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Zero Point Correction

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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

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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

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Zernickes

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Time Series of Zernicke Coefficients

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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

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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?