Speaker
Description
The FRAM (F/Photometric Robotic Atmospheric Monitor) telescopes
are small robotic instruments deployed at astroparticle physics
observatories for continuous atmospheric monitoring using wide-field
stellar photometry. At the Cherenkov Telescope Array Observatory (CTAO),
FRAM measures the vertical aerosol optical depth (VAOD) at optical
wavelengths.
We extend the original single $B$-filter (440~nm) method to simultaneous
measurements in $B$, $V$ (540~nm), and $R$ (620~nm) using Gaia~DR3
synthetic photometry, enabling the determination of the Ångström
coefficient --- which describes how aerosol extinction varies with
wavelength. We derive the Ångström coefficient at three CTAO sites over
2021--2022 and validate the results against independent AERONET
sun-photometer measurements at La Palma, where both
instruments independently recover the same two aerosol populations
corresponding to episodic Saharan dust intrusions and clean maritime
background conditions. This agreement between two completely independent
instruments operating on different physical principles provides
confidence in the FRAM method for nighttime aerosol characterisation
at the CTAO sites.