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Titan Occults a Double Star!

Titan occulting one of the binary stars in the system NV0435215+200905.

The 200-inch Hale Telescope on Mt. Palomar is not only huge – it is  hugely lucky! Or at least a couple astronomers using it were, as detailed in a recent paper they published. The challenge they had accepted was to make a movie of Titan, Saturn’s largest moon, as it occulted a star. But it was going to take quite a bit of luck just for them to see the occultation from Mt. Palomar. And how lucky were they?  How about extremely lucky times two!   Not only were they exactly were they had to be to see Titan occult what any amateur would consider a very close double, but instead of Titan going right between the two stars, it managed to score a direct hit on both of them.   Here’s the video of the event, followed by some more fascinating details.

It’s amazing to watch as first one star disappears and its light is refracted around the upper edge of Titan, and then the second one disappears behind the moon, and is refracted around the lower portion of it.  When I read the paper written about this event, I found that the observers assumed they were dealing with an occultation of a single star by Titan.  It wasn’t until the star came into view in the camera that they realized it was a double – a very pleasant surprise because it gave them a chance to “double” the amount of data they could collect from the event.

Although the stars are 1.5″ apart, they’re actually beyond the reach of all but the largest of amateur scopes because they both have magnitudes of 13.8 – but they should be a breeze for two hundred inches of aperture.  However, even for that telescope, adapative optics were necessary to film the event in the detail shown in the video.  Theoretically, the Mt. Palomar scope can resolve down to .02 of an arcsecond, but because of the thickness of our atmosphere, on good nights the 200 inch scope is limited to .5 of an arcsecond, which is four percent of it’s potential.

Adaptive optics – impossible without computers – counters the effects of the atmosphere, thereby allowing a ground-based telescope to approach it’s theoretical limit of resolution.  On  his web site, Jim Kaler explains that adaptive optics “de-twinkles” starlight,  meaning that “as the star jumps one way as a result of refraction in the Earth’s atmosphere — the basis of twinkling — the telescope optics move in the other way to counter the apparent stellar motion.”

As a result, large telescopes equipped with adaptive optics are now able to take photographs that rival those of the Hubble telescope for resolution and detail.    And as long as we’re discussing optics, if you’re wondering about how wide the field of view was for this observation, how does 8″ x 4″ strike you?  And I consider anything under 30″ claustrophobic.

And consider also how high the odds were against this occultation ever taking place where it could be viewed from Mt. Palomar in the first place.  When a total eclipse of the sun by the moon occurs, full totality can only be seen along a narrow path situated directly below the event.  The width of that path is determined by the size of the sun and moon as seen from earth, which is about one half of a degree, or thirty arcminutes.

If you look closely at the photo from the video at the top of the page, you can see that Titan would just fit between the 1.5 arcseconds separating the two stars with a bit of space left over, a scene which you can actually see in the video.  So when viewed from the earth, that makes Titan about one acrsecond wide, which is 1/1800 the width of the moon.   Going through some old Sky and Telescope magazines, I found the width of the average path of totality during last year’s total eclipse over China was about 160 miles.  In the case of Titan, what that means is that the line along which this occultation could be viewed on earth was about nine-tenths of a mile wide!

The original predictions put Palomar within 80 kilometers of that line, plus or minus another 250 kilometers.  And when the observers discovered that Titan was occulting a double star, not a single star, they quickly realized that the two stars could easily pass to the north and south of Titan!

So all in all, a fantastic film of a fantastic event with huge odds stacked against it ever being seen from Mt. Palomar – an example of stellar luck if ever there was one.


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