Very Large Telescope

Interferometry and the VLTI

Most interferometry will be done using 1.8 meter Auxiliary Telescopes (ATs), which will be dedicated to full-time interferometric measurements. The first observations using a pair of ATs were conducted in February 2005, and additional ATs are expected to be installed soon. For interferometric observations on the brightest objects, there is little benefit in using 8 meter telescopes rather than 1.8 meter telescopes.

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In its interferometric operating mode, the light from the telescopes is reflected off mirrors and directed through tunnels to a central beam combining laboratory. The VLTI is intended to achieve an effective angular resolution of 0.001 arcsecond at a wavelength of 1 µm. While this is comparable to the resolution achieved using other arrays such as the Navy Prototype Optical Interferometer and the CHARA array, the collecting power of the big telescopes permits much fainter objects to be observed. Also, the VLTI is working in a fully integrated way, so that interferometric observations are actually quite simple to prepare and execute.

Related Topics:
Angular resolution - Arcsecond - Wavelength - µm - Navy Prototype Optical Interferometer - CHARA array

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Because of the many mirrors involved in the VLTI system, a significant fraction of the light is lost before reaching the detector. Additionally, the interferometric technique is such that it is very efficient only of objects that are small enough that all their light is concentrated. For instance, an object with a relatively low surface brightness such as the moon cannot be observed, because its light is too diluted. Only targets which are at temperatures of more than 1000 °C have a surface brightness high enough to be observed in the mid-infrared, and objects must be at several thousands of degrees for near-infrared observations using the VLTI. This includes most of the stars in the solar neighborhood and many extragalactic objects such as bright active galactic nucleii, but this sensitivity limit rules out interferometric observations of most solar-system objects. Although the use of large telescope diameters and adaptive optics correction can improve the sensitivity a small amount, this cannot extend the reach of optical interferometry beyond nearby stars and the brightest active galactic nucleii.

Related Topics:
Surface brightness - C - Active galactic nucleii - Interferometric - Adaptive optics

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The first two instruments at the VLTI were VINCI (a test instrument used to set-up the system) and MIDI, which only allowed two telescopes to be used at any one time. With the installation of the three-telescope AMBER closure-phase instrument in 2005, the first imaging observations from the VLTI are expected soon. In 2008 the PRIMA instrument will further enhance the imaging capabilities of the VLTI by allowing phase-referenced imaging.

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After falling drastically behind schedule and failing to meet some specifications, in December 2004 the VLT Interferometer became the target of a second ESO recovery plan. This involves additional effort concentrated on more rapid improvements to fringe tracking and the performance of the main delay lines. Note that this only applies to the interferometer and not other instruments on Paranal. In 2005, the VLTI was up to speed, routinely producing observations.

Related Topics:
2004 - ESO

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