The zero point is defined as the apparent magnitude of a star than gives 1 count per second. We plan to determine the zero points for the different instrument configurations to have an empirical measurement of the throughput of the instrument plus telescope and transmission of the atmosphere. It is determine as follows:
ZP = mag + ext + 2.5log(S)
where mag is the apparent magnitude of a standard star in the relevant band taken from the literature, ext is the atmospheric extinction in magnitudes for the standard star (= airmass x zenith extinction coefficient) and S is the flux in counts per second received from the standard star, i.e. the measured value:
S = (counts - sky background - dark current) / total exposure time
(Colour term has not been taking into account).
| filter | Zero Points (magnitudes) |
Extinction Coefficient (mag./airmass) |
|---|---|---|
| J | 23.73 ± 0.07 | 0.107-0.099 |
| Km | 23.0 ± 0.1 | 0.096-0.085 |