On the nights of 15–17 May 2011, astronomers successfully applied a new observing technique on ESO’s Very Large Telescope Interferometer (VLTI). The new procedure is called “blind observation mode” and will allow interferometric observations to be made of targets significantly fainter than before. It has already produced detailed observations of the spectrum of a quasar, 10 times fainter than the best currently known results. The result was obtained by combining the light from three of the VLT’s 8.2-metre Unit Telescopes in the AMBER  beam combiner.
The quasar 3C 273 — an active galaxy with a supermassive black hole in its core — was the first target successfully observed with this new procedure. The team’s observations allow them to study the size, structure, and movement of the innermost material surrounding the central black hole. The new blind observation technique has a huge potential and, by pushing the capabilities of current interferometers to fainter limits, is expected to bring a number of other previously unobservable targets within reach.
- Interferometric fringes from an observation of 3C 273 with the VLTI
- An interferometric observation of the quasar 3C 273 with the VLTI. The columns marked UT1, UT2, and UT4 show the data from the individual 8.2-metre Unit Telescopes, which were then combined interferometrically. The resultant fringes are shown in the column marked VLTI. The bright feature is the Paschen-α emission line from atomic hydrogen gas, redshifted to a wavelength of 2.17 µm due to the quasar’s distance of about two billion light-years.
This image is a rare example of a five-second exposure that reveals the fringes. In real use, multiple much shorter 0.3-second observations must be made. It is thanks to the new “blind observation mode” that these shorter exposures can be successfully combined.
Credit : ESO/A. Merand
AMBER data processing (and its fringe detection) are based on a frame-by-frame data processing called “Pixel to Visibility Matrix” (P2VM, see Tatulli et al. 2007). This algorithm is efficient for high-flux sources, but is difficult to apply when the signal-to-noise ratio (SNR) per frame is clearly smaller than one.
This was the motivation to try “blind observation” on the AMBER instrument, by recording data where only detector noise is visible in single frames. The key point of this mode is that the fringe pattern remains well contrasted as long as the optical path difference (OPD) between the beams of the interferometer is maintained smaller than the light coherence length. At medium spectral resolution (R=1500), this coherence length is of the order of 3mm, which guarantees a fringe pattern life time quite larger than half an hour with the current VLTI delay lines stability. At low resolution, with a coherence length of less than 70 μm, the fringe contrast is degraded in less than 60s and lost in a few minutes.
This "blind" observation mode opens new perspectives for the VLTI used at medium and high spectral resolution, since it offers a “safe” way to observe 1 or 2 magnitudes beyond the current limits. The preliminary performance estimate is that we will be able to observe in medium spectral resolution beyond K=10 in outstanding conditions and between K=8 and K=9.5 in average conditions. This is a real breakthrough with regard to the current limit set by the fringe tracker cophasing magnitude K=7. Our goal now is to develop and provide an end-to-end data reduction software adapted to this new observing strategy.
Romain Petrov : Romain.Petrov@unice.fr
Laboratoire H. Fizeau at Université de Nice — Sophia Antipolis, Observatoire de la Côte d’Azur, and CNRS
Fizeau page on the result
CNRS Press Release