows : Red (A, 6230) Baird No. V plus Wratten No. 9 (K3) Green (X 5458) Baird No plus Wratten No. 16. The Wratten filters were used to remove the unwanted second- ary peaks usually found in interference filters. Such a combina- tion gives a narrow band-pass approximately 100 Â wide for 50 percent of maximum transmission. During the first and last portions of the eclipse a small dia- phragm, of diameter 0.22 inch, was placed in front of each ob- jective to reduce the amount of light entering the telescope. The diaphragms were removed during totality in order to obtain usable signals. Detectors were 1P21 photomultiplier tubes operated at 880 volts. The outputs from the photomultipliers were fed into sepa- rate linear DC amplifiers of a type described in Reference 1. The response of the amplifiers departs less than 1 percent from line- arity over the range of the Esterline-Angus recording milliam- meters with which they were designed to be used. Amplifier gains could be varied as necessary to give readable recorder pen 431 Its

2 432 ELMO C. BRUNER, JR. deflections. The photomultipliers were operated well within the linear portion of their characteristic curves. 3 The photometer was provided with a motor-driven altazimuth mounting arranged to scan continually in altitude over a small angle. This vertical sweeping, together with the rotation of the earth, caused the telescope to trace out a zigzag path in the sky (see Fig. 1). Before each observation the telescopes were set a Fig. 1. Diagram showing method of scanning moon with photometer. While the telescope is scanning vertically, the apparent motion of the moon carries it through the area being scanned. little west of the moon and then allowed to sweep vertically and gradually drift across it. Each time the path of the telescope crossed the moon, deflections of the two recorder pens were produced. These readings rose to a maximum when the telescope was crossing the center of the illuminated portion of the disk and then fell gradually to zero as the telescope passed the eastern rim. When the moon was completely out of the field, the telescope was moved west in azimuth and the process repeated. Each such observation yielded 100 to 200 readings and required 5 to 10 minutes. The central readings from each observation were averaged and recorded, to-

3 PLATE I View of photometer showing the two telescopes and the motor-driven altazimuth mounting. The 2" X 2" interference filters can be seen in front of the objectives of the telescopes.

4 PLATE II View of photometer, amplifiers, power supplies, and recorders.

5 PHOTOMETRIC OBSERVATIONS 433 gether with the time at the center of the observation. Figure 2 shows the milliammeter record of a typical observation. A total Fig. 2. Sample portion of photometer record. The chart speed was about 3 divisions per minute. of 30 observations in each wavelength were made during the eclipse. Average readings for each observation during the eclipse were compared to the average reading obtained when the moon was uneclipsed. Corrections for atmospheric extinction were less than.05 mag. and were neglected. The derived data, expressed in magnitudes, are given in Table I, and are plotted in Figure 3. In TABLE I The Lunar Eclipse of Nov , 1956 Pacific Differences in Standard Magnitude Time red green 20 h 51 m h 29 m Pacific Differences in Standard Magnitude Time red green 23 h 36 m h 46 m

6 434 ELMO C. BRUNER, JR. this figure, the solid curve and the points marked with circles denote the À 6230 record while the broken curve and points marked by plus signs are for the À 5458 record. The sizes of the circles and plus signs indicate an estimate of the probable error. Fig. 3. Graph showing variation of the average brightness of the moon during the eclipse. The vertical lines show the predicted times of first, second, third, and fourth contacts. At the predicted time of first contact, penumbral darkening had reduced the average brightness of the moon to about 65 per- cent of its original value. After first contact, the average bright- ness decreased rapidly until at second contact the red had de- creased to percent and the green to percent of the original values.

7 PHOTOMETRIC OBSERVATIONS 435 During totality, the moon is illuminated by light which has been refracted into the umbra by the earth s atmosphere. Scat- tering of the shorter wavelengths by the atmosphere causes the red to be enhanced, giving the moon an appearance often de- scribed as copper colored. The present measurements indicate that at the center of totality, light in the green had changed by 11.4 magnitudes while that in the red had decreased by only 10.8 magnitudes ; a difference of 0.6 magnitude. The increase in brightness between third and fourth contacts was not quite symmetrical to the decrease between first and second contacts. At fourth contact, the average brightness was 52 percent of that of the uneclipsed moon. Penumbral darkening was noticeable until approximately 1:15 PST. The records may have been affected somewhat during ingress by light cirrus clouds. No clouds were observed during egress and the air was sufficiently dry that no difficulty was experienced with dew forming on the lenses. The author wishes to express his gratitude to his wife, and to Dr. Pierre St.-Amand, Mr. E. V. Ashburn, Mr. C. P. Pen- toney, and Mr. Z. W. Hohanshelt of the U.S. Naval Ordnance Test Station for their assistance in assembling the apparatus and carrying out the observations. 1 P. St.-Amand, Dissertation, Dept, of Geological Science, California Institute of Technology, F. E. Roach and H. B. Pettit, /. Geophys. Research, 56, 325, R. W. Engstrom, J. Opt. Soc. Am., 37, 420, 1947.