Monday, 4 February 2013

Observing at NASA Infrared Telescope Facility

Why Mauna Kea?

A combination of factors make the summit of Mauna Kea on the Big Island of Hawaii ideal for planetary observing, particularly in the infrared.  The altitude means that the amount of water vapour above the telescope is low (water is opaque in the infrared so you can't see through it), plus an inversion layer sits below us that keeps the moist cloud cover well beneath the observatories.  The air here is also really stable, limiting the atmospheric turbulence that degrades seeing, which means the stars don't 'twinkle' as much as they do elsewhere.  We're also well away from any towns or cities, meaning that the skies are clearer than I've ever seen them.  All this means that there are thirteen observatories of different flavours, from the optical to the radio, sat up here on the summit of an extinct volcano.

Infrared Telescope Facility

Sunset over IRTF on February 4th 2013
Our workhorse for planetary astronomy has always been the Infrared Telescope Facility (IRTF), operated by the University of Hawaii under an agreement with NASA.  It features a 3-m diameter primary mirror with a classical Cassegrain telescope, so light bounces off the primary mirror, back to the top to a small secondary mirror, before being focussed onto whatever instrument is pushed into the MOM (multiple instrument mount), which allows different instruments to be slid into place.  The telescope is mounted on a large, heavy English yoke equatorial mount (the yellow thing in the pictures) to limit sensitivity to any vibrations.  The observatory was constructed in the late 1970s to support the Voyager missions to the outer solar system, and has since been used to support many planetary missions (Galileo, New Horizons, Cassini) and for the Shoemaker-Levy 9 impact back in 1994.  About 50% of its observing time is dedicated to planetary astronomy.

The telescope and dome before the start of our run.

Inside the Control Room

The observers and telescope operator tend to work a 12-hour night, coming up at 6pm as the sun sets and then heading back down at 6am, or thereabouts.  The telescope operator is in control of the telescope and where its pointing (i.e., what target, and whether we use stars to guide the telescope for added stability on the target), and sits at a large control panel in front of a big window looking into the dome itself (blacked out when we're actually observing).  The observers then sit at a series of computer terminals on the other side of the room, controlling the instruments themselves.  The instrument can make small corrections of a few arcseconds to move the target around on the arrays (i.e., for subtraction of background emission or bad pixels from the final images), but most of our time is spent setting up the wavelengths of interest, moving filters in and out, deciding how long to integrate for to get clean-looking spectra, and which spectral settings are needed to achieve the desired science.

Inside the control room, observing Jupiter.
When we observe with a visitor instrument such as TEXES, the instrument specialists (this time John Lacy and Tommy Greathouse) arrived early to start cooling the instrument with liquid nitrogen and liquid helium down to the optimum temperatures of 4K (i.e., 4 degrees above absolute zero), and to ensure everything is mounted and working properly.  Each morning and night we add more liquid N2 to the dewar to keep things cool, and top up the (very expensive) liquid helium once every other morning.  Then we come into the control room, sit down at the terminals, and start to run through the pre-planned targets and spectral settings (this time mostly Jupiter and Saturn, with some Io and starburst galaxies thrown in too).  Some of the terminals are for operating TEXES, others are for transfering data between discs, and another is used Here we stay, staying hydrated (and for some of us, caffeinated) for the night, and trying to keep each other awake during the longer integrations when we're just sat adding up photons from dim sources, or executing mapping procedures.  Hale Pohaku prepare bagged 'night lunches' for us to stave off the hunger.  At the end of the night the instrument is shut down, the dome closed and the telescope yoke returned to zenith, and we all head down to get some much needed sleep!

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