Archive for November 8th, 2009


Galileo Club, Part 3: Callisto in eclipse

November 8, 2009

There were two chances to see a Galilean moon entering or exiting Jupiter’s shadow from California tonight. At 6:22 PM Europa came out of the planet’s shadow, and at 8:50 Callisto went into it. I missed the first one but caught the second one.

I cheated a little bit; in addition to observing the disappearance of the moon at under 20x as required by the rules, I also photographed it at higher magnification in my 6″ reflector.

Now, when I first started observing, there were four little moons, two on each side of Jupiter, and as I watched, the inner one on the right got dimmer and then disappeared. But that requires a little unpacking. If you punch up Jupiter this evening in Stellarium or Celestia (follow the links on the right to download ’em if you haven’t already–they’re free), you’ll see that Callisto was the inner moon on the left as viewed from Earth. It was the inner moon on the right in the telescope because Newtonian reflectors rotate the image by 180 degrees. No big whoop, but if you watch the un-flipped version in Stellarium, the mechanics of the process are a lot clearer.

Callisto eclipse diagram

If you face south to see Jupiter, the sun is off to your right, having just set. That means the shadow of Jupiter forms a cylinder sticking out into space to the left of the planet as viewed from Earth. Since most stuff in the solar system orbits in a counter-clockwise direction* when viewed from above (Earthly north), Callisto must be on the far side of Jupiter from Earth. Callisto came out from behind the planet (1), was briefly visible from Earth, then entered the planet’s shadow (2), and will re-emerge in a little less than two hours (3).

* Neptune’s largest moon, Triton, is a notable exception, and its backward motion indicates that it is almost certainly a captured Kuiper Belt object and not a “true” moon. In fact, it is possible that all of Neptune’s moons are captured objects.

So how did the pictures turn out? Well…the listed time of 8:50 PM turned out not to be the midpoint of the entrance of the moon into the shadow, but the  completion. Which I guess makes sense, but I was prepared to start my observations 10 minutes early and run 10 minutes after. In fact, by 8:40 Callisto was already noticeably dimmer than the other moons, and at 8:50 the show was over. “Noticeably dimmer” in the eyepiece means “almost impossible to photograph”, at least with my setup (Orion XT6 telescope, 25mm Sirius Plossl eyepiece, and Nikon Coolpix 4500 camera). I thought I had missed it completely. My only shot of Callisto this evening:

Callisto 1 - nothing to see

Can’t see it? Neither could I. But I blew out the contrast and look who showed up, if only just:

Callisto 2 - ugly but there

By using that image as a template, I was able to copy, paste, and lightly sharpen my best Jupiter image of the evening, and replace the moon blobs with tiny little brush-dots that approximate their actual size and brightness, to create this much prettier and more representative, but less real, image:

Callisto 3 - too pretty to be real

Jupiter looks lousy compared to previous efforts because I was looking through most of the LA light dome and attendant haze, but still: eat yer heart out, Galileo. One required task down, ten to go!

(If you haven’t bagged a moon eclipse yet, there are still several opportunities this week.)


Hack that scope

November 8, 2009

A time-honored tradition in amateur astronomy is amateur telescoping making, or ATMing. For many decades, most people didn’t buy their first scope, they built it, sometimes using optics or mirror grinding kits supplied by Edmunds and others, sometimes just hacking with whatever they happened to have lying around.

I’ve messed around with this a little; a couple of years ago I bought a National Geographic brand 76mm reflector at Target at a deep, deep discount. The scope had good mirrors but the mechanics were terrible; in particular, the eyepieces seemed to have been designed by someone who wanted to discourage people from looking skyward. Why, National Geographic, why? The old rule still applies: never buy a telescope anywhere that sells underwear (that includes Toys ‘R Us!). Unless, like me, you only want the scope for its parts, to build it into something better, and you can get it super-cheap.

Anyway, the full saga of the oft-rebuilt 76mm reflector will be a story for another day–not least because I am contemplating rebuilding it for, let’s see, the fourth time.


Today I’m writing to bring to your attention this very cool rig built by frequent commenter David DeLano. It’s basically a board with three holes, but this  simple device allows him to mount his GalileoScope (tricked out with a diagonal and helical focuser from StellarVue) and his SkyScout (a handheld computerized planetarium-star pointer-thingy) on the same tripod. And he’s thinking of adding binoculars on top! His inspiration came from this cool tripod adapter from astronomy hacker extraordinaire, Rob Nabholz.

What do you have laying around the house that might make your observing life easier? Give it a think, and if you come up with any cool ideas, let me know!