The VLT Interferometer - ESO

72 CHAPTER 3. OPTO-MECHANICAL ASPECTS
• increase the integration time T to more than the atmospheric coherence
time to (sensitivity gain (JT Ito) of several hundredths for T = 1 hour)
In contrast to .fringe tracking Fringe Acquisition and Maintenance cannot be
avoided. Even when all internal distances inside the interferometer are measured
carefully, two beams from two separated telescopes are in general out of
phase by many wavelengths so that fringes have first to be found. For a given
flux from the star, the time necessary to acquire and maintain fringes depends
on their contrast and on how good the metrology of the interferometer is, and
much less on the seeing.
3.6.1 Different Situations depending on the Object's Magnitude
In the section 2.1 the sensitivities have been calculated for several situations
depending on the source used to acquire and/or track fringes. We summarize
here the different cases, in order of increasing magnitude:
1. Enough light is available from the object itself during to and the contrast
of the fringes for the selected baseline is high enough to allow cophasing
measurements : the white light fringe can be actively tracked using a
large spectral band. Depending on the time scale needed to obtain enough
tracking information, the atmospheric contribution to the phase shifts can
be controlled within a fraction of a wavelength (cophasing).
2. There are not enough photons at the selected wavelength but the signal
at another wavelength is enough to locate fringes during to. It may also be
even possible to record the phase error signal together with the "flying"
fringes at the selected wavelength and use it later, provided that the
detector noise is not dominant.
3. The object is too faint whatever the spectral bandpass, or too well resolved
(this means that it is observed at a spatial frequency which yields
too Iowa fringe visibility to allow detection without integration). However
a bright enough source exists in the coherent field of view (the isoplanatic
patch is assumed to be equal or larger). Off-axis fringe tracking is possible
and long time integrations become possible since the referencing is now
independant of the object under study. Unfortunately this situation will
be only exceptional, the density of celestial objects of visual magnitude
::; 11 in the field-of-view and/or isoplanatic patch (a few arcsec in the
visible) is much less than 1.