Abstract

Contributed Talk - Splinter JungeAG

Tuesday, 16 September 2025, 15:10

Evaluating Natural Magnitudes in the Photometric System of Astronomical Plates

Maryam Raouph, Andreas Schrimpf
Philipps Universotät Marburg

Aim: Accurate photometric calibration of astronomical photographic plates remains a fundamental challenge in astronomy, especially when bridging historical photographic data with modern observations. We demonstrate how Gaia Data Release 3 (DR3) Blue Photometer (BP) and Red Photometer (RP) low-resolution spectral data can be used to derive consistent natural magnitudes for celestial sources within the intrinsic photometric systems of astronomical photographic plates. Objective: Our primary objective is to eliminate zero-point offsets caused by the varying spectral color sensitivity response of photographic plate emulsions, which can lead to inaccuracies in the natural magnitude measurements, using the common method of color term correction. Methods: The methodology involves compiling Spectral Characteristic Curve data for each emulsion and filter used in photometric observations using glass plates. These data represent sensitivity across different wavelengths, indicating relative responses to the blue, green/yellow, and red regions of the spectrum. The collected color sensitivities for emulsions are then used to compute accurate magnitudes and fluxes of objects through synthetic photometry, utilizing a catalog of Gaia BP/RP spectra and synthetic magnitudes derived from the spectral energy distribution (SED) over the 330–1050 nm range (XP spectra). This process is carried out using GaiaXPy, a Python library designed to handle Gaia DR3 spectral data. Conclusion: By presenting the synthetic photometry calibration method that operates independently of the color spectral sensitivity variations of emulsions, we achieved more accurate and reliable magnitude estimates for astronomical plate objects. This approach overcomes the limitations of traditional color-term corrections and enables the integration of photometric data from historical plates across global astronomical plate collections, thereby supporting time-domain studies.