Detection of scattered light in telescopes

Abstract

We present a simple, but powerful method to test for the presence of scattered light in telescopes. Scattered light can seriously hamper any attempt to achieve a proper flat fielding of CCD data. The method consists of making a pinhole camera (camera obscura) by placing a pinhole mask in front of the CCD window. Exposing the CCD in this configuration gives an image of the light sources in the optical path. Tests made at the Nordic Optical Telescope have led to a reduction of large scale flat field variations due to scattered light from a total of ∼ 5% to below 1% by improving the telescope baffles.