Optimising background-limited observing during bright-moon phases and twilight

Abstract

For the majority of optical observing programs, the sky brightness provides the fundamental limit to signal detection such that the scientific feasibility is largely dictated by the moon phase. Since most observatories do not have the resources to build expensive high-resolution or infrared instruments, they are increasingly at a loss as to how to exploit bright time. We show that, with due consideration of the field and moon position, it is possible to undertake `dark time' observing programs under `bright time' conditions. Our recommendations are particularly appropriate to all-sky survey programs. In certain instances, there are gains in observing efficiency with the use of a polariser, which can significantly reduce the moonlight (or twilight) sky-background flux relative to an extraterrestrial flux. These gains are possible in background-limited cases because the sky background can be highly polarised, due to scattering, around ninety degrees away from the moon (or sun). To take advantage of this, only minor modifications to existing instruments are needed.