1. 1 Zero Point Correction

2. 2 Basic Steps - I For a given site on a given day: 1.Create a magnetic flat field from the east & west calibration images. 2.Using the magnetic flat, create a “reference magnetogram” from the calibration solar image and the observations near the calibration sequence 3.Create 10-min averages of the magnetograms during the day. 4.Register, rotate and remap the averages and reference to a common scale and time

3. 3 Basic Steps II 5.Subtract the reference from each 10-min average. 6.Fit the differences with Zernicke polynomials. 7.Fit the time series of Zernicke coefficients with 24-hr and 12-hr sinusoids. 8.Smooth the harmonic fit coefficients over several days. 9.These final coefficients can be used to compute a correction for the magnetogram at any time

4. 4 Zernickes

5. 5 Time Series of Zernicke Coefficients

6. 6 How well does it work? Before processing, zero point varies across image by about 5-10 G. After processing, variation is about 1 G at best Not good enough, want 0.1-0.3 G Seeing noise is a significant problem

7. 7 Questions Will a software correction be needed after the new modulator is installed? If so, will the current method be adequate? If not, how can it be improved? How can it be tested?