Exposure Time Calculator

The Exposure Time Calculator is preliminary. It is based on instrumental efficiency, detector characteristics, and further assumptions that will be refined with on-sky tests. The presently used characteristics are the following:

RON 5 electrons
DC 0.007 electron/s
Velocity bin 2.28 km/s
Individual exposure time 5.4 s
Extracted pixels 35
Efficiency Y:4% J:7% H:14% K:13%
Stellar Templates Phoenix library
Atmospheric Templates Maunakea from ATRAN

The ETC can be used both ways: 1) to calculate the SNR per 2.28-km/s bin for a set of magnitude, star temperature, seeing, and total integration time (left example below) or 2) to estimate the exposure time needed to reach a given SNR per 2.28-km/s bin (right example below). The SNR of the center of each order is given in the detailed output. Regions of low SNR correspond to low-transmission bands of the Earth atmosphere. The variations of the SNR with airmass and H2O column density are low, according to the used models.
Note that no graphical output of the ETC is yet available.



For M stars, the ETC also provides the user with an estimate of the radial-velocity precision. This quantity depends on several parameters:

- the signal-to-noise ratio per velocity bin (as calculated with the tool described above)
- the spectral line content of the spectrum, from a library of stellar spectra (Figueira et al, 2016). Tabulated values were used and interpolated accross the effective temperature range from 2500 to 4000K.
- the projected rotational velocity of the star (Bouchy et al, 2001). We used rotational velocities of 1 and 10 km/s to give a range.
- the precision at which telluric lines are corrected. We used two extreme values (min and max sigma RV, correcting versus removing tellurics), refering to methods 2 and 3 of Figueira et al, 2016. The actual correction level will likely lie within this range, close to the best-case precision scenario (Artigau et al 2014).


Theoretical predictions as those cited above, and SNR estimates, allow us to foresee the following performance in terms of radial-velocity accuracy:
Again, these performances will be refined and validated when SPIRou gets on-sky data.