5.3.3 Aperture and orientation corrections

The measurement of total galaxy magnitudes requires that contributions from all parts of a galaxy can be measured. This is not possible when

- parts of the galaxy are not included by the measuring process

- parts of the galaxy are hidden.

Maguitude measurements may require corrections for the limited size of the measuring aperture or uncertain sky correction in the case of apertures which are too large.

The aperture effect is mentioned implicitly by Whitford (1936) in the first report about photoelectric magnitude measurements of galaxies. The more detailed paper by Stebbins and Whitford (1937) lists magnitude measurements made with different apertures. Form their tabulated values the increase in total brightness with increasing aperture size is clearly apparent.

Hidden light due to intervening matter is discussed above. Light is also hidden as a function of orientation: inclinations of the plane of the galaxy relative to the plane of the sky near 90° require the largest corrections. Regions of low surface brightness may be lost in the sky background.

Hubble (1932) presented an interesting effect which is related to the above phenomenon: objects of low surface brightness are lost in the background as a function of surface brightness (Fig. 28).

Figure 28. Relation between surface brightness and diameter of threshold images. (Hubble 1932).