[Paper Review] Transformation of Pan-STARRS1 gri to Stetson BVRI magnitudes. Photometry of small bodies observations

A. Kostov, T. Bonev|arXiv (Cornell University)|Jun 19, 2017

Stellar, planetary, and galactic studies12 citations

TL;DR

This paper presents a set of linear transformation equations to convert Pan-STARRS1 gri magnitudes to the Stetson BVRI photometric system, enabling accurate relative photometry of small Solar System bodies using PS1 as a secondary standard. The method achieves sub-magnitude precision, validated on asteroid 48220 with V-magnitude consistent with ephemeris data and neutral colors typical of C-type asteroids.

ABSTRACT

The UBVRI broad band photometric system is widely used in CCD astronomy. There are a lot of sets of standard stars for this photometric system, the Landolt's and Stetson's catalogues being the most precise and reliable. Another photometric system, recently considerably spread in CCD observations is ugriz, which originates from the Sloan Digital Sky Survey (SDSS) and has now many variations based on its 5 broad-band filters. One of the photometric systems based on it is The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). In this paper we compare the BVRI magnitudes in the Stetson catalogue of standard stars with the magnitudes of the corresponding stars in the Pan-STARRS1 (PS1) grizyw catalogue. Transformations between these two systems are presented and discussed. An algorithm for data reduction and calibration is developed and its functionality is demonstrated in the magnitude determination of an asteroid.

Motivation & Objective

• To establish a reliable transformation between Pan-STARRS1 gri and Stetson BVRI photometric systems for use in small-body photometry.
• To overcome the limitations of USNO B1.0, which lacks photometric accuracy, by leveraging the high-precision, all-sky coverage of Pan-STARRS1.
• To enable accurate relative photometry of moving objects like comets and asteroids when standard stars are absent in the field of view.
• To develop a robust, repeatable calibration procedure using instrumental magnitudes and transformation coefficients for scientific use.

Proposed method

• Cross-identified 15,074 stars between Stetson's BVRI catalogue and the Pan-STARRS1 catalogue using astrometric and photometric criteria.
• Used seven transformation equations (linear fits) relating gri to B'V'R'I' magnitudes, incorporating color terms to account for filter response differences.
• Derived photometric zero points (Z) and extinction coefficients (S) per image by comparing instrumental magnitudes with transformed standard magnitudes.
• Applied the transformation algorithm to calibrate instrumental magnitudes of a moving object (asteroid 48220) in the NGC5904 field.
• Validated the method on three Stetson standard fields, confirming consistency and reliability of the transformation coefficients.
• Used ASTROMETRICA software to extract and calibrate the asteroid's instrumental magnitudes, applying the derived transformations.

Experimental results

Research questions

• RQ1 Can Pan-STARRS1 gri magnitudes be reliably transformed into the Stetson BVRI system for use in small-body photometry?
• RQ2 What are the optimal transformation equations that account for differences in filter response functions between the gri and BVRI systems?
• RQ3 How accurate are the derived transformation coefficients when applied to real observational data, particularly for moving objects?
• RQ4 To what extent do stellar population effects (e.g., Population II stars) or extreme colors affect the transformation accuracy?
• RQ5 Can the method be reliably applied to calibrate instrumental magnitudes of asteroids when no standard stars are present in the field?

Key findings

• The transformation equations produced reliable results, with linear fits showing low residuals and high consistency across multiple test fields.
• The V-magnitude of asteroid 48220 was measured as 18.29 ± 0.08, in excellent agreement with the JPL HORIZONS ephemeris.
• The derived B-V, V-R, and R-I colors of 48220 (0.829 ± 0.165, 0.364 ± 0.109, 0.317 ± 0.131) indicate a neutral reflectance typical of C-type asteroids.
• The transformation method successfully enabled photometric calibration of a moving object without standard stars in the field, demonstrating its utility for small-body observations.
• Minor deviations from linearity were observed for extremely red stars, and Population II stars in NGC6341 slightly affected results, but these did not compromise overall accuracy.
• The method is applicable to a wide range of photometric tasks, including variable stars, exoplanets, and optical transients, where standard stars are unavailable.