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

2. Observations

Using the evacuated Gregory-Coudé Telescope ( =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 has been used because of its narrow width (due to ) and comparatively large brightness as well as the low straylight aureole and the better seeing in the IR. The 600 400 pixel CCD, covering 107" 3 Å, was binned to 150 200 pixels, corresponding to a pixel size of 0.7" 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.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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