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%.
