  DECal is a new calibration system for the CTIO 4 m Blanco telescope. It is
currently being installed as part of the Dark Energy Survey and will provide
both broadband flat fields and narrowband (about 1 nm bandwidth)
spectrophotometric calibration for the new Dark Energy Camera (DECam). Both of
these systems share a new Lambertian flat field screen. The broadband flat
field system uses LEDs to illuminate each photometric filter. The
spectrophotometric calibration system consists of a monochromator-based tunable
light source that is projected onto the flat field screen using a custom
line-to-spot fiber bundle and an engineered diffuser. Several calibrated
photodiodes positioned along the beam monitor the telescope throughput as a
function of wavelength. This system will measure the wavelength-dependent
instrumental response function of the total telescope+instrument system in the
range 300 < lambda < 1100nm. The spectrophotometric calibration will be
performed regularly (roughly once per month) to determine the spectral response
of the DECam system and to monitor changes in instrumental throughput during
the five year Dark Energy Survey project.
