Measurements of solar irradiance are performed simultaneously at four wavelengths. High photometric accuracy is achieved by integration of photodiode signals over 13 seconds, using an ultra-low-noise current-to-voltage amplification stage and a digitization scheme with a voltage-to-frequency converter coupled to a high stability time base. The apparatus is equatorially mounted, its tracking is servo-controlled and other parameters such as errors on both axes, temperature measurements at the detectors and at voltage-to-frequency converters are also recorded.
The instrument used for this work is the Solar Luminosity Oscillation Telescope (SLOT), (Andersen et al. 1988a), built at the Space Science Department at ESTEC and devoted to the detection of solar intensity oscillations (Jiménez et al. 1987, 1988, 1990) over five continuous years (1984 to 1989) and for a seasonal atmospheric conditions study (Andersen et al. 1988b). During the campaign, the sets of filters were changed four times to check different observing possibilities and improve the detection of solar intensity oscillations. Table 1 (click here) shows filter sets used during the campaign. Nine different filters were used, but only 5 have been used in the present analysis: 680 nm (common to all filter sets), 500 nm, 450 nm, 870 nm and 770.2 nm. Channel #4 of filter set #1, at 680 nm, was a redundant channel to assess the instrumental noise. Channel #2 with filter sets #1 and #2 at 1060 nm presented a low frequency trend due to temperature variation because the photometer lacked a cooling system needed for near infrared measurements. Channel #2 with filter sets #3 and #4 at 517.8 nm and channel #3 of filter set #3 at 516.2 nm were very similar, the only difference was in their bandwidths. This was an attempt to increase the signal-to-noise ratio by dividing the signal in both channels (as suggested by Harvey 1986). For atmospheric extinction analysis, the behaviour of these channels is the same as the one at 500 nm (channel #3 with filter set #1 and #2) which has been used in this work.
| Filter set 1 | Filter set 2 | Filter set 3 | Filter set 4 | |
| (84 Aug. / 85 Jun.) | (85 Jun. / 87 Aug.) | (87 Aug. / 88 Dec.) | (88 Dec. / 89 Jun.) | |
| Channel |  ( ) nm | () nm
| () nm |
() nm |
| 1 | 680 (10) | 680 (10) | 680 (10) | 680 (10) |
| 2 | 1060(10) | 1060(10) | 517.8 (3.5) | 517.8 (3.5) |
| 3 | 500 (10) | 500 (10) | 516.2 (9.1) | 450 (10) |
| 4 | 680 (10) | 870 (10) | 770.2 (4.7) | 770.2 (4.7) |