La Palma sky brightness

The night-sky magnitudes adopted for the TNG Exposure Time calculators (ETC), were derived from recent measurements (June and August 2002) made with Dolores@TNG during moonless photometric nights. We also added estimates for the Johnson U and Gunn z bands, though with poorer accuracy.

Our values (dark sky) are in good agreement with the values given by the La Palma sky-brightness Calculator, that adoptes average values for Ecliptic and Galactic latitudes and maximum Solar Activity (sky is up to ~0.4mag brighter at solar maximum; latest solar minimum was in 1994-96). Our values are also in good agreement with those found at ESO.

The ETCs also use average values for the Atmospheric Extinction Coefficients at the Roque de Los Muchachos Observatory: U=0.49 mag, B=0.25, V=0.15, R=0.11, I=0.07, z Gunn=0.07 (see La Palma Technical Note No. 31 for further details).
A brightening of the sky background with airmass (sky is ~0.25 mag brighter at a zenithal distance of 45 deg. than at the zenith itself) is also taken into account by the ETCs at optical and NIR wavelenghts (excepting the K' and K bands).

The Sky-brightness magnitudes at various lunar phases (greater or equal 3 days) were calculated by offsetting the dark values for the differences (Dark-Bright) as derived from La Palma sky-brightness Calculator at the ING site. In the case of the Gunn z filter we simply adopted the sky parameters for the I-band filter.

Please note that calculated moon contribution could be affected by large uncertainty, as stated in the relevant web page.

In the following table the adopted night sky brightness magnitudes at various lunar phases are listed.

Days from new moon U B V R I z Gunn
0 22.5 22.8 21.8 20.9 19.7 19.3
3 22.3 22.5 21.6 20.8 19.6 19.2
7 21.4 21.4 20.7 20.2 19.1 18.7
10 20.4 20.3 19.6 19.2 18.2 17.8
14 19.3 19.1 18.5 18.2 17.3 16.9

The adopted values for the IR sky are shown in table 2 (see also Nics pages).
Note that due to a negligible influence of moonlight in the infrared, these values are to be taken irrespectively of the moon phase.

Table 2. IR sky brightness.
1 µ Js J H K' K
16.5 15.5 15.5 14.0 13.2 12.6

For any comments please contact Marco Pedani.