Download EFOSC2 Manual 3.1

104
EFOSC2 USER’S MANUAL - 3.1
t=
LSO-MAN-ESO-36100-0004
F∗ + Fsky
SN R2
g × F∗2
(24)
For example, with a seeing of 100.5 and during the new Moon, to reach a SN R = 100 with a star of
V = 20.0 an exposure time of about 90 seconds is required.
If the objects are very faint, the P SN from the sources is also negligible with respect to the noise
generated by the sky background. In this regime, if we assume that the detection limit is given by
a SN R = 5, we can easily compute the limiting magnitude of an image with an exposure time of t
seconds:
mlim ' m0 − 2.40 − 2.5 logF W HM − 1.25 log
ssky
t
(25)
The limiting magnitude with a seeing of 100.5 in a V exposure of 1800 seconds taken during the new
Moon is Vlim ' 25.1. If we assume the limiting SN R = 3 the correspondent limiting magnitude
increases by 0.56 mag. Note that the integrated SN R for a point source does not depend on the
binning factor b, while the SN R in a single pixel scales directly as b.
In the case of extended sources (i.e. when the angular size of the object is much larger than the
seeing) it is more convenient to treat each pixel separately instead of computing an integrated SN R.
If the surface brightness of the source is m∗ mag arcsec−2 , the expected SN R is:
SN Rext ' 0.16 b √
√
s∗
g×t
s∗ + ssky
(26)
where s∗ = 100.4×(m0 −m∗ ) .
An imaging exposure time calculator for EFOSC2, developed by J. Brewer, can be found online at:
http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=EFOSC2+INS.MODE=imaging.
A.2
Spectroscopy
In planning their observing strategy users can refers to the EFOSC2 exposure time calculator available
at:
http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=EFOSC2+INS.MODE=spectro
A.2.1
Relative Grism sensitivities
The relative Grism throughputs/sensitivities are displayed in Fig. 32. These are all measured relative
to Grism 1, which is the EFOSC2 grism with the widest spectral range. For their measurements,
spectra were taken with all the grisms with an internal lamp with fixed intensity. With the spectrum
for Grism 1 normalized to have a value of 1 everywhere, the throughputs of different grisms relative
to Grism 1 are calculated.
Fig. 32 is useful for finding the most efficient grism at the spectral range that you want. For example,
at 8000Å Grism 2 is the most efficient.
Beware that in Fig. 32 for some Grisms there seems to be an upturn or bump in the blue extreme
of the curve. This is due to reflected light in the instrument and not due to a real improvement
in sensitivity. As the sensitivity in UV drops dramatically, and the internal lamp is rather red, the
spectrum of the internal lamp is quite faint in the blue. Thus a small amount of reflected light
contamination gives a bump in the blue extremes of some of the curves.