ON THE COMPARISON OF FILTER MAGNETOGRAPHS AND THE ADVANCED STOKES POLARIMETER

Lites et al. (1994) (hereafter LMS) have recently published a comparis on of the response of their magnetograph (the Advanced Stokes Polarime ter (ASP)) to that they expect from filter-based magnetographs (FM). N ot surprisingly, they conclude the ASP is better. They claim that only their instrument is ''quantitative,'' and others are not, and that th e transverse field strength and azimuth measured by filter magnetograp hs may be in error by up to 50%. While the calculation is formally cor rect it ignores the high sensitivity attained by real FM's accumulatin g thousands of difference frames. Further, FM's have been cross-compar ed 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 tot ally erroneous results when the field rapidly changes direction within its resolution element, as occurs in delta spots.