Lambert: a novel compact calibration solution for superior telescope flat fielding

Abstract

Traditional dome flat fielding methods typically have difficulties providing spatially uniform illumination and adequate flux over a telescopic instrument's entire spectral range. Traditional flat fielding screens, with an illumination source at least the size of the primary, can be difficult or impractical to mount and uniformly illuminate. The Las Cumbres Observatory Global Telescope Network (LCOGTN) will consist of approximately 50 robotic telescopes of 0.4 m, 1.0 m, and 2.0 m apertures with instrument bandwidth ranging from 350 -- 1800 nm. The network requires a robust flat-field solution to fit in compact enclosures. A scanning illuminated flat fielding bar, Lambert, was developed to meet these requirements. Illumination is from a linear arrangement of sources that are spatially dispersed by a narrow holographic or glass diffuser equal in length to the primary's diameter. We have investigated a linearly scanning, enclosure mounted, deployable unit, and a rotary scanning, telescope mounted unit. For complete visible-light bandwidth, a set of different color LEDs is used. The source density, scan speed, and variable intensity tunes the flux to the instrument wavelength and bandwidth. The Lambert flat fields in comparison to sky flats match pixel to pixel variations better than 0.5%; large scale illumination differences, which are stable and repeatable, are ~1%.