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Astron. Astrophys. 340, 569-578 (1998)

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5. First measurements

TESOS was installed at the VTT in November 1996. During a few short commissioning campaigns in 1997 the hard- and software configuration were improved. First regular observations were made in late autumn of 1997.

5.1. Stability

TESOS is installed in an air-conditioned laboratory with an ambient temperature of 19 oC which is kept constant within [FORMULA] 1 oC. Sunlight entering the instrument is a major heat source: the power at the prefilter is about 15 W, but most of the light is reflected from the filter surface back to the telescope. All motors of the instrument are switched off after completion of the setup to prevent additional warming. The wavelength stability of TESOS is determined by the stability of the spacings of the two FPIs. Plate separation and parallelism of the interferometers are controlled by a closed loop servo system. Despite this servo control, there are residual drifts of the effective plate separation leading to wavelength instabilities. These drifts result from changes of the dielectric constant of the air (due to changes of temperature, pressure and humidity), thermal effects of the capacitor elements and electronic drifts within the CS100 controller. The instrument stability was tested with long-time measurements using a HeNe laser and the telluric O2 lines in the solar spectrum at 630.2 nm. Fig. 10 shows two examples of such measurements with the laser and with the O2 line showing a wavelength drift between 1 and 1.3 pm/oC. This leads to a typical ET50-FS stability of 0.1-0.3 pm/h during solar observations. These values are lower than the typical drifts of a grating spectrograph, but larger than in the Italian panoramic monochromator, a combination of an Universal Filter with a FPI (Cavallini 1997). They achieved a stability of 0.002 pm/h.
It should be noted that the stability values given in this section were achieved without a large effort. The wavelength drifts are small enough to allow for observing sequences with a duration of a few hours without re-tuning the instrument.

[FIGURE] Fig. 10. Wavelength stability of TESOS. Upper panel: Night time measurement with an HeNe laser. Lower panel: Measurement of the telluric O2 line (630.1 nm) during solar observation. The dotted line shows the temperature inside the instrument.

5.2. Solar observations

The examples shown in Figs. 11 to 15 are preliminary results from observations in July and December 1997. Data of the granulation image were taken at disk center in a quiet region using the magnetically insensitive line Fe I 569.1 nm with an integration time of 250 ms and a cycle time of 1.5 frames/s. The spectral scan consists of nine points and covers only the center of the line. Fig. 11 is the average of the 9 broadband continuum images taken simultaneously with the filtergrams. The rms intensity fluctuation of the averaged image is 3.3%. The Doppler map was obtained by fitting a polynomial to each observed profile. The rms-velocity is 370 m/s. The left panel of Fig. 13 shows the solar spectrum around the Fe I 569.1 nm line measured through the prefilter with an FWHM of 0.3 nm.. The central part of the line and the wavelength scan positions are displayed in the right part of Fig. 13. Figs. 14 and 15 show the continuum and the line-of-sight velocity of a sunspot very close to the solar limb. Velocity values are given relative to the mean velocity of the spot. With 11 wavelength positions per scan and an integration time of 600 ms and a frame rate of 0.9 Hz one spectral scan was completed in about 12 s.

[FIGURE] Fig. 11. Broadband continuum image (630 nm), average of 9 single frames (one line scan). Data of Figs. 11 and 12 have been taken on 2 July 1997.

[FIGURE] Fig. 12. Doppler map obtained from single scan (9 wavelength positions, step width 2 pm) through the Fe I 569.1 nm line.

[FIGURE] Fig. 13. Scan (using integrated light) through the full prefilter range around the Fe I 569.1 nm line (left ) and the observed line profile averaged over the field of view of Fig. 12 (right ).

[FIGURE] Fig. 14. Continuum image of a sunspot close to the solar limb. Observations were made in December 1997, integration time was 600 ms

[FIGURE] Fig. 15. Line-of-sight velocities of the sunspot image, derived from a single scan through the Fe I 557.6 nm line. The mean spot velocity is set to zero.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998