A method for deriving Cousins R magnitudes from UCAC4 data

We describe a method for deriving Cousins Rc magnitudes from r, B, and V stellar photometry in the UCAC4 catalogue, valid in the range 9 < Rc < 16. The source of the majority of UCAC4 reference star photometry is the AAVSO Photometric All-Sky Survey (APASS) catalogue. Version 433 of the software Astrometrica is updated so that, provided the UCAC4 catalogue is selected as the reference, Rc magnitudes are calculated using our derived transformation, namely: Rc= r – 0.108(B–V) – 0.132. This relationship is especially valid in the colour index range +0.4 < (B–V) < +1.4. A typical uncertainty of 0.03-0.04 magnitudes is obtained for a single star measurement. In practice, the actual accuracy obtained depends on the number of stars of intermediate colour in the analysed image and the quality of the APASS data for the region concerned.


Since 2002 the software Astrometrica, authored by Herbert Raab, has been widely used by both amateurs and professionals for measuring the positions and brightnesses of asteroids and comets. Although originally written as an astrometric tool, it has become increasingly accurate in performing photometry as newer versions have been able to access more modern star catalogues. One of these catalogues is the United States Naval Observatory (USNO) 4th CCD Astrographic Catalog, commonly referred to as UCAC4. This is especially useful for photometry of small solar system bodies, as it incorporates Version DR6 of the AAVSO Photometric All-Sky Survey (APASS), which provides photometry for more than 45 million stars accurate to 0.02–0.05 magnitudes in a variety of filter passbands, more especially Sloan r (red), and Johnson V and B (Visual, Blue).

According to the Minor Planet Center (MPC), the preferred catalogue for photometry is UCAC4. During much of the 20th Century, most observations were made using blue-sensitive photographic plates and so derived magnitudes corresponded to B magnitudes unless otherwise stated. With the arrival of photomultiplier detectors, V photometry became the standard for asteroid magnitudes and the absolute (H) magnitude was defined in this passband.

More recently, with the introduction of CCD cameras, comet observations began to be reported as ‘R’ magnitudes because cometary dust reflects strongly in this passband, which also excludes most of the light from emission lines in the spectrum caused by ionised gas. Also, CCDs are particularly sensitive in the red. So for comet observation, the de facto standard has become the Cousins R or Rc magnitude; yet the UCAC4 catalogue does not include photometric measurements in this passband. We therefore need to be able to transform data obtained in other filters to the equivalent Cousins R values. How best can we achieve this?

Definitive work has been published in the literature for transforming magnitudes from the Sloan filter system to the earlier Johnson–Cousins system and vice versa. An account of the Sloan magnitude system is given by Smith et al. who in Table 7 of their paper included… (continued…)

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