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The low-resolution, Cassegrain mounted, FORS spectropolarimeter of the ESOVery Large Telescope is being extensively used for magnetic field surveys. Someof the new discoveries suggest that relatively strong magnetic fields may playan important role in numerous physical phenomena observed in the atmospheres aswell as in the circumstellar environments of certain kinds of stars. We show indetail how small instabilities or data-reduction inaccuracies represent analternative explanation for the origin of certain signals of circularpolarisation published in recent years. With the help of analyticalcalculations we simulate the observation of a spectral line inspectropolarimetric mode, adding very small spurious wavelength shifts, whichmay mimic the effects of seeing variations, rapid variations of the stellarradial velocity, or instrument instabilities. As a case study, we then re-visitthe FORS2 measurements that have been used to claim the discovery of a magneticfield in the A0 supergiant HD 92207. In addition, we present new observationsof this star obtained with the HARPSpol instrument. Both calibration andscience data show compelling evidence that photon-noise is not the only sourceof error in magnetic field measurements, especially in sharp spectral lines.Non-photon noise may be kept under control by accurate data reduction andquality controls. Our re-analysis of FORS2 observations of HD 92207 shows noevidence of a magnetic field, and we are able to reproduce the previous FORSdetection only by degrading the quality of our wavelength calibration. OurHARPSpol spectropolarimetric measurements show no evidence of a magnetic fieldat the level of 10 G.

astronomy and astrophysics

The importance of non-photon noise in stellar spectropolarimetry. The  spurious detection of a non-existing magnetic field in the A0 supergiant HD  92207