We describe TCal, a mobile spectrophotometric calibration system that will be used
to characterize the throughput as a function of wavelength of imaging systems at observatories
around the world. TCal measurements will enhance the science return from follow-up
observations of imaging surveys such as LSST (Large Synoptic Survey Telescope) and
DES (Dark Energy Survey) by placing all tested imaging systems on a common photometric
baseline. TCal uses a 1 nm bandpass tunable light source to measure the instrumental
response function of imaging systems from 300 nm to 1100 nm, including the telescope,
optics, filters, windows, and the detector. The system is comprised of a monochromator-based
light source illuminating a dome flat field screen monitored by calibrated photodiodes,
which allows determination of the telescope throughput as a function of wavelength.
This calibration will be performed at 1-8m telescopes that expect to devote time towards
survey follow-up. Performing the calibration on these telescopes will reduce systematic
errors due to small differences in bandpass, making follow-up efforts more precise
and accurate.
