THE CALIBRATION OF THE ADVANCED-STOKES-POLARIMETER

We describe and apply the methods that have been developed to calibrat e the Advanced Stokes Polarimeter and to compensate for the polarizati on effects introduced by the Vacuum Tower Telescope at the National So lar Observatory/Sunspot. A seven-parameter model of the telescope is f itted to data obtained at a variety of mirror angles using observation s of both the center of the solar disk and that point within a sunspot umbra at which the magnetic field is oriented as close to the line of sight as possible. The response matrix of the polarimeter itself is d etermined by the use of polarizing calibration optics that modify the polarization state of the beam exiting the telescope but before enteri ng the polarimeter. A global least-squares solution is obtained simult aneously for the response matrix and the telescope parameters. A detai led gain-correction procedure is described that reduces the multiplica tive gain errors in the spectral images to typically less than 1%. We have successfully recovered net-linear polarization profiles with peak amplitudes of 1 x 10(-3)I(c) against an instrumentally produced backg round polarization of similar or equal to 1-5 x 10(-2)I(c). Net-polari zation signals smaller than similar or equal to 3 x 10(-4)I(c) are los t, even with sufficient averaging, in a background due to photometric and other calibration errors.