CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
19 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
Hartmann WFS, used for example in NACO. However, CRIRES, as well as other ESO
MACAO systems, relies on a curvature WFS: it is designed to measure the WF curvature
as opposed to the WF slope. This is achieved by comparing the irradiance distributions of
two planes placed behind and in front of the focal plane. In practice, a variable curvature
mirror (membrane) is placed in the telescope focus. By vibrating, inside and outside focus
blurred pupil images can be imaged on a detector array: in the case of CRIRES, a lenslet
array feeds avalanche photo-diodes (APDs). The modulation frequency of the membrane
corresponds to the temporal sampling frequency of the WFS. The di
ff
erence between the
inside and outside pupil image measures the local WF curvature.
The performance of an AO system is related to the number of lenslets in the lenslet array,
the number of actuators behind the DM, and the rate at which WF errors can be measured,
processed and corrected (the server-loop bandwidth). The performance of an AO system is
also linked to the observing conditions. The most important parameters are the seeing, the
coherence time, the brightness of the reference source used for WFS and the distance
between the reference source and the object of interest. In case of good conditions (i.e.,
seeing < 0.8” and coherence time > 3 ms) and a bright (i.e., R < 7), nearby (i.e., within ~5
“) reference source, the correction is good, and the resulting point spread function (PSF) is
very close to the di
ff
raction limit. A good correction in the K-band typically corresponds to a
SR larger than 30%. At shorter wavelengths (particularly in the J-band) or in the case of
poor conditions or a faint, distant reference source, the correction is only partial - the SR
may only be a few percent.
Figure 11: Top view of the warm optics of the MACAO-CRIRES system. From f/15 Nasmyth
focus and after the optical derotator, one notices the deformable mirror and the tip-tilt mount
assembly. Light enters from the dichroic to the cold and warm part of the instrument. On the
right the wavefront sensor and some analysis tools are visible.
