  With the increasingly large number of type Ia supernova being detected by
current-generation survey telescopes, and even more expected with the upcoming
Rubin Observatory Legacy Survey of Space and Time, the precision of
cosmological measurements will become limited by systematic uncertainties in
flux calibration rather than statistical noise. One major source of systematic
error in determining SNe Ia color evolution (needed for distance estimation) is
uncertainty in telescope transmission, both within and between surveys. We
introduce here the Collimated Beam Projector (CBP), which is meant to measure a
telescope transmission with collimated light. The collimated beam more closely
mimics a stellar wavefront as compared to flat-field based instruments,
allowing for more precise handling of systematic errors such as those from
ghosting and filter angle-of-incidence dependence. As a proof of concept, we
present CBP measurements of the StarDICE prototype telescope, achieving a
standard (1 sigma) uncertainty of 3 % on average over the full wavelength range
measured with a single beam illumination.
