  Historical Ca II K spectroheliograms (SHG) are unique in representing
long-term variations of the solar chromospheric magnetic field. They usually
suffer from numerous problems and lack photometric calibration. Thus accurate
processing of these data is required to get meaningful results from their
analysis. In this paper we aim at developing an automatic processing and
photometric calibration method that provides precise and consistent results
when applied to historical SHG. The proposed method is based on the assumption
that the centre-to-limb variation of the intensity in quiet Sun regions does
not vary with time. We tested the accuracy of the proposed method on various
sets of synthetic images that mimic problems encountered in historical
observations. We also tested our approach on a large sample of images randomly
extracted from seven different SHG archives. The tests carried out on the
synthetic data show that the maximum relative errors of the method are
generally <6.5%, while the average error is <1%, even if rather poor quality
observations are considered. In the absence of strong artefacts the method
returns images that differ from the ideal ones by <2% in any pixel. The method
gives consistent values for both plage and network areas. We also show that our
method returns consistent results for images from different SHG archives. Our
tests show that the proposed method is more accurate than other methods
presented in the literature. Our method can also be applied to process images
from photographic archives of solar observations at other wavelengths than Ca
II K.
