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Astron. Astrophys. 321, 921-926 (1997)

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2. Observations

Using the evacuated Gregory-Coudé Telescope ([FORMULA] =45 cm; f=25 m; Wiehr et al., 1980) of the 'Istituto Ricerche Solari' near Locarno/Switzerland (Bianda & Wiehr 1994), several prominences were observed on July 31, August 3, 4, 13, 14, 16, 1996, cf. Table 1. The emission line Ca [FORMULA] has been used because of its narrow width (due to [FORMULA]) and comparatively large brightness as well as the low straylight aureole and the better seeing in the IR. The 600 [FORMULA] 400 pixel CCD, covering 107" [FORMULA] 3 Å, was binned to 150 [FORMULA] 200 pixels, corresponding to a pixel size of 0.7" [FORMULA] 14.5 mÅ. This spatial scale is adapted to the slit width of correspondingly 1.5", required for an acceptable integration time of 2 sec. Depending on the temporal stability of the individual prominences, time series up to 2 hours were taken with cycle times between 6.5 and 60 sec, cf. Table 1.


Table 1. Characteristic parameters of the prominences observed

A 'one-dimensional' guiding perpendicular to the slit is realized by a plan-parallel glass-plate directly fixed to the axis of a stepping motor. It is driven by a photo-diode monitoring the white light limb which is oriented parallel to the slit by means of a Dove prism. A second photo-diode inside the solar disc gives a reference intensity accounting for transparency variations. This prototype of a limb-guider is still limited to a few Hertz thus removing only low frequency image motion, but very effectively drifts of the limb perpendicular to the slit with an accuracy of about one arcsecond.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998