1. Introduction

In 1971, about one hundred photographs of the Southern Milky Way were taken at La Silla with the Bochum Super-Wide-Angle Camera (Schlosser & Schmidt-Kaler 1978; Schlosser et al. 1975). The majority of the frames were exposed in spectral passbands with filter/emulsion combinations, which matched as closely as possible Sandage & Smith's (1963) photoelectric UBVR system. While these U, B, and V passbands are quite similar to the Johnson (1955) system and thus could be transformed into that standard system by linear colour equations, things are different with the red passband. Sandage & Smith employed a red spectral region, which contains H. This we did not consider as an appropriate base for stellar work, since H from dispersed interstellar hydrogen is practically omnipresent in the night sky and makes stellar aggregates "looking older" than they really are. Therefore this hydrogen line was omitted and the R-band was restricted to wavelengths above 656.3 nm. The finally adopted photographic system U, V, and R (excluding H) is given in Table 1 (click here).

 

Passband U V R

Filter/Schott UG1 GG475 RG665

Emulsion/Kodak IIaO 103aG 103aF

356 534 681

FWHM/nm 52.5 83.5 21.5

350 555 690

Table 1: Photographic Colour System. taken from Johnson (1955) (U and V), and Sandage & Smith (1963) (R)

 

Additionally we used the following colour equations to transform our photoelectric magnitudes (index "pe") to the Johnson standard system:

Although our R-photometry has been calibrated using stellar colours from Tereshchenko & Kharitonov (1977), one should always remember that - in a strict sense - our R-map of the Southern Milky Way (Figs. 1c, 4) represents a colour system of its own.