The VLT Interferometer - ESO

52 CHAPTER 3. OPTO-MECHANICAL ASPECTS
As the number of interferometer apertures increases, or when their distance
is small compared to their size (the opposite of a diluted aperture), both effects
tend to merge, and the combined fringed/speckle image grows in a new
speckled image with speckle sizes corresponding to that of the diameter of the
interferometric array.
Interferometry using multi speckle images is much less sensitive than that of single
speckle diffraction limited systems. In the highest sensitivity, broad spectral
band mode, the sensitivity for multispeckle imaging also becomes very insensitive
to the size of the telescope with the signal-to-noise ratio increasing only as
D 1 / 6 • For narrow band observations the sensitivity increases, however, as D2 so
that the VLTI will probably be used in that case in the multispeckle mode as
long as adaptive optics in the visible can not be used. It is therefore necessary
to examine the factors listed in Table 3.1 also for the multispeckle case:
1. Fringe Position Variations with Time and with Wavelength affect the
multi speckle observations in ways very similar to that of the diffraction
limited images.
2. Fringe Position Variations across the Pupil just modify the atmospheric
phase disturbances, and hence the speckle pattern. As long as the atmosphere
dominates the wavefront disturbances, there is no effect on the
interferometer sensitivity. When the optics wavefront disturbances dominate,
they will cause an increase in the speckle pattern size and hence
deteriorate the sensitivity.
3. Also Polarization Effects, Unequal Beam Intensities, Detector Resolution,
and Pupil Transfer Geometry affect the multispeckle observations in ways
very similar to that of the diffraction limited images.
3.2 Polarization Effects in Interferometers
3.2.1 General
In most optical interferometers there are a number of components in the optical
trains of each leg (telescope and beamcombiner optics) which affect the polarization
state of the light. The most common of these are non-normal reflections
which act both as partial polarizers (diattenuation) and linear retarders to the
light which they reflect. Multiple reflections can also cause rotation of the polarization
direction (also called here: rotation of Polarization Frame-of-Reference)
in the same way as they can cause image rotation. All three will modify the
polarization state of the light and as a result may decrease the contrast and
change the position of the interferometer fringes. In the ideal interferometer