2. Observations, instrumental parameters and set-up
The observations were obtained at the Sacramento Peak Observatory Vacuum Tower Telescope (VTT) on September 18th, 1994, from 15 h 32 mn to 17 h 22 mn UT. The observed region was a square of 350" side, centered at N 17:O3, E 1:O3. The data were acquired using a specially prepared set-up at one of the auxiliary focus of the VTT. After a re-imaging system demagnifying the field of view (FOV), a pixel CCD camera was placed at the exit of the Universal Bi-refringent Filter (UBF). Before the UBF, a beam splitter allowed the re-imaging of the field through an H Zeiss filter (25 pm FWHM). Four images (true continuum and MgIb1 blue wing, H red and blue wing) were taken sequentially every 15 s, up to a total of 110 for each wavelength, with the parameters listed in Table 1. The time interval between consecutive images at the same wavelength was therefore 1 mn. The optical setup yielded a spatial resolution at its focal plane of 0:0034 per pixel. Several sequences of "dark current" and "flat field" were used to correct the raw images in order to reach the high photometric accuracy allowed by the high level of counts (about 25 000 counts per pixel in average). We used only a sub-region of the FOV of 600x600 px to avoid the remaining vignetting after the flat field correction.
Table 1. Parameters of the observations for the 4 sequences of images. is the spectral FWHM, and the exposure time
The bandwith we used with the UBF (see Table 1) yields true measurements in the continuum, not affected by line brightenings with a precision better than 0.1%.
The very high correlation between magnetogram signal and brightness in the wing of the MgIb1 line in small scale magnetic field is well known (Beckers 1976 , Dara-Papamargaritis & Koutchmy 1983 ). Moreover, this line is not affected by the cancellation of fields of opposite polarity within the resolution element, so one can use the MgIb1 brightness as a very good proxy for small scale magnetic fields. At the wavelength used (see Table 1) the height of formation is approximately 140 km (Stellmacher -private communication- and Stellmacher & Wiehr 1991 ).
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997