User Manual
Doc. Number: ESO-323064
Doc. Version: 2
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instrument. The APD counts are recorded by the APD counter module, synchronously with
the membrane signal. The front-end assembly of the fiber bundle is shown in Figure 14.
Figure 14: Front-end assembly of the 60 fiber bundle which guide the light to the sensors.
3.2.2.3 The Control Loop
The oscillating membrane produces a signal modulated proportional to the local wavefront
curvature. This signal, collected by APDs, is sent to the RTC. The RTC computes this
modulation and retrieves the voltages to be applied to the mirror and tip-tilt mount to
optimally compensate for the local curvature measured. For this, a precise calibration of the
system is required, which includes synchronization of the membrane mirror, determination
of the membrane curvature, pupil alignment and interaction matrices.
3.2.2.4 The Membrane Setting
The membrane mirror curvature represents an optical gain for the aberration
measurements. A way to increase the performance of the system is therefore to increase
the curvature of this mirror. Increasing the curvature, however, requires increasing the field
of view of the wavefront sensor optics as well. This and some other non-linear effects can
degrade the estimate of the curvature. For the same reason, extended sources will a
ff
ect
the quality of curvature measurement, and lead to a di
ff
erent optimal gain. In some extreme
cases, the system can be unable to close the loop (extended
6’’
planetary nebula with a
faint blue-white dwarf in the middle, or a faint star close to the Moon, for example). A trade-
off is needed, and an optimal optical gain has to be determined. This optimal gain mainly
depends on the seeing, and marginally on the star magnitude and other factors. It is
tabulated in the configuration of the software and is transparent to the user.
3.2.3 The New Calibration Unit
The calibration unit itself consists of an integrating sphere illuminated by a continuum,
Halogen lamp for flat-fielding and, together with a gas-cell, for wavelength calibration. An
IR- emitter lamp used for technical tests, a Kr/Ne lamps and the Fabry-Perot Etalon System
(FPI) fiber are also attached to the integrating sphere (see Figure 15). The integrating
sphere provides uniform illumination of the entrance slit of the spectrometer and its flux can
be adjusted by a moving baffle.