  The design and later use of modern spectropolarimeters and magnetographs
require a number of tolerance specifications that allow the developers to build
the instrument and then the scientists to interpret the data accuracy. Such
specifications depend both on device-specific features and on the physical
assumptions underlying the particular measurement technique. Here we discuss
general properties of every magnetograph, as the detectability thresholds for
the vector magnetic field and the line-of-sight velocity, as well as specific
properties of a given type of instrument, namely that based on a pair of
nematic liquid crystal variable retarders and a Fabry-P\'erot etalon (or
several) for carrying out the light polarization modulation and spectral
analysis, respectively. We derive formulae that give the detection thresholds
in terms of the signal-to-noise ratio of the observations and the polarimetric
efficiencies of the instrument. Relationships are also established between
inaccuracies in the solar physical quantities and instabilities in the
instrument parameters. Such relationships allow, for example, to translate
scientific requirements for the velocity or the magnetic field into
requirements for temperature or voltage stability. We also demonstrate that
this type of magnetograph can theoretically reach the optimum polarimetric
efficiencies of an ideal polarimeter, regardless of the optics in between the
modulator and the analyzer. Such optics induces changes in the instrument
parameters that are calculated too.
