Physics – Optics
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres...23.&link_type=abstract
ESO Press Release, 11/2001
Physics
Optics
Scientific paper
First Scientific Results with Combined Light Beams from Two 8.2-m Unit Telescopes
Summary
It started as a preparatory technical experiment and it soon developed into a spectacular success. Those astronomers and engineers who were present in the control room that night now think of it as the scientific dawn of the Very Large Telescope Interferometer (VLTI) .
On October 29, 2001, ANTU and MELIPAL , two of the four VLT 8.2-m Unit Telescopes at the ESO Paranal Observatory, were linked for the first time. Light from the southern star Achernar (Alpha Eridani) was captured by the two telescopes and sent to a common focus in the observatory's Interferometric Laboratory.
Following careful adjustments of the optical paths, interferometric fringes were soon recorded there, proving that the beams from the two telescopes had been successfully combined "in phase" . From an analysis of the observed pattern (the "fringe contrast"), the angular diameter of Achernar was determined to be 1.9 milli-arcsec. At the star's distance (145 light-years), this corresponds to a size of 13 million km. The observation is equivalent to measuring the size of a 4-metre long car on the surface of the Moon.
This result marks the exciting starting point for operations with the Very Large Telescope Interferometer (VLTI) and it was immediately followed up by other scientific observations. Among these were the first measurements of the diameters of three red dwarf stars ("Kapteyn's star" - HD 33793, HD 217987 and HD 36395), a precise determination of the variable diameters of the pulsating Cepheid stars Beta Doradus and Zeta Geminorum (of great importance for the calibration of the universal distance scale), as well as a first interferometric measurement of the core of Eta Carinae , an intriguing, massive southern object that may possibly become the next supernova in our galaxy.
This milestone is another important step towards the ultimate goal of the VLT project - to combine all four 8.2-m telescopes into the most powerful optical/infrared telescope system on Earth. When ready, it will be able to reveal at least 15 times finer details in astronomical objects than what is possible with any existing, single ground-based telescope.
PR Photo 30a/01 : Overview of the VLT Interferometer . PR Photo 30b/01 : "Joint" stellar light-spot produced via ANTU and MELIPAL at the VLTI focus. PR Photo 30c/01 : Interferometric fringes from the star Achernar . PR Photo 30d/01 : Time sequence of fringes from Achernar. PR Photo 30e/01 : "Visibility curve" of the star Psi Phoenicis . Scientific Appendix First VLTI observations with two 8.2-m telescopes
ESO PR Photo 30a/01
ESO PR Photo 30a/01 [Preview - JPEG: 357 x 400 pix - 82k] [Normal - JPEG: 713 x 800 pix - 208k] [Hi-Res - JPEG: 2673 x 3000 pix - 1.4M]
ESO PR Photo 30b/01
ESO PR Photo 30b/01 [Preview - JPEG: 400 x 350 pix - 57k] [Normal - JPEG: 800 x 700 pix - 176k]
Caption : PR Photo 30a/01 : Overview of the VLT Interferometer as it was operated when the light beams from two of the 8.2-m telescopes were combined. The VINCI instrument that was used for the present test, is located at the common focus in the Interferometric Laboratory. PR Photo 30b/01 shows one of the first "joint" light-spots from a star as seen at this VLTI focus and resulting from the superposition of light collected with the 8.2-m VLT ANTU and MELIPAL telescopes. Despite the long optical paths (about 200 m), the quality is excellent (FWHM = 0.45 arcsec). Note that this is not (yet) an image of the stellar surface.
At 1 o'clock in the morning of October 30, 2001, ESO astronomers and engineers working in the VLTI Control Room successfully combined the light from ANTU and MELIPAL , two of the four 8.2-m VLT Unit Telescopes at the Paranal Observatory. The same night, a series of high-resolution test observations with the VINCI instrument [1] at the focus of the VLT Interferometer (VLTI) proved that this complex system was functioning extremely well, and within the technical specifications .
Following about seven months after the moment of "VLTI first light" during which the light beams from two small test telescopes were combined - as described in detail in ESO Press Release 06/01 - this accomplishment above all serves as a demonstration of the possibilities and potential of interferometric observations with the four giant VLT telescopes.
The two large telescopes used for the present test are separated by 102 metres. In order to properly combine the starlight received by them, a train of 25 mirrors is needed . All of them must be adjusted with a precision of one thousandth of a millimetre or better.
As can be seen on PR Photo 30a/01 , the light from the observed star is first directed towards the Nasmyth focus by three mirrors in the telescope tube. From here, it continues towards the intermediate Coudé focus below the telescope and then onwards through a subterranean light duct to the VLTI Delay Lines that are installed in the Interferometric Tunnel . At the end of this long chain of mirrors and after traveling a distance of approximately 200 metres, the light finally reaches the VINCI instrument in which the two beams interact coherently (in phase) to produce "interferometric fringes".
The tests have shown that the starlight arrives at the VINCI instrument with a pointing accuracy of about 1 arcsecond and, even more important, with a long-term tracking stability of the order of 0.2 arcseconds per hour.
In fact, the image quality measured at the focus of VINCI is essentially identical to that of the individual telescopes at the Nasmyth (and Cassegrain) foci. Stellar images as sharp as 0.4 arcsec (note that this is the size of the "seeing disk" FWHM, not yet a real image of the stellar surface; the VLTI will start producing two-dimensional images of stars and other objects at a later stage) have been obtained at the interferometric focus, cf. PR Photo 30b/01 . The installation of an Adaptive Optics system (see below) will later reduce the image size to the theoretical limit of 0.057 arcsec (for observations with an 8.2-m telescope in the infrared K-band at wavelength 2.2 µm (or 0.032 arcsec in the J-band at 1.2 µm). First scientific results already during the test observations
ESO PR Photo 30c/01
ESO PR Photo 30c/01 [Preview - JPEG: 400 x 368 pix - 50k] [Normal - JPEG: 800 x 736 pix - 136k]
ESO PR Photo 30d/01
ESO PR Photo 30d/01 [Preview - JPEG: 400 x 332 pix - 168k] [Normal - JPEG: 800 x 663 pix - 440k]
Caption : PR Photo 30c/01 shows the interferometric fringes of the star Achernar , as observed on the computer screen in the VLTI Control Room, at the moment of "First Light" with two 8.2-m VLT telescopes. PR Photo 30d/01 displays the time evolution of the interferometric fringes obtained on Achernar . Each horizontal scan represents a recorded fringe pattern, with time running vertically from bottom to top. PR Photo 30c/01 was extracted from one of these scans.
The technical demonstration being so successful, the ESO astronomers and engineers involved in the development of the VLTI immediately decided to go one step further. And indeed, the interferometric fringes recorded with the light beams from two 8.2-m VLT telescopes during these initial technical tests have already led to some very valuable scientific results.
The first star to be observed - the brightest star in the southern constellation Eridanus (The River) and known as Alpha Eridani or Achernar - is quite different from our Sun. It is estimated to be several times more massive and, with a surface temperature of about 20000 degrees, it is about three times hotter than our local star.
The distance to Achernar has been measured by the ESA HIPPARCOS satellite as about 145 light-years, and from its apparent brightness, it is found to be almost 1000 times more luminous than the Sun. Consequently, it depletes its energy resources much faster and has a much shorter life expectancy (about 100 million years) than the Sun (about 10,000 million years).
The new measurement with the VLTI found the angular diameter of Achernar to be 0.00192 ± 0.00005 arcs
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