The VLT Interferometer - ESO

8.2. DESCRIPTION OF THE ADVANCED ARRAY 145
6. Four delay lines with a ::::::: 50 meter range (::::::: 100 meter path compensation)
usable either with the Auxiliary Telescopes only or in combination
with the 8 meter telescopes.
7. Beam combination through a reflective system in a wind shielded, thermally-controlled
air optical path designed to minimize seeing effects. This
does not eliminate the option of using mono-mode fibers in beamcombination.
It does however delay the incorporation of an evacuated light
optical path to the Extended Array if it turns out to be needed after all.
Air paths do imply the incorporation of longitudinal atmospheric chromatic
dispersion compensation.
8. A wavelength range to cover 420 nm through 25 pm. This is more than envisaged
in the VLT proposal but appears within the range of throughput
limitations set by coatings, fringe contrast limitations set by vibration of
the telescopes and delay lines, and sensitivity increase limitations due to
the partial functioning of adaptive optics.
9. An unvignetted field-of-view in the combined coherent focus of at least 1.5
arc sec diameter. This allows enough area for multispeckle interferometry
and image acquisition.
10. A coherent or cophased field-of-view of at least 1 arcsec diameter. This
will allow interferometry of compact multiple objects or extended objects.
11. The use only with objects which give fringes with sufficient strength to
allow fringe tracking. That includes fringe tracking on relatively bright
objects which are not fully resolved. This fringe sensing object can either
be the object under study or a nearby object within the coherent FOV.
Fringe sensing does not have to be done either at the wavelength being
used for the interferometric observations, it can be done at any wavelength
at which there is sufficient fringe strength.
12. A limited number of general user instruments in the beamcombining laboratory.
Not yet defined, one might consider, for example, a visible and
near LR camera in the image recombination mode and a Fourier transform
spectrometer for the near LR in the pupil combination mode.
13. A laboratory at the beamcombining station to provide for experimentation
by the experimental astrophysics community.
14. A data recording and quick look analysis capability at the beamcombining
station for data collection, systems performance check and data quality
assessment.