X-shooter
User Manual
Doc:
Issue
Date
Page
VLT-MAN-ESO-12000-0115
1
01.03.2009
17 of 60
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
Optical layout
The optical layout of the UVB spectrograph is presented in Figure 6. Light from the entrance
slit, placed behind the plane of the figure, feeds a 5˚ off-axis Maksutov-type collimator
through a folding mirror. The collimator consists of a spherical mirror and a diverging fused
silica corrector lens with only spherical surfaces. The collimated beam passes through a 60˚
silica prism twice to gain enough cross-dispersion. Main dispersion is achieved through a
180 grooves/mm échelle grating blazed at 41.77˚. The off-blaze angle is 0.0˚, while the off-
plane angle is 2.2˚. After dispersion, the collimator creates an intermediate spectrum near
the entrance slit, where a second folding mirror has been placed. This folding mirror acts also
as field mirror. Then a dioptric camera (4 lens groups with CaF2 or silica lenses, 1 aspherical
surface) reimages the cross-dispersed spectrum at f/2.7 (plate scale 9.31”/mm) onto a
detector that is slightly tilted to compensate for a variation of best focus with wavelength. The
back focal length is rather sensitive to temperature changes. It varies by ~22.7
µ
m/˚C which
corresponds to a defocus of 9
µ
m/˚C or ~0.08”/˚C. This is automatically compensated at the
beginning of every exposure by moving the doublet of the camera by -10.9
µ
m/˚C.
Detector
The UVB detector is a 2048
×
4102, 15
µ
m pixel CCD from E2V (type CCD44-82) of which
only a 1800
×
3000 pixels window is used. The CCD cryostat is attached to the camera with
the last optical element acting as a window. The operating temperature is 153K. The CCD
control system is a standard ESO FIERA controller shared with the VIS CCD. The list of
readout modes offered for science observations is given in Table 5.
One more readout mode (1000
×
1000 window, low gain, fast readout, 1x1 binning)
exclusively used for flexure measurement and engineering purposes is also implemented.
Measured properties and performances of this system are summarized in Table 6. The
associated shutter, located just after the slit is a 25mm bi-stable (2 coil, zero dissipation)
shutter from Uniblitz (type BDS 25). Full transit time is 13ms. Since the slit is 2.8mm high
(11” at f/6.5), the illumination of the detector is homogenous within <<10ms.
Table 5: List of detector readout modes offered for science observations
Readout mode
Gain [e-/ADU]
Speed
Binning
name
UVB
VIS
[kpix/s] Spatial dir. Dispersion dir.
100k/1pt/hg
1
1
100k/1pt/hg/1x2
1
2
100k/1pt/hg/2x2
High
[0.67]
High
[0.64]
Slow
[100]
2
2
400k/1pt/lg
1
1
400k/1pt/lg/1x2
1
2
400k/1pt/lg/2x2
Low
[1.75]
Low
[1.5]
Fast
[400]
2
2
