If we consider for example a point source with a uniform emission of magnitude 15, in K band, being the sky airmass 1.2, and a seeing of 0.5, using a slit width of 2 arcsec, in low resolution, with the red grism inserted, and the large field, for an integration time, we could get:
where we have used data in Tab. 2.3 and eq. (2.5), (2.6) and expressed the result in Joule units.
Using eq. (2.7):
With the previously specified parameter we can get from formulae in Tab. 2.1:
The ETC for FORS1 with the input parameter as specified above predicts NETC=515 cntsbin.