The influence of diffuse scattered light. I. The PSF and its role in observations of the edge-on galaxy NGC 5907

All telescopes and instruments are to some degree affected by scattered light. It is possible to estimate the amount of such scattered light, and even correct for it, with a radially extended point spread function (PSF). The outer parts of the PSF have only rarely been determined, since they are faint and therefore difficult to measure. A mostly complete overview of existing properties and measurements of radially extended PSFs is presented, to both show their similarities and to indicate how bright extended objects can be used to measure the faintest regions. The importance of the far wings of the PSF and their possible temporal variations are demonstrated in three edge-on galaxy models. The same study is applied to the first edge-on galaxy where earlier observations reveal a halo, NGC 5907. All PSFs were collected in two diagrams, after they were offset or normalized, when that was possible. Surface-brightness structures of edge-on galaxies were modelled and analysed to study scattered-light haloes that result when there is an exponential disc. The models were convolved with both a lower-limit PSF and a more average PSF. The PSF of the observed data could be used in the case of NGC 5907. The comparison of the PSFs demonstrates a lower-limit r^-2 power-law decline at larger radii. The analysis of the galaxy models shows that the outer parts of the PSF also are important to correctly model and analyse observations and, in particular, fainter regions. The reassessed analysis of the earlier measurements of NGC 5907 reveals an explanation for the faint halo in scattered light, within the quoted level of accuracy.