CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
22 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
The wavefront sensor box consists of 4 mirrors, which provide parallel beam to image the
pupil on the lenslet array. First, the beam is collimated by a spherical mirror. It is then folded
by a flat mirror and injected in the beam expander, which adapts its diameter to the lenslet
array (14 mm). The optical path of the wavefront sensor box is shown in Figure 13.
Figure 14: Front-end assembly of the 60 fiber bundle which guide the light to the sensors.
The lenslet array intercepts the beam and divides the flux in 60 sub-apertures. Each sub-
pupil is imaged on a fiber, with a 100 μm core diameter. When the membrane mirror
vibrates, the pupil image is projected on both sides of the lenslet array plane. The
normalized difference between the intra- and extra-pupil flux collected by each sub–
aperture is proportional to the local wavefront curvature, which provides the wavefront error.
The fibers drive the signal from the fiber feed module to the APD cabinet, mounted on the
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.
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.
3.2.3 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, Kr/Ne lamps, a He-Ne laser for spectral resolution
measurements and the Fabry-Perot Etalon System (FPI) fibre 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.
