April 21, 2019

Astro-Vid of the Week: Chasing Occultations

The Moon approaches Lambda Geminorum (arrowed).

The morning was damp with dew and the hour was late. I silently hoped for batteries and gear to hang on, and dared not jitter the scope or camera, for fear of bumping off kilter the perfect alignment of optics and settings that was now producing an exquisite image of a waxing gibbous Moon with a flicker of a star just off of the approaching dark limb. WWV radio clicked away the moments in the background night.

Welcome to the wacky world of occultation recording. Just this past week, we had an occultation – one of the best for 2014 – pass over Astroguyz HQ here in Hudson Florida. The occultation involved the Moon and the +3.5 magnitude star Lambda Geminorum:

I get lots of questions on how I shoot these, and the rig is exceedingly simple: I’m simply aiming a JVC video camera afocally aimed though a widefield (42mm) eyepiece of our trusty Celestron C8 which is tracking the Moon via an equatorial mounting. The time signal in the background in courtesy of WWV Radio out of Fort Collins, Colorado (I find I pick ‘em up best at 1500 MHz at night here in Florida) and we simply start the video running about two minutes prior to the scheduled event.

I’d say the toughest part of the whole affair is initial focusing and acquisition. For faint stars such as Lambda Geminorum I frequently have to take the shutter speed down a bit to 1/60 to 1/100th of a second. This overexposes the Moon a bit, but brings out our quarry: the target star.

Why study and document occultations of stars by the Moon? Well, a graze event can give us some insight as to the craggy profile of the lunar limb, and several stars have revealed themselves to be binary in nature during such events. Unfortunatel,y no such discovery occurred during last week’s occultation, although Brad Timerson over at the IOTA was kind enough to examine our video and use it to generate a light curve:

When is the next lunar occultation occurring in a sky near you? Check out this nifty list of coming events for 2014.

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  1. [...] The more adventurous might want to try actually catching the numerous occultations of bright stars on video. And U.S. and Canadian west coast observers are well placed to catch the Moon cross right though [...]

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