[Paper Review] Empirical Color Transformations Between SDSS Photometry and Other Photometric Systems

Katrin Jordi, E. K. Grebel+1 more|Springer Link (Chiba Institute of Technology)|Sep 5, 2006

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper presents empirical color transformations between the Sloan Digital Sky Survey (SDSS) ugriz photometric system and the Johnson-Cousins UBVRI and Becker's RGU systems, using actual SDSS data from the 2.5-m telescope and well-calibrated standard stars. The transformations are linear and show a slight metallicity dependence, particularly affecting very blue and red stars, offering improved accuracy over prior theoretical or indirect methods.

ABSTRACT

We present empirical color transformations between the Sloan Digital Sky Survey (SDSS) ugriz photometry and Johnson-Cousins UBVRI system and Becker's RGU system, respectively. Owing to the magnitude of data that is becoming available in the SDSS photometric system it is particularly important to be able to convert between this new system and traditional photometric systems. Unlike earlier published transformations we based our calculations on stars actually measured by the SDSS with the SDSS 2.5-m telescope. The photometric database of the SDSS provides in a sense a single-epoch set of 'tertiary standards' covering more than one quarter of the sky. Our transformations should facilitate their use to easily and reliably derive the corresponding approximate Johnson-Cousins or RGU magnitudes. The SDSS survey covers a number of areas that were previously established as standard fields in the Johnson-Cousins system, in particular, fields established by Landolt and by Stetson. We used these overlapping fields to create well-photometered star samples on which our calculated transformations are based. For the RGU photometry we used fields observed in the framework of the new Basel high-latitude field star survey. We calculated empirical color transformations between SDSS photometry and Johnson-Cousins UBVRI and Becker's RGU system. For all transformations we found linear relations to be sufficient. Furthermore we showed that the transformations between the Johnson-Cousins and the SDSS system have a slight dependence on metallicity.

Motivation & Objective

• To provide accurate, empirical color transformations between the SDSS ugriz photometric system and traditional Johnson-Cousins UBVRI and Becker's RGU systems.
• To improve cross-system photometric calibration by using actual SDSS observations from the 2.5-m telescope, rather than synthetic or indirect data.
• To assess whether metallicity affects the transformation relations, especially for metal-poor stars.
• To enable reliable magnitude conversions for SDSS objects into standard photometric systems for broader astronomical use.
• To establish a robust, data-driven reference for tertiary photometric calibration using SDSS stars as standard stars.

Proposed method

• Utilized overlapping fields observed in both SDSS and Johnson-Cousins UBVRI systems, specifically Landolt and Stetson standard fields.
• Collected SDSS ugriz magnitudes from the 2.5-m telescope at Apache Point Observatory and matched them with Johnson-Cousins and RGU photometry from the same stars.
• Applied linear regression to derive color transformation equations of the form: m_SDSS = m_standard + a0 + a1*(color)
• Used high-latitude fields from the Basel survey for RGU system calibration, ensuring consistency with SDSS filter responses.
• Tested for metallicity dependence by comparing transformations using metal-poor (Draco dSph, globular clusters) and metal-rich (Landolt) stars.
• Validated results by comparing residuals and assessing nonlinearity, rejecting two-color corrections as ineffective.

Experimental results

Research questions

• RQ1 Can empirical color transformations between SDSS and Johnson-Cousins UBVRI photometry be derived using actual SDSS data from the 2.5-m telescope?
• RQ2 Do transformations between SDSS and Becker’s RGU system show consistent linear behavior, and can they be reliably applied?
• RQ3 Is there a measurable dependence of the transformation coefficients on stellar metallicity, particularly for metal-poor stars?
• RQ4 How do these empirical transformations compare in accuracy to previously published theoretical or synthetic transformations?
• RQ5 Can SDSS stars serve as effective tertiary standards for photometric calibration in traditional systems?

Key findings

• Empirical linear transformations between SDSS ugriz and Johnson-Cousins UBVRI systems were successfully derived using actual SDSS data from the 2.5-m telescope.
• The transformations for the RGU system are also linear and show consistent behavior, with the first such transformations reported in the literature.
• A slight metallicity dependence was detected: transformations for metal-poor stars (e.g., in Draco dSph) differ slightly in slope from those for metal-rich stars, affecting very blue and very red stars most.
• Systematic differences of up to ~0.1 mag were found when comparing with earlier theoretical or synthetic transformations, due to differences in filter–detector–telescope combinations.
• Nonlinear corrections using two color terms did not improve the fit, indicating that linear models are sufficient for both systems.
• The SDSS photometric database provides a reliable, high-density set of tertiary standards for photometric calibration across the sky.