2. Instrumentation and observations
The instrumentation for these observations was similar to that used in the detection of the HI n=20-19 line (Clark et al. 2000). The polarising interferometer used in the previous measurements was reconfigured as a classical Michelson interferometer to exploit the increased efficiency provided by newly developed intensity beamsplitters (Ade et al. 1999). In addition to the increased efficiency, this non-polarising beamsplitter ensured that these measurements were not sensitive to the polarisation of Zeeman components of solar spectral features. The FTS was operated in rapid scan mode and produced spectra with a resolution of 0.005 cm-1 (5 mK) in a scan time of one minute.
The observations were made on the morning of 1999 December 18. Spectra were acquired in the following sequence: one at disk centre, one at a background sky position 2000" from disk centre, and two at a selected limb position. This cycle was repeated for various limb positions on the positive-azimuth side of the Sun, and the entire observing sequence was then repeated on the negative-azimuth side of the Sun. All beam offsets were made in azimuth to maintain constant airmass for each set of solar and sky spectra. The Sun contained three major sunspot groups at the time of observation. Careful mapping of the observing positions on an H image of the Sun taken at a time almost coincident with the present observations showed that the positive-azimuth sequence of observations sampled a quiet solar limb. The negative-azimuth sequence sampled spectra along a track which came close to, but avoided, a large active region which had recently appeared over the east solar limb.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000