Stray-light effect on magnetograph observations

To examine the stray-light effect in magnetograph observations, we have det ermined the point spread functions of the vector magnetograph mounted on th e Japanese Solar Flare Telescope based on two indirect methods, one analyzi ng the solar limb intensity profile, and the other using the Fourier power spectra of photospheric intensity distributions. Point spread functions con sist of two parts: a blurring part which describes seeing and small-spread- angle stray light, and a scattering part which describes large-spread-angle stray light. The FWHM spatial resolution is typically 3.0 ", and the amoun t of scattered light is about 15% on clear days. We find that the blurring part is well described by a Moffat function whose Fourier transform is give n by an exponential function. Our results indicate that polarization measur ements of low-intensity magnetic elements like sunspots may be significantl y underestimated due to the large-spread-angle stray light, and polarizatio n measurements of magnetic elements which are smaller than 5-7 " appear to be disturbed by small-spread-angle stray light.