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An accurate measurement of magnetic field is very important for understandingthe formation and evolution of solar magnetic fields. Currently there are twotypes of solar magnetic field measurement instruments: the filter-basedmagnetographs and the Stokes polarimeters. The former gives high temporalresolution magnetograms and the latter provides more accurate measurements ofmagnetic fields. Calibrating the magnetograms obtained by filter-basedmagnetographs with those obtained by Stokes polarimeters is a good way tocombine the advantages of the two types. Our previous studies have shown that,compared to the magnetograms obtained by the Spectro-Polarimeter (SP) on boardHinode, those magnetograms obtained by both the filter-based Solar MagneticField Telescope (SMFT) of the Huairou Solar Observing Station (HSOS) and by thefilter-based Michelson Doppler Imager (MDI) aboard SOHO have underestimated theflux densities in their magnetograms and systematic center-to-limb variationspresent in the magnetograms of both instruments. Here, using a sample of 75vector magnetograms of stable alpha sunspots, we compare the vectormagnetograms obtained by the Helioseismic and Magnetic Imager (HMI) aboard SDOwith co-temporal vector magnetograms obtained by SP/Hinode. Our analysis showsthat both the longitudinal and transverse flux densities in the HMI/SDOmagnetograms are very close to those in the SP/Hinode magnetograms and thesystematic center-to-limb variations in the HMI/SDO magnetograms are veryminor. Our study suggests that using the filter-based magnetograph to constructa low spectral resolution Stokes profile, as done by HMI/SDO, can largelyremove the disadvantages of the filter-type measurements and yet still possessthe advantage of high temporal resolution.

A comparison of co-temporal vector magnetograms obtained with HMI/SDO and SP/Hinode