2. Observations and image processing
Observations were obtained at the R.B. Dunn telescope of the Sacramento Peak Observatory with a 512 by 512 pixel CCD camera and the UBF filter. The pixel spatial resolution was 0.26". A large isolated sunspot was observed at N14.7, E26.0 on August 15, 1997. In this work we analyze images obtained in the magnetically non sensitive line Fe I (557.6099) 0.012 nm. Note that the precision of the UBF filter is of the order of 0.1 pm, while the FWHM is about 12 pm near Fe I (557.6099). The time interval between successive images of the same wavelength was 28 seconds, while the time difference between opposite Fe I wings was 4 seconds. We computed Dopplergrams in Fe I 0.012 nm by red/blue wing subtraction.
In Paper I, in order to study the properties of umbral oscillation and running waves, we took a cross section of every image of a time series, starting from the center of an oscillating element and directed outwards, at right angles to the propagating wavefronts. We created a new image with the distance from the center of the oscillating elements as one axis and time as the other (method I). In order to improve this method, we computed the average intensity along circular arcs at right angles to a line directed from the center of an oscillating element outwards (method II). The line is obtained at right angles to the wavefronts so that the arcs are parallel to the wavefronts. The arcs correspond to a stable angle so that their length smoothly increases with distance from the center of the spot. This is a more appropriate method since the waves expand in larger arcs as they propagate outwards. The new method gives significantly better results, much less noise and the oscillating frequency can be determined with significantly higher confidence.
In order to remove the sharp intensity gradient between the umbra and the penumbra and enhance time varying phenomena in intensity images, we subtracted from each image the average image integrated over the whole time series. Details about the method can be found in Paper I and references therein. We should note that we applied this method only to intensity images and not to the Dopplergrams or the images from which we produced the Dopplergrams. The method that we have used makes the waves much more noticeable without introducing artifacts (see Fig. 1 of Paper I).
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000