CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
54 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
http://www.eso.org/observing/etc/
The ETC returns an estimate of the on-source exposure time necessary to achieve the
requested Signal-to-Noise ratio (S/N ratio) given the
Target
properties, the
Instrument
setup
and the constraints set, i.e.,
Sky
,
Seeing
and IQ
(see Figure 40).
A detailed description of all ETC input and output parameters is given on the
web page
,
however below we provide some general guidelines, and advise on the main ETC features.
Figure 40: Screenshot of the CRIRES ETC Exp.Time tab. User requests the desired S/N
ratio by using a given DIT and the ETC returns the total exposure time (DITxNDIT) needed.
Target
tab:
•
The target input magnitude can be specified for a point or an extended source. For
the latter, the input parameter corresponds to the magnitude per square arcseconds.
Also input flux can be specified as surface brightness. If the
Emission line
option is
chosen, it corresponds either to the total line flux, for a point source, or to the surface
brightness for an extended source.
•
If the observing date is known, it may be useful to compute the radial velocity shift
due to orbital and rotational movement of the Earth. Alternatively, the tool allows the
user to select the best time of the year to observe an object so that the targeted
feature avoids a specific telluric line.
Sky
tab:
•
The sky conditions are defined in terms of airmass, moon phase (FLI) ranging from
0 (New Moon) to 1 (Full Moon) and PWV (see Sections 5.3.2 - 5.3.5).
•
By specifying the vertical amount of PWV in the atmosphere, the ETC uses the
appropriate emission and transmission spectra for the PWV and airmass chosen.
This functionality allows one to determine if the S/N ratio will be or not affected by
water vapor. One should note that the effect of water vapor lines also depends on
the temperature at the time of observation, as well as the altitude of the layers where
the water vapor is concentrated, therefore limiting the accuracy of the S/N ratio
determination to a few 10%.
