A calibration standard above the atmosphere.
The CELESTE spacecraft doubles as the most precisely characterised photometric reference source observable from any equipped ground telescope — transforming the systematic floor of survey cosmology.
Imagery · ESO / G. Vecchia
Key Science Topic
Flux
Flux
calibration.
A spaceborne calibration source enables absolute photometric standards an order of magnitude beyond current limits — observable through the same atmospheric column as ground science targets.
Almost every cosmological measurement made in the next decade depends on the absolute and relative photometric calibration of major survey facilities. Type Ia supernova cosmology, weak-lensing photometric redshifts, and exoplanet atmospheric retrievals all share the same systematic floor: how well we know the flux of the standard sources used to calibrate the instruments.
The CELESTE spacecraft can carry an onboard, well-characterised photometric source. Above the atmosphere, the source is unaffected by terrestrial extinction, water vapour, and aerosol variability. Observed from the ground through the same optical path as a science target, it constrains the instrumental and atmospheric transmission of the host observatory at the sub-percent level — an order of magnitude better than what is achievable with terrestrial standard stars alone.
The Brief
i.
Science Objectives
Deliver absolute spectrophotometric standards traceable to a laboratory radiance scale, observable through the same optical path as ground-based science targets. Provide a calibration anchor for survey programs on Roman, Rubin LSST, ELT, and TMT at the 0.1% level.
ii.
Observables & Measurements
A space-borne calibration source with a well-characterised spectrum across the 0.4 – 1.0 μm CELESTE band. Paired observations: calibration source and science target observed through the same atmospheric column within the same orbit.
iii.
Key Functional Requirements
Source spectral energy distribution known to < 0.1% in the 0.4 – 1.0 μm band. Pointing stability sufficient to place the calibration beam within the field of the ground instrument. Orbit geometry that allows paired observations with major survey observatory sites.
iv.
CELESTE Uniqueness
No existing or planned facility provides an above-atmosphere, absolutely calibrated photon source that can be pointed at any sky location and observed simultaneously with ground science targets through the same atmospheric column. This is unique to the CELESTE hybrid architecture.