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Astron. Astrophys. 338, 1-7 (1998)

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2. Observations

The observations were made in November 1996 with a 36 cm telescope in La Silla (Chile). This telescope, equipped with a scanning Fabry-Perot interferometer, is devoted to a Survey of the Milky Way and Magellanic Clouds (Amram et al. 1991, Le Coarer et al. 1992). The field of view is [FORMULA]; the spectral resolution was 11.5 km [FORMULA] with the interferometer used here, and the sampling step was either 4.6 km [FORMULA], or 2.3 km [FORMULA] (i.e. 0.10 Å or 0.05 Å), depending on the scanning process adopted (24 or 48 channels over the free spectral range of 115 km [FORMULA], i.e. 2.5 Å, of the Fabry-Perot interferometer). The H[FORMULA] line observed was selected through a 8 Å FWHM interference filter with 70% transmission, centered at 6563 Å for the observing conditions. The lines passing through the filter are the Galactic H[FORMULA] line we are looking for, the geocoronal H[FORMULA] emission and the OH night sky line at 6568.78 Å. These two parasitic lines are brighter than the Galactic H[FORMULA] line, the geocoronal line being typically twice as bright as the OH line and 10 times brighter than the Galactic line. The filter (3 cavities) transmission function is steep enough on the edges so that the two nearby, bright OH lines, at 6553.62 Å  and 6577.28 Å, are effectively suppressed by the filter and may be neglected; all the more since the brighter line (6553.62 Å) is brought into coincidence with the OH line at 6568.78 Å, their separation being exactly 6 times the free spectral range of the Fabry-Perot.

In order to compare the Galactic H[FORMULA] emission fluctuations with the South Pole results (Schuster et al. 1993; Gundersen et al. 1995), we selected fields at declinations of [FORMULA] (corresponding to SP91) and [FORMULA] (corresponding to SP94). Our fields were separated by 15 mn in right ascension, which is about [FORMULA] on the sky, thus offering a fair coverage of each band. We observed 19 fields at [FORMULA] (from [FORMULA] = [FORMULA] to [FORMULA] = [FORMULA]) and 6 fields at [FORMULA] (from [FORMULA] = [FORMULA] to [FORMULA] = [FORMULA]). Some of these fields were observed twice, on different nights, to check the reproducibility of our measurements, and also at times with a different spectral sampling. Table 1 summarizes the observations parameters and Table 2 gives the details of these observations with exposure times and the number of scanning steps. Fig. 1 shows the profiles observed in the 25 fields. The observing conditions were fairly good, with some faint cirrus clouds on the nights of November 8th, and 10th to the 13th. Only two exposures had to be cut because of heavy clouds (number 14 and 25 in Table 2), and the corresponding fields were re-observed in good conditions with the [FORMULA] exposure time currently adopted.


Table 1. Observations parameters


Table 2. Observed fields

[FIGURE] Fig. 1. Summary of the observed profiles for the different fields. The reference numbers are those of Table 2.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998