Wait… what? First images of stars? But astrophotography goes way back. Let’s clarify.
Last week, astronomers unveiled the first images of stars using optical interferometry*
(other than the sun) where the star is actually resolved. Even in the best astronomical images to date, every star other than the sun has been nothing more than a point of light on the detector with no discernible features. Astronomers using the Very Large Telescope Interferometer have released images of a double star in the Orion Nebula’s Trapezium and an expanding shell of gas around another star, T Leporis. I encourage you to check out the great coverage at Universe Today, Bad Astronomy, The Spacewriter’s Ramblings, and Simostronomy. But why is optical interferometry so important?
Resolution is an important concept in observational astronomy. Better resolution allows us to see finer detail in our. For example, you can change your screen resolution to see more or less detail on the display. Or, if you’ve ever driven on a long, flat desert highway, a car coming your way at night might seem to only have one headlight, until the point when the car is close enough that the headlight suddenly splits into two! That is the point at which the car is close enough that your eyes can resolve the two separate headlights. Resolution is determined by the wavelength of the light that you are observing and the size of the optical system you are using to measure it. In the headlight example, the wavelength is that of visible light, and the optical system is your eye. There is actually an equation that describes this: resolution = 1.22 * wavelength / diameter of telescope. You want your resolution to be small, so your telescope should be big!
Image from Nick Strobel’s Astronomy Notes. Go check out his lecture on resolution!
Single telescopes can only be built so big before they are mechanically unwieldy. Resolution was especially a problem for early radio astronomers, since the wavelengths are so long to begin with. The largest single-dish radio telescope is in fact built into a valley, that being the Arecibo Telescope in Puerto Rico. It’s not entirely unmovable, since the subreflector above the main dish can be moved around to point at different parts of the sky.
Radio astronomers built interferometers, or arrays of radio dishes that together act like one big telescope, in order to improve resolution. Although interferometers are less sensitive than if you actually had a telescope the full size, the resolution that they achieve can be stunning. Whereas ground-based optical telescopes have sub-arcsecond** resolution, and Hubble is quoted as having a resolution of 0.085 arcseconds, single-dish radio telescopes can only get resolutions of a few arcminutes. However, the Very Large Array in New Mexico can achieve resolutions down to 0.05 arcseconds. The Very Long Baseline Array, stretching from Hawaii to the Caribbean, regularly produces images with milli-arcsecond resolution. That’s like seeing my desk on the Moon.
Aperture synthesis is much more difficult, it turns out, for optical and infrared wavelengths than it is for radio. First, radio signals can be “mixed down” to frequencies that electronics and waveguides can easily transfer and manipulate. Optical signals, however, have to be transported as a beam of light with very little loss of information in the waveguides and beam combiner. By the time the interferometric fringes are measured, very few photons are left! Also, optical interferometers have to deal with atmospheric turbulence, just like their single-telescope counterparts.
Optical interferometry for astronomy was explored in the 1970s, and aperture synthesis was applied to optical wavelengths in the 1980s. Today, a number of telescope arrays are in use or being developed, such as the Naval Prototype Optical Interferometry, which I got to visit in 2004 and 2005, and the Magdalena Ridge Observatory. These, along with the two-element interferometers such as the Keck Observatory, contribute valuable observations and science such as astrometry, stellar diameters, binary star orbits, stellar accretion disks, stellar mass loss, sunspots on other stars, and more! However, most work so far has involved measuring the fringes, or the pattern of spatial frequencies that the interferometer actually measures, and making a model from that. This is valuable, and a fantastic achievement considering the technological hurdles that optical interferometers face. But, it is not the same as imaging.
Images released by the VLTI are being described as the first actual interferometric images of stars, not just models based on the fringes. They combined data from multiple runs of their four-element interferometer with the small telescopes in different positions in order to “synthesize” a telescope 100-meters across. The resulting images achieve resolution on the order of milli-arcseconds. We are entering an era when optical and infrared images with such fine resolution can be compared to similar images in the radio.
* Phil Evans points out in the comments that Hubble has been able to resolve Betelgeuse! See, I’m just biased towards interferometers
** One arcsecond is 1/60th of an arcminute, and one arcminute is 1/60th of a degree. A degree on the sky can be approximated by the width of your index finger held out at arms length!
- 9th Imaging Synthesis Summer School, with
lectures on optical interferometry by Chris Haniff and Michelle Creech-Eakman.
- (pdf) Optical Interferometry: A Brief Introduction by Markus Scholler
- Links to astronomical interferometers from Wikipedia