Download FLAMES User Manual

FLAMES User Manual
VLT-MAN-ESO-13700-2994
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The flatfield spectra provide a good correction for the blaze function of the echelle. They
correct the pixel-to-pixel variation in CCD sensitivity as a function of the wavelength of
the light. In the red part of the spectra (λ ≥ 650 nm) narrow fringes with peak-to-valley
amplitudes up to 30% are present on the EEV CCD of the mosaic. On the MIT-LL CCD, the
fringes are less sharp and of smaller amplitude.
In the fibre mode of UVES, long slit spectra are acquired with a slit longer than the extent
of the fibre slit to ensure that even in case of small shifts between the observation and the
calibrations, the detector area covered by the fibres is covered by the long slit spectra. Pixelto-pixel variations can be reduced in this way.
5.7
Fibre to Fibre Transmission and Sky Subtraction issues
When dealing with fibre spectra proper sky subtraction is a concern. For some science cases
this may be the limiting factor on the quality of the reduced data. In the present scheme
(i.e., without nod and shuffle technique) it is critical to characterize the fibre to fibre relative
transmission with excellent accuracy. This task is not always trivial, because fibres may
develop photometric instabilities which depend on their history and on the way they are
routed. In FLAMES we have ensured that in normal operating conditions, the fibre system is
constant to better than 1% stability. At this point, the most critical issue is to find a way to
uniformly illuminate the fibres. This task is done by the positioner.
Other steps to obtain a correct sky subtraction involve a) the knowledge of the transmission
of the corrector, which is given in Table 4 as a function of wavelength and position on the field
of view and b) a good spatial distribution of the fibres, which can account for sky variations
in the field of view. It is also important that enough fibres are allocated to the recording of
the sky.
Fig. 21 shows an example of a GIRAFFE image of exposure of 2750-s taken with Carreras at
a wavelength of L881.7. Aside from the many cosmic rays present, the attention of the reader
is drawn to the many sky emission-lines present at this and other red settings. Fig 22 shows
extracted spectra which emphasise the strength of the sky lines.
Before submitting a proposal PIs should consider downloading previous GIRAFFE spectra
from the ESO archive to see how badly their spectra will be contaminated by sky lines. We
note that the current version of the GIRAFFE pipeline does not perform sky subtraction.
PIs should consider referring to the following articles (amongst others) on how to remove sky
features in FLAMES data:
• Battaglia et al., (2008), MNRAS, 383, 183. Contains a detailed description of how sky
lines can removed from FLAMES-GIRAFFE spectra.
• Koch et al. (2007), AJ, 134, 566. An estimate in the final accuracy of sky subtraction
of ∼3 per cent is given for Leo spectra.
• Koch et al. (2006), AJ, 131, 895. An estimate in the final accuracy of sky subtraction
of ∼2 per cent is given for Carina spectra.