Introduction

Observations of standard stars are of crucial importance in stellar photometry. Currently two main sets of standard stars are used for astronomical broad band UBVRI photometry. One is the E-region standards centered at declination -45 which provide fundamental standards for UBVRI photometry in the southern hemisphere. These regions were developed to calibrate photographic magnitudes where the north polar sequence was not observable. The establishment of the E-region standard star photometry was performed by Dr. A. W. J. Cousins. Astronomers at the South African Astronomical Observatory (SAAO) continue to refine the accuracy (Menzies et al. 1989) and to extend the colour range (Kilkenny et al. 1998) of the standards. Although E-regions provide the most accurate standards available currently, there are several limitations. E-regions are available only in the southern hemisphere; many E-region standards are too bright and too sparsely distributed, and therefore not well suited for modern CCD observations.

The other main set is the Landolt standards (Landolt 1973, 1983, 1992) along the celestial equator. Initially the system comprised only UBV, but has since been extended to Cousins' RI. Landolt standards have several strong points. The first is the availability from both hemispheres. The second is the inclusion of several blue and red stars in a small area which makes it easier to obtain the transformation coefficients. Thirdly, the magnitude coverage was extended to faint stars to provide standard stars for CCDs on large telescopes.

Cousins (1984a,b) found some small systematic differences between his photometry and that of Landolt for some stars in common in the equatorial regions. Later, Menzies et al. (1991) performed extensive photometry of Landolt equatorial standards and demonstrated the extent and nature of the systematic differences with respect to the E-region standards. Bessell (1995) derived transformation relations between the SAAO system (and the Bessell 1990b extension for the reddest stars) and the Landolt system.

Both systems were established with photoelectric photometers, while most current photometry is undertaken using CCDs. In most cases, linear transformations to the standard system are employed for the CCD photometry. Linear transformations are possible when the combined response of filter and detector for the two photometric systems are very similar. The bandpasses of the wide band UBVRI system are relatively well defined (see Bessell 1990a) as are reasonable glass filter combinations. The quantum efficiency of CCD chips are also usually similar across the V, R and I bands but can vary greatly in the UV and blue due to different processing steps in CCD manufacture. Some CCDs have zero UV and low blue quantum efficiency (QE) because they are 'thick' CCDs. Others are thinned, but the surface treatment and anti-reflection coatings differ resulting in different UV and blue responses. Thick (and thin) CCDs can also be coated with a fluorescent material such as Lumogen that absorbs UV and blue photons and converts them into green ones better detected by the CCD. As a result of all these effects, the combined response of filter and detector can produce drastically different U and B bands for different CCDs. The existence of discontinuities in stellar energy distributions or the presence of emission lines may then give rise to non-linear terms in the transformation equations.

In this work, we will discuss the transformation of CCD photometry to the standard systems based on observations of standard stars made during observing runs at Siding Spring Observatory (SSO) in 1996 and 1997. In 1996 August, November and 1997 January, Feb/Mar we observed Landolt standard regions. In 1997 May/Jun, August and November, SAAO E-regions (Menzies et al. 1989), SAAO measurements of equatorial standards (Menzies et al. 1991) and blue and red standards (Kilkenny et al. 1998) were observed. In Sec. 2, we will discuss the atmospheric extinction coefficients. The transformation relations will be dealt with in Sec. 3. Some comparisons will be made in the same section. The results will be summarized in Sec. 4.

© Copyright Astronomical Society of Australia 1997