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The design and later use of modern spectropolarimeters and magnetographsrequire a number of tolerance specifications that allow the developers to buildthe instrument and then the scientists to interpret the data accuracy. Suchspecifications depend both on device-specific features and on the physicalassumptions underlying the particular measurement technique. Here we discussgeneral properties of every magnetograph, as the detectability thresholds forthe vector magnetic field and the line-of-sight velocity, as well as specificproperties of a given type of instrument, namely that based on a pair ofnematic liquid crystal variable retarders and a Fabry-P\'erot etalon (orseveral) for carrying out the light polarization modulation and spectralanalysis, respectively. We derive formulae that give the detection thresholdsin terms of the signal-to-noise ratio of the observations and the polarimetricefficiencies of the instrument. Relationships are also established betweeninaccuracies in the solar physical quantities and instabilities in theinstrument parameters. Such relationships allow, for example, to translatescientific requirements for the velocity or the magnetic field intorequirements for temperature or voltage stability. We also demonstrate thatthis type of magnetograph can theoretically reach the optimum polarimetricefficiencies of an ideal polarimeter, regardless of the optics in between themodulator and the analyzer. Such optics induces changes in the instrumentparameters that are calculated too.

Assessing the Behavior of Modern Solar Magnetographs and Spectropolarimeters