Typical sky brightness at La Palma, in mags per square arcsec, are given below, in `dark', `grey' & `bright' time. These data refer to clear conditions (no cloud or dust; low aerosols; extinction at V between 0.10-0.16 mag/airmass).
Table 18: La Palma Sky Brightness (Mag/Square Arcsec)
In the Table, `Dark' means Moon below the horizon; `Grey' means above the horizon with phase 0.58-0.67, and `bright' means above the horizon with phase 0.90-0.96. The sky data are guidelines; precise values depend critically on angular distance from the Moon, the elevation of the Moon, and the atmospheric aerosol & dust content.
The dark-sky variation between 1984-9 may be partly due to a correlation with phase of the 11-year Solar cycle; a variation of 0.5mag has been reported by the UK Schmidt Telescope, but because of the lower geomagnetic latitude of La Palma a slightly smaller amplitude is expected here.
Flux-calibrated, low-resolution night sky (airglow) spectra have been presented by C.R. Benn (Gemini, 35, p20, 1992) and by C. Jenkins & S. Unger (ING La Palma Technical Note No 82). Note that in the red, emission lines from OH and H O molecules dominate the spectrum, and so the broad-band magnitudes given above do not give a good idea of the sky continuum flux at any particular wavelength.