2 The camera

 The GAUSS camera (GAlactic Ultra Wide Angle Schmidt System) is an improvement of the Bochum Super Wide Angle Camera, which has been successfully used for the former, earthbound $U,\ B,\ V,\ R$ photometries (Paper VII and Paper VIII). It has been especially designed for the investigation of surface brightnesses. This is achieved by the combination of the short focal length $f_{\rm center} = 11\,\rm mm$ with the high focal ratio f/2. As main progress, the former spherical, concave mirror has been changed for a hyperbolic, convex one, and the imaging component is now a Schmidt system. This combination results in a flat focal plane without astigmatism. The mirror surface is made of aluminum with a protective covering of LiF. In Fig. 1, a sketch of the main components of the camera is given. A ring diaphragm is employed for broad band photometry, and for the colour images. Interference filters centered on 122nm, 175nm, 217nm, 280nm and 357nm are combined with the Schmidt correctors and separate different passbands in the ultraviolet spectral range. All these non-reflective components are made of MgF₂ or SiO₂ depending on the wavelength. Figure 2 shows the transmission curves of the five filters, the sensitivity of the detector can be regarded as constant. Two detectors have been chosen for the UV-images: Kodak SO647 and Kodak 101-01, the latter one being more sensitive by a factor of four. Additionally, a couple of commercial negative and reversal colour films have been exposed. The idea and function of the camera and its linear mounting has been described in detail by Schlosser (1993) and by Schlosser & Koczet (1995).

  

Figure 1: The geometric concept of the GAUSS camera

  

Figure 2:  The transmission of the GAUSS colour system