Visibility of magnetic features on filtergrams and spectroheliograms

An expression is derived for the contrast variation of a hypothetical magnetic element observed with a tunable monochromatic filter or spectroheliograph in a triplet Zeeman line. The theory is valid for a magnetic field of arbitrary strength and direction and for polarizing and non-polarizing optics. Numerical results are presented for both a stationary and a moving magnetic element over a range of field strengths and directions. They provide a simple diagnostic means of inferring the presence or, alternatively, the absence of strong fields, such as the kilogauss network fields postulated by Stenflo and others on the basis of indirect evidence. The proposed method of measuring strong fields is capable of very high spatial resolution and is free of the saturation effects encountered in the conventional magnetograph. (Auth.)