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Basic Steps for Zero Point Correction in Magnetogram Analysis

This guide outlines the essential steps for performing zero point correction on magnetograms collected from a specific site on a given day. It includes creating a magnetic flat field using east and west calibration images, generating a reference magnetogram, and averaging 10-minute data. The process involves subtracting the reference from the averages, fitting Zernicke polynomials, and smoothing harmonic coefficients to enhance accuracy. The goal is to reduce variations in magnetogram readings to within 0.1-0.3 G and address software correction needs with the new modulator.

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Basic Steps for Zero Point Correction in Magnetogram Analysis

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  1. Zero Point Correction

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

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

  4. Zernickes

  5. Time Series of Zernicke Coefficients

  6. 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 How well does it work?

  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?

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