Global High Resolution H-alpha Network

Data Reduction

IMAGE ACQUISITION

Before starting the daily high-cadence observations with a frequency of usually 1 frame per minute, a dark frame and several flat field frames are obtained. From these frames a flat field image, a quiet Sun image, and a contrast enhanced H-alpha image are computed - see the sections below and the paper by Denker et al. (1999).

The typical exposure time for the H-alpha full-disk images obtained at BBSO is 30 ms. The exposure time of the calibration frames and full-disk images is usually the same during the daily observing run.

DARK CURRENT AND FLAT FIELD

Once a day - usually early in the morning - a dark current frame is taken and a flat field frame is computed from one centered solar image and 7 to 8 solar images shifted with respect to the center of the camera chip by applying the method of Kuhn, Lin and Loranz (1991). The shifted frames are obtained by off-pointing the telescope from Sun center.

The basic idea of the Kuhn-Lin-Loranz technique for calibrating spatially non-uniform image array detectors is to compute the horizontal and vertical derivatives of the logarithm of the gain function from multiple displaced full-disk images. Once the gradient of the log-gain function is determined, integration of this function leads to the logarithm of the gain within an additive constant. Finally, an iterative least-square algorithm is used to solve the pixel gain function for various displacement vectors.

Computing the gain table from the full-sized flat-field frames would take a lot of computer time. Therefore, we reduce the size of the centered and shifted calibration frames, which are corrected for dark current, by a factor of 8 before computing the flat field image. The resulting miniature gain table is subsequently enlarged to original frame size.

Example: Dark and flat field frames for June 19, 2000 ...
  Dark Frame Centered Shifted 1 Shifted 2 Shifted 3
  DARK CENTERED SHIFTED 1 SHIFTED 2 SHIFTED 3
  Shifted 4 Shifted 5 Shifted 6 Shifted 7 Shifted 8
  SHIFTED 4 SHIFTED 5 SHIFTED 6 SHIFTED 7 SHIFTED 8
  ... and the resulting flat field, showing the pattern of the shutter:
  FLAT FIELD Please click on the images to view the full-sized versions.

CENTER AND RADIUS

In the centered image of the Sun the radius and coordinates of the solar disk are determined. This is done by using the Sobel function to determine the limb pixels and a subsequent iterative least-square circle fit. In this way we can eliminate the influence of active regions (filaments, plages and spots on the disk, prominences at the limb) on the resulting radius and center values. In order to calculate the gain table these values have also to be determined in the shifted flat field frames .

Example: Center and radius of the solar disk in the centered image for June 19, 2000:
  CENTER & RADIUS Please click on the image to view the full-sized version.

QUIET SUN CENTER-TO-LIMB VARIATION

Using a 200 x 200 arcsec^2 area at disk center, the median disk center brighness is derived. Then a bilinear interpolation algorithm is applied to transform the centered H-alpha image from cartesian coordinates to polar coordinates resulting in one radial disk profile per 1 degree in azimuth. Computing the median value at each radial position yields an average radial profile. A second order least-square polynomial fit to the average disk profile is used to replace the inner 50% of the average disk profile. The average disk profile is further smoothed by means of a floating average using wider smoothing kernels at the flatter part of the profile at disk center. Finally, the smoothed average disk profile is linearly interpolated onto a two-dimensional cartesian grid, thus yielding the two-dimensional center-to-limb variation of the quiet Sun.

Example: Quiet Sun center-to-limb variation for June 19, 2000:
  QUIET SUN Please click on the image to view the full-sized version.

CONTRAST-ENHANCED IMAGES

Contrast-enhanced full-disk H-alpha images are obtained by subtracting the quiet Sun center-to-limb variation from the dark current and flat field corrected observations.

Example: Contrast enhanced H-alpha image for June 19, 2000:
  ENHANCED Please click on the image to view the full-sized version.

REFERENCES

  • Denker C., Johannesson A., Marquette W., Goode P.R., Wang H., and Zirin H., 1999, Solar Phys. 184, 87
  • Kuhn J.R., Lin H., and Loranz D., 1991, Publ. Astron. Soc. Pac. 103, 1097