Doc:
Issue
Date
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VLT-MAN-ESO-14650-4942
P96
24.06.2015
57 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.4.4 Ghosts
Spurious reflections from the rear surfaces of the dichroics towards the first surface and back
again produce a secondary image of the object on the slit that is displaced from its parent by
few arcsec and leads to almost in focus
ghost spectra in the bottom part of the spectra.
For a centered object the ghost is located on the edge of the orders but when a bright object
is placed on the top part of the slit (positive x) it moves in and becomes particularly
noticeable in the dichroic cut-off region between UVB and VIS arms. It is strongest in the last
order of the UVB spectrum in the wavelength range of the dichroic reflectivity cut-off (see
Figure 13, left). In the VIS, the ghost is noticeable in several orders and its intensity is <0.5%
of the parent spectrum (see Figure 13, right). It is particularly relevant when observing a
bright object with the nod on slit template.
Figure 13: ghost spectra in UVB and VIS produced by back reflection in the two dichroics
A possible ghost seems also to exist in the NIR arm (Figure 14), it lies at the bottom-edge of
some orders (at ~5”) when the observed object is bright. It counts for less than 1%.
Figure 14: Ghost spectrum in the NIR arm.
