Colorful solar instrument treks east for a new career

The Fourier Transform Spectrometer — one of the premier instruments for laboratory spectroscopy, high-resolution solar spectroscopy, and other research — will now dissect the chemistry of comets and stars.
By | Published: February 8, 2012 | Last updated on May 18, 2023
The enclosure for the Solar Fourier Transform Spectrometer, commonly called the FTS Tank, at the McMath-Pierce Solar Facility on Kitt Peak. The tank is shown open, with some of the equipment racks required to operate the FTS visible in the background. Credit: Tom Eglin, Mark Hanna/NOAO/AURA/NSF
One of the world’s most colorful solar instruments is moving across country for a new life dissecting the chemistry of comets and stars.

The Fourier Transform Spectrometer (FTS) recently ended its career with the National Solar Observatory (NSO) at Kitt Peak, Arizona, and is being shipped to Old Dominion University in Norfolk, Virginia, with delivery scheduled for February 9. There it will operate under the direction of Peter Bernath, chair of the ODU Department of Chemistry and Biochemistry. He is a long-time FTS user whose work includes discovering traces of steam inside sunspots.

“The FTS is one of the premier instruments for laboratory spectroscopy, high-resolution solar spectroscopy, and other research,” Bernath said. “It is a fantastic instrument.”

The FTS was built in 1971-76 and used almost continuously through 2001 at NSO’s McMath-Pierce Solar Telescope at Kitt Peak for solar and atmospheric physics and as a stand-alone laboratory spectrometer. Pressures on the NSO budget have made it impossible to keep operating the FTS, and other operators at Kitt Peak have expressed interest in its lab space.

Millions of people have seen one of its products, a reconstruction of the Sun’s spectrum mottled with black absorption lines representing the fingerprints of chemicals in the solar atmosphere. The colorful image only hints at the power of the FTS, which can measure the intensity of light in incredibly narrow slices of the spectrum from 250 nm (ultraviolet) to 18,000 nm (18 microns; far infrared). Human vision spans just 380-770 nm (violet to deep red).

The solar spectrum image came from the 1984 “Solar Flux Atlas from 296 to 1300 nm” (ultraviolet to near-infrared). This Atlas, “Atmospheric Transmission Above Kitt Peak, 0.5 to 5.5 microns” (orange to mid-infrared), and other products still are used worldwide. Still more can be done with FTS.

“I would like to use the FTS in a laboratory here at ODU to make measurements related to molecules in the atmospheres of planets, comets, and stars,” Bernath said. For example, Bernath has used the FTS to analyze an unusual cousin of cyanide, CN molecules found in interstellar clouds. His particular interest is molecules made of rare carbon-13 or nitrogen-15 isotopes or both. The slight mass difference causes subtle changes in infrared emissions from the more common version, shedding light on the compositions of comets, planets, and stars.

While he seeks funding to continue that work, the FTS will be stored at ODU in custom crates built in the latter half of 2011 as the staff at Kitt Peak carefully readied the FTS for shipment. This was a delicate process since the slightest nick or contamination could end the FTS’s value.

The heart of the FTS goes back to 1887 and early relativity experiments involving the speed of light by A. A. Michelson. Mechanical yardsticks won’t do, so Michelson used light. Folding a beam of light back on itself forms light and dark patterns as the light waves interfered with each other. Changes along the path alter the pattern in a measurable fashion. This allows measuring the intensity of light at extremely fine fractions of a wavelength.

Such precision required special carriages that slide FTS’s optics on fine oil bearings atop a polished granite table, all inside a vacuum chamber as a laser measures their ever-changing positions. The Fourier Transform part of the name refers to a sophisticated type of math used to reconstruct the spectrum. While more complex than a grating that spreads light much like a CD creating the rainbow, the delicate system gave the FTS exquisite precision that made it invaluable in the history of solar physics, and should make it an asset for continued exploration of the cosmos.