15. April 2011: Occultation of 87 Leo

Manfred Rudolf

In the evening hours of 15 April 2011, the almost full moon (age 12.3 days) occulted the star 87 Leo (4.7 mag). The occultation which took place on the non-illuminated limb of the moon was recorded from Munich with a C11 telescope and a Mintron video camera. A
TIM-10 video time inserter was used to obtain the exact occultation time.

The chart below shows the star and the moon shortly before occultation. The white frame represents the approximate field of view of the Mintron video camera.

The MP4 video (30 seconds, 2.5 MB) showing the occultation event can be seen here.

The occultation time can be estimated from consecutive images of the video sequence:

Date and time (UTC) are shown in the last line in the format YYMMDD : HHMMSS. The last three figures in the middle row indicate the milliseconds of the start time when the image was recorded. Unfortunately, the framegrabber I used was not capable to separately record the two individual half-frames which make up the TV image, but added the two half-frames to the full TV image. Therefore the millisecond values indicating the begin of exposure of each half frame are superimposed and appear blurred in the composite image (above: millisecond values 937 and 957 superimposed), and estimation of occultation time is slightly complicated.

After "LO" and "LA" in the first and second lines, the geographic longitude and latitude of the observation site are shown.

The image above (start of exposure: first half-frame at 21:31:10.937, second half-frame at 21:31:10.957) shows the star completely unocculted, in normal brightness.

The next frame, start at 21:31:10.977 (first half-frame) and 21:31:10.997 (second half-frame), shows the star clearly dimmer than in the previous image. Careful inspection reveals that the star image is not a blob, as in the previous picture, but is composed of "stripes" (see magnified insert to the upper left in the image below). Apparently, the star was visible during the first half-frame of that image, but has been occulted when the second half frame was recorded. Thus the occultation took place between 21:31:10.977 (begin of first half-frame) and 21:31:10.997 (begin of second half-frame).

The star has completely disappeared in the next frame, taken at 21:31:11.017 / 21:31:11.036.