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Astron. Astrophys. 341, 610-616 (1999)

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2. Observations and image processing

The observations were obtained with the EIT experiment on board SOHO in the frame of Joint Observing Program 57 (Delannee et al. 1997). In this work we analyze He II 304 Å images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) and H[FORMULA] CCD images obtained with the UBF at the focus of the Vacuum Tower Telescope of the Sacramento Peak Observatory, on December 11 and 13, 1996. The duration of the simultaneous sequences of observations was about 1 hour on each day. The time step of EIT images was about 1 minute. In H[FORMULA] it was 3 sec on December 13 and 18 sec on December 11.

The raw images were corrected for dark current and flat field and carefully aligned. We furthermore normalized the images by computing the average intensity over a large area and dividing the intensity of each pixel by this value. We used a technique developed by Georgakilas et al. (1997, 1998) to correct the images for limb darkening and enhance their spatial resolution. Fig. 1 shows images in He II and H[FORMULA] before and after the correction.

[FIGURE] Fig. 1a-d. Simultaneous H[FORMULA] and He II 304 Å filtergrams, obtained on December 13, 1996, near the North pole. a and b are the original images, while c and d show the effect of the correction for limb darkening, which greatly enhances the visibility of fine structures.

In Fig. 2 we compare a He II image with almost simultaneous images at H[FORMULA] center, H[FORMULA]+0.4 Å and H[FORMULA]0.4 Å. We notice that some spikes are so faint that they can be better observed at the center of H[FORMULA] or in one of the wings. In order to further enhance the signal/noise ratio of H[FORMULA] features, we added H[FORMULA], H[FORMULA]+0.4 Å and H[FORMULA]-0.4 Å filtergrams and examined the quality of the sum image. If the seeing in one of the images we added was lower compared to the others, that image was not used.

[FIGURE] Fig. 2a-d. Almost simultaneous filtergrams in He II 304 Å a , H[FORMULA] center b , H[FORMULA]+0.4 Å c and H[FORMULA]0.4 Å d . We have marked with white lines He II features that have H[FORMULA] counterparts and with black lines the ones that have no obvious corresponding structures. Due to the image correction technique we can observe most features in all three H[FORMULA] wavelengths. However, a careful examination of the figures reveals intensity differences due to mass motions.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998