Pluto. In all but the largest of telescopes, an object so small and distant it cannot be distinguished from a dim star. But it's a remarkable planet (dwarf planet in the current nomenclature), with a moon large enough to make the pair qualify as binary planets, and a complex surface and thin atmosphere. One way that atmosphere was detected and partially characterized was by studying the change in brightness that occurs when Pluto passes in front of a distant star. Such occultations are not uncommon, although observing them requires the luck to be in the right spot on the planet, with dark skies and clear weather. Cloudbait had such fortune on 18 March 2007. I joined an imaging campaign with other astronomers located along the predicted occultation path. Data from this and several other occultations led to an interesting paper in Astronomy and Astrophysics about Pluto's atmosphere.

On March 18, 2007, Pluto occulted a 15th magnitude star (UCAC2 25823784), and the resulting shadow crossed the central U.S. I was able to record this event from Colorado at 4:55 AM MDT (UT 10:55).

Initial analysis (March 26) shows that the shadow path was significantly north of the original predictions, indicating that the ephemeris model used for Pluto needs some correction, and probably should be updated every year or two.

Pluto centered in its star field. There is nothing visually to distinguish it from a star.

I observed a drop in brightness of about 0.45 magnitude when Pluto blocked the light of the star. The duration of the occultation was about five minutes. These data were collected under extremely poor seeing conditions (FWHM 5.5 arcsec), so the photometric signal is noisy. At the scale of the images, Pluto and the star it occults cannot be resolved as separate sources. The magnitude scale on the plot is approximate; the camera was unfiltered.

Images were collected over 25 minutes with an SBIG ST-8 camera binned 2x2, attached to a 300 mm aperture, 2306 mm focal length SCT (1.61"/pixel). The exposure time of each image is 5 seconds. The light curve is normalized against 8 surrounding reference stars.

If you watch carefully, you can see the brightness of the Pluto/star pair drop as Pluto blocks the star's light.