Doc:
Issue
Date
Page
VLT-MAN-ESO-14650-4942
P96
24.06.2015
24 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.2.1.5 The dichroic box
Light is split and distributed to the three arms by two highly efficient dichroic beam splitters.
These are the first optical elements encountered by the science light. The first dichroic at an
incidence angle of 15˚ reflects more than 98% of the light between 350 and 543 nm and
transmits ~95% of the light between 600 and 2300 nm. The second dichroic, also at 15˚
incidence, has a reflectivity above 98% between 535 nm and 985 nm and transmits more
than 96% of the light between 1045 and 2300 nm. The combined efficiency of the two
dichroics is shown in Fig. 6: it is well above 90% over most of the spectral range.
2.2.1.6 The flexure compensation tip-tilt mirrors
Light reflected and/or transmitted by the two dichroics reaches, in each arm, a folding mirror
mounted on piezo tip-tilt mount. These mirrors are used to fold the beam and correct for
backbone flexure to keep the relative alignment of the three spectrograph slits within less
than 0.02” at any position of the instrument. They also compensate for shifts due to
atmospheric differential refraction between the telescope tracking wavelength (fixed at 470
nm for all SLIT X-shooter observations) and the undeviated wavelength of the two ADCs (for
UVB and VIS arms) and the middle of the atmospheric dispersion range for the NIR arm.
In case of IFU observations, one can select the telescope tracking wavelength.
Figure 6: The combined efficiency of the two dichroic beam splitters.
In blue
: reflection
on dichroic 1;
in orange
: transmission through dichroic 1 and reflection on dichroic 2;
in
red
: transmission through dichroics 1 & 2.
