Measurement of telescope transmission using a Collimated Beam Projector

TL;DR

This paper introduces the Collimated Beam Projector (CBP) as a new method to measure telescope transmission more accurately, reducing systematic errors in flux calibration for supernova cosmology.

Contribution

The paper presents the CBP technique, which uses collimated light to more precisely measure telescope transmission and mitigate systematic uncertainties in flux calibration.

Findings

Achieved 3% uncertainty in transmission measurement over the wavelength range

Demonstrated CBP on the StarDICE prototype telescope

Improved handling of systematic errors like ghosting and filter angle dependence

Abstract

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.