3.3. Corrections to the magnitude scale
As mentioned earlier, in one of the most important papers of the decade, Stebbins, Whitford, and Johnson (1950) showed that corrections to the mpg magnitudes in the Mount Wilson Catalog for SA 57, SA 61, and SA 68 begin as bright as mpg = 16 and increase with increasing faintness. They further showed that the North Polar Sequence, long the principal source of faint standards in the decades before 1950 (Seares 1915, 1922a, b; Seares and Humason 1922) was accurate in scale to better than the 0.1 mag level to the limit of its tabulation.
Later work extended the result to additional selected areas where it was shown that the needed corrections were generally ubiquitous and in some areas reached 1.5 mag at mpg(Seares) = 18.5. Examples for four Selected Areas are in Figure 3, taken from an early summary of a systematic program to determine the corrections to the faintest level of the Catalog (Sandage 1983, 1998) in a dozen Selected Areas. The corrections begin near mpg = 15 and increase to an average of 0.7 mag at mpg = 18.
Figure 3. Comparison of the apparent magnitudes in the Mount Wilson Catalog of Selected Areas with the listed mpg magnitudes by Seares et al. (1930). The modern B magnitudes have been measured photoelectrically in a program at Mount Wilson in the 1970s. Diagram from Sandage (1983).