StarDICE IV: correcting visible photometry from atmospheric gray extinction using thermal infrared observations

TL;DR

This paper tackles the challenge of achieving mmag-level photometric calibration from the ground in the presence of clouds that introduce gray extinction. It introduces a method that jointly uses optical photometry and simultaneous infrared radiometry, combined with a forward atmospheric model and Gaia DR3 based stellar SEDs, to infer and correct cloud-induced attenuation on an image-by-image basis. The gray extinction is linked to radiance excess via a radiometric model fitted per exposure, yielding extinction maps with ~2 arcmin resolution and ~0.01 mag accuracy, and improving per-source extinction corrections to as low as 0.025 mag under highly variable conditions. These results demonstrate the feasibility of extending usable observational uptime and precision for future time-domain surveys, with implications for instruments like the Rubin Observatory LSST and CALSPEC-based flux calibration transfers.

Abstract

Next-generation ground-based surveys such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time require photometric calibration that is both long-term stable and spatially uniform at the sub-percent level, even during non-photometric conditions. Achieving this precision motivates new approaches to characterize atmospheric transmission, particularly to mitigate gray extinction from clouds. The StarDICE experiment aims to establish a metrology chain linking laboratory standards to astrophysical fluxes with 1 mmag accuracy in the $\textit{griz}$ bands, a goal for which controlling variable atmospheric effects is essential. We present a method that corrects photometric measurements using simultaneous radiometric information from an infrared thermal camera. The gray-extinction model is fit on an image-by-image basis using thermal radiance excess and the difference between synthetic and instrumental fluxes of calibration stars, without requiring assumptions about the spatial structure of extinction. The method relies on a forward model that incorporates environmental monitoring, radiative-transfer simulations, and Gaia DR3 stellar catalogs. Using data from a remote observing system that repeatedly monitored two fields under diverse atmospheric conditions, we show that the corrections reduce residuals between corrected and reference magnitudes and produce extinction maps with 2-arcmin resolution and $\sim$0.01 mag accuracy. Using this technique, we can recover data acquired under non-photometric conditions with a precision comparable to data obtained under photometric conditions. For the most affected exposures, the mean absolute error improves from 0.64 to 0.11 mag, and temporal extinction variations can be reduced to 0.025 mag per source. We discuss the implications of this technique for future surveys and outline directions for further refinement.

Figures (13)

• Figure 1: Synthetic sky down-welling radiances (solid curves) against instrument transmission (red dashed curve) as a function of wavelength in the long-wave infrared (LWIR) band. The radiances are simulated at airmass $X$ = 1 with libRadTran for a typical atmosphere encountered at OHP site (650 m ASL, $\mathrm{P_{\text{atm}}}$ = 937.5 hPa, $\mathrm{O_{3}}$ = 350 DU, PWV = 12 mm) using a summer seasonal profile. The blue curve depicts the standard atmosphere including the water vapor content whereas the gray curve illustrates the dry component. The orange dash-dotted curve corresponds to the radiance of a blackbody at 273.15K. The green and purple regions illustrate wavelength ranges dominated by ozone and carbon dioxide emissions, respectively.
• Figure 2: Flowchart of the analysis presented in this work. Data products are indicated by rounded boxes, while discrete stages of our analysis are indicated with rectangular boxes and blue text.
• Figure 3: Synthetic transmission curves for the elements of the instrumental chain used for photometry. The optical transmission curve (dotted green curve), filter transmission curve (solid orange curve), the detector's quantum efficiency (dashed black curve) and the atmospheric transmission curve (solid blue curve; same parameters as the simulated curve of Fig. \ref{['fig:libradtran_sky_spectral_radiance_chemicals']}) are also shown. The solid bold red curve correspond to the theoretical instrumental transmission $\mathcal{T}^{\text{inst}}(\lambda)$ without atmosphere defined in Eq. \ref{['eq:instr_transmission']}. The dashdotted purple curve corresponds to the product of the instrument and atmosphere transmission curves. The vertical axis represents the global standardized transmission.
• Figure 4: Panel a: The photometric image of the test field near BD+28 4211, centered approximately at R.A. = 329° and DEC. = 29°. The field contains roughly 425 stars after selection cuts. Magenta boxes and green circles indicate the positions of training and test sources, respectively. Panel b: Stacked radiometric image captured simultaneously with the photometric image. The white rectangle delimits the photometric field-of-view overlaid onto the radiometric image. Higher IR radiances due to clouds are shown in red. Panel c: Time series of radiance for the three color-matching sources marked above. Radiance values are extracted from 73 IR images acquired during a single 20 s optical exposure. The horizontal dashed lines represent the mean radiance, while the shaded areas indicate the interval of $\pm$ 1- around the mean.
• Figure 5: Linear relation between "reference" top-of-atmosphere magnitudes of all the stars in our dataset and the transformed Gaia DR3 r-band magnitudes computed with Eq. \ref{['eq:r_gaia']}. The linear curve is fitted using a robust least-squares method with a smooth approximation to absolute value loss function (soft_l1) to prevent contamination from a small amount of outliers. The gray shaded area on the lower panel is delimited by $\pm \, \sigma_{\text{Gaia}}$ = 0.03776 mag, which corresponds to the Gaia DR3 photometric system conversion function uncertainty. Stars identified by red crosses are excluded from the catalogs.