Occultations of stars by the Moon

Solar eclipses are rare, but occultations of other stars by the moon happen on a  daily basis. They are most easily observed  when the dark limb of the moon move before the star.

The video on the left, which is reduced to 340x240 pixels, shows the occultation of the magnitude 6.2 star 78 Aquarii by an almost full moon on 22 October 2007.  The bright upper right corner is the sunlit part of the moon. The lunar limb is visible close to the star.

The video was recorde with a Watec 902-H video camera on my Televue 102 refractor. The time signals are superimposed by the use of a GPS receiver and a KIWI-OSD time inserter. About 3 cm to the left of 78 Aqr a magnitude 9.5 star is visible as a blinking start. This is much better on the original recording. It demonstrates the sensitivity of the Watec video camera. This star is probably to weak to be properly timed with my equipment, but I have succesfully recorded a occultation fo a 8.4 magnitude star.

A video in PAL format consists of 25 images or frames per second. Each frame consists of two fields: one for the ood and one for the even lines of the frame. The camera thus records 50 fields per second.

The KIWI-OSD time inserter superimposes a time stamp on every single field. The upper field is taken on 22 October between 20:05:40.79 and 20:05:40.81 and the lower field between 20:05:40.81 and 20:05:40.83. All times are UT.

The star is clearly visible on the upper frame, and hardly anymore on the lower frame. This means that during the beginning of the recording of the lower frame the star disappeared behind the moon. The time this happened was 20:05:40.82 0.01 sec.

The average speed of the moon in its orbit around the earth is about 1 km/s. During the recording of one frame (0.02) the moon move about 20 m. This corresponds to an angular movement of about 0.01" (arc seconds - an arc second is 1/3600 of a degree). I find it amazing that such a small movement can be observed with a 10 cm telescope and relatively inexpensive equipment.

The picture on the left shows the camera on the telescope. The red plug is for the video signal, and the black is for power. The camera is hardly larger than a regular eyepiece.

 

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