CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
18 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
3.2.1.1 Atmospheric Turbulence
The VLT theoretical di
ffr
action limit is 1.22×l/D = 0.07 arcsec at a wavelength of 2.2 μm.
However, temperature inhomogeneities in the atmosphere induce temporal and spatial
fluctuations in the air refractive index and cause fluctuations in the optical path. This leads
to random phase delays that corrugate the wavefront (WF). The path differences are, to a
good approximation, achromatic. Only the phase of the WF is chromatic. The coherence
time of WF distortions is related to the average wind speed V in the atmosphere and is
typically of the order of ~60 ms at 2.2μm for V = 10 m/s.
3.2.1.2 Adaptive Optics
A technique to overcome the degrading e
ff
ects of atmospheric turbulence is real-time
compensation of the deformation of the WF by adaptive optics (AO, Figure 10).
The wavefront sensor (WFS) measures WF distortions which are processed by a real-time
computer (RTC). The RTC controls a deformable mirror (DM) to compensate the WF
distortions. The DM is a continuous thin plate mirror mounted on a set of piezoelectric
actuators that push and pull on the back of the mirror. Because of the significant reduction
in the WF distortions by continuous AO correction, it is possible to record near diffraction-
limited images with exposure times that are significantly longer than the turbulence
coherence time. One of the main parameters characterizing this image quality is the Strehl
ratio (SR), which corresponds to the amount of light contained in the di
ff
raction-limited core
relative to the total flux.
Figure 10: Principle of Adaptive Optics. Note that in practice, and contrary to this schematic
design, CRIRES has no dedicated tip-tilt mirror, but performs low- and high-order correction
with a single deformable mirror mounted on a tip-tilt stage (see Figure 12).
An AO system is a servo-loop system working in closed loop. The DM flattens the incoming
WF and the WFS measures the residual WF error. A commonly used WFS is the Shack-
