ON THE COMPARISON OF FILTER MAGNETOGRAPHS AND THE ADVANCED STOKES POLARIMETER

Lites et al. (1994) (hereafter LMS) have recently published a comparison of the response of their magnetograph (the Advanced Stokes Polarimeter (ASP)) to that they expect from filter-based magnetographs (FM). Not surprisingly, they conclude the ASP is better. They claim that only their instrument is ''quantitative,'' and others are not, and that the transverse field strength and azimuth measured by filter magnetographs may be in error by up to 50%. While the calculation is formally correct it ignores the high sensitivity attained by real FM's accumulating thousands of difference frames. Further, FM's have been cross-compared and tested empirically, without any such errors appearing. We point out that the two instruments have different roles, but the functional use of the FM is far superior to the ASP for solar research. The ASP may give accurate results for unresolved unipolar fields; it gives totally erroneous results when the field rapidly changes direction within its resolution element, as occurs in delta spots.