Exposure time calculator for IFOSC and sky background estimation

Abstract

This paper describes a spectroscopy exposure time calculator for the faint object spectrograph, the \IUCAA Faint Object Spectrograph" (IFOSC). It is intended to provide reasonable estimates of exposure times for observations. The background sky brightness is modeled for different phases and angular distance from the Moon. The code automatically calculates the Lunar Ephemerides from the date and time of observations and uses it for sky brightness computations. We adopt a new technique of scaling the Rayleigh and the Mie scattering functions, according to the corresponding extinction terms. This estimation of the sky brightness with moonlight ¯ts reasonably well with the observed sky brightness measured at the IUCAA telescope site. A simple component model is used for the optical elements. Each compo- nent is described by simple wavelength dependent extinction tables. At present, a blackbody/power-law spectrum is chosen as the input spectrum. Model or actual spectrum can also be used. The input blackbody spectrum is computed using the stellar \Spectral type" and scaled by the apparent V magnitude of the star. The programme is easily con¯gurable for di®erent sites and instruments. The code is written in ANSI C and thus portable to any system. A graphically interactive interface using HTML-cgi script has also been developed.