A Night at the Observatory
March 25, 2013
by Ryan Wyatt
I’m writing this from more than 8,000 feet (around 2,500 meters, for the more metric-ly inclined) above sea level, in the control room of one of the twin 6.5–meter Magellan telescopes at Las Campanas Observatory, near the southern end of Chile’s Atacama Desert. I’m tagging along on a night of observing with Jackie Faherty and Chris Tinney as they measure distances and chemical compositions of exotic objects known as brown dwarfs. For the next three Science Today entries, I’ll try my best to tell the story of this one night of observing and to give a sense of what Faherty and Tinney are attempting to learn about these tiny, faint stellar wannabes.
The night’s work starts in the afternoon. The instruments require calibration, which can take place long before the sky gets dark. Because the observations will involve taking both images (basically photographs) and spectra (a “fingerprint” of the light) of the brown dwarfs, they will use both the FourStar camera and the FIRE spectrograph. Astronomers have a more fastidious approach to their images than, say, your average Instagram user, so they carefully characterize the camera’s responsiveness and uniformity. For the spectrograph, they create a map of how the light splits into its constituent wavelengths using the equivalent of neon billboard lights aimed at the instrument.
At sunset, a few clouds in the southwest cause some concern: astronomers prefer their sunsets dull, unimpressive, and cloud-free. The worry passes, however, and as the sky darkens, the work begins in earnest.
Only four days from full, the moon brightens the sky considerably. For astronomers who observe in visible wavelengths (what we see with our eyes), this would ruin a perfectly good night. Consequently, many seek out “dark time,” defined as the first few nights before or after the new moon. Luckily, brown dwarfs show up best in infrared light, so tonight’s observations can take place in the “bright time,” three to five nights before or after the full moon. Indeed, the astronomers appreciate not having to deal with pitch-black observing conditions: “It’s inconvenient. You can’t see the clouds, and you trip over things,” Tinney notes.
A little more calibration occurs as the sky darkens, including pointing and focusing the telescope, and then the observations begin. “The focus at the beginning of the night changes rapidly because the temperature is dropping,” Faherty explains. “So we take shorter exposures, and continually monitor the images for out-of focus stars, which look like little donuts.”
Ultimately, Faherty and Tinney want to determine each object’s precise location in the sky—a process known as astrometry—as well as its light fingerprint—a process known as spectroscopy.
Particularly for this kind of project, astronomers need excellent “seeing,” which refers to “the blurring the atmosphere produces,” as Tinney describes succinctly. More blurring means the light gets spread out over a larger area of the detector, making precision work on faint brown dwarfs far more challenging.
Astronomers describe the quality of seeing in terms of the apparent angular diameter of a star. Optimal observing conditions at Las Campanas can yield seeing of 0.4 arcseconds or better—equivalent to the diameter of a penny observed from a distance of twelve miles (nearly twenty kilometers). This evening started with seeing around 0.5 arcseconds, but as the night wears on, the seeing drops to nearly 0.3 arcseconds! A great night! (Or perhaps simply observational karma: on Faherty’s last visit to the Magellan telescope, the seeing averaged 1.4 arcseconds, and the observatory shut down because of high winds. C’est l’astronomie.)
Amazingly, these high-quality observations can translate into even more impressive precision when it comes to locating the brown dwarfs relative to the other stars in the image. The resolution of the detector (about 0.16 arcseconds per pixel for FourStar) combined with good seeing means they can pinpoint an object’s location down to a few milliarcseconds—that’s right, 4% the apparent size of the object itself! Such excellent conditions also make it possible to tease apart the atmospheric properties of some of the faintest compact sources in the vicinity of the Sun.
Tomorrow, I’ll share a little more about brown dwarfs and the particular challenge that Faherty and Tinney plan to address, and on Wednesday, I’ll give a summary of how the evening’s work went and what it could mean for the next steps in brown dwarf science.
Ryan Wyatt is the director of the Morrison Planetarium and Science Visualization at the California Academy of Sciences.
Image: Karl Schultz