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

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4. Spectrometer design

The optical scheme of TESOS is shown in Fig. 5. Only the main optical train and the basic elements are shown. The lettering used corresponds to that in Fig. 6. An exploded view of the mechanical layout including all calibration elements and optional features is given in Fig. 6. The telescope forms an image at the field stop (IP0 ). The sensor of the IAC/KIS Correlation Tracker(CT ) is located near the telescope's main focus (Schmidt & Kentischer 1995).

[FIGURE] Fig. 5. Optical scheme of TESOS. Lettering is the same as in Fig. 6.

[FIGURE] Fig. 6. Mechanical layout of TESOS. Abbreviations are explained in the text. Dimensions of the optical table are 2500 mm [FORMULA] 1200 mm. The two positions of FPI1 are indicated, which correspond to different fields of view (50 or 100 arcsec diameter).

The field stop contains four apertures: FOV 50 arcsec, FOV 100 arcsec, a target for alignment, focus adjustment and scale factor measurements and a pinhole for alignment purposes.

Lens L1 collimates the light and forms an image of the telescope entrance aperture at the shutter . An intermediate image near the FPIs is formed by L2 (L3 for the small field of view). Etalon FPI1 and the pre-filter wheel PF are mounted on a motorized rotating stage. FPI1 together with the filter wheel are moved to different positions depending on the FOV chosen. The second interferometer FPI2 remains fixed. The solar image is located in between FPI1 and the prefilter wheel PF . The reimaging lenses L4 and L5 form the final image at the CCD camera. L5 is mounted on a motorized stage to adjust for chromatic focus shifts. For the white-light reference image, 10% of the light is extracted by the beam splitter BS1 . A set of lenses (L6 to L9 ) forms an image and an exchangeable broadband interference filter IF2 allows to choose a convenient spectral band for that image. The white-light image is formed next to the filter image on the camera. The filter and white-light images have the same shutter and are using the same CCD, which guarantees strict simultaneity. The intensity of the white-light channel is adjusted using two crossed polarisators (POL1 ). TESOS can be equipped with a polarization optics located behind mirror U5 . In that configuration the light passes through a super-achromatic quarter wave plate (Lambda/4 ) and is split by a Wollaston prism Wol ). In this case the camera records two Stokes-V circular polarized filtergrams and one white-light image (Fig. 7). Internal polarization within TESOS is minimized by two sagittal [FORMULA] reflections followed by two tangential [FORMULA] reflections. For monitoring purposes beam splitter BS2 feeds 10% of the light to a video camera. To adjust the band-passes of the two FPIs on each other, the folding mirror U5 is removed and the etalon plates are reimaged onto a photomultiplier via L15 . A HeNe laser is used to adjust the parallelism of the FPIs. The light from a HeNe laser, followed by a beam expander (L13, PH, L14 ) is fed to the etalons either by U1 or by U2 depending on the FOV used. Both FPIs are mounted onto motorized stages, so they can be adjusted separately. The second CCD camera shown in Fig. 6 together with the folding mirror (U10 ), imaging lens (L16 ) and a diversity sensor(PD15 ) (Tritschler et al. 1997) are an auxiliary feature of TESOS for image reconstruction purposes.

[FIGURE] Fig. 7. CCD chip illumination in the polarimeter mode of TESOS. The optical axis (OA) is in the chip center.

4.1. Interference filters

TESOS in its present tandem configuration requires narrowband interference filters with an FWHM of 0.3 nm. The filters are mounted telecentrically just in front of the first FPI. In this position it is possible to tilt the filter in order to adjust the wavelength passband. If the interference filter is located in a pupil image, any tilt would result in a field-dependent shift of the passband This problem had been discussed e.g. by Atherton, Taylor et al. (1982) in relation with their Fabry-Perot spectrograph TAURUS. With a four position filter wheel one can easily switch between several spectral lines in a very short time (typical 1-2 seconds). We use Andover filters with a 2-cavity design, an FWHM of 0.3 nm and a typical transmission of 25%. The manufacturer guarantees the specified center wavelength to [FORMULA] 0.1 nm. Each filter can be slightly tilted with a motorized mechanism within the filter wheel, to fine-tune the passband to the precise wavelength of the chosen spectral line. The tilt range of [FORMULA] corresponds to a passband shift of +0/-0.2 nm (only blue-shift is possible).

4.2. Ghost images

Back reflections of the light between the two FPIs and the interference filter lead to ghost images. The integrated intensity of such ghost images is 12% of the main passband intensity for the first reflection and 5% for the second one. By tilting the interferometers by a small angle, this parasitic light is reimaged in the pupil plane, separated from the ordinary pupil and then baffled. The tilt angles for each interferometer are [FORMULA]/[FORMULA] for the case of the 50/100 arcsec fields of view. Tilting the etalons introduces slight intensity gradients within the pupil image in one direction caused by a slight shift of the two FPI passbands.

4.3. Detector

In polarimetric mode, three different frames are simultaneously imaged on the CCD: Two Stokes-V circular polarized filtergram images and one white-light reference image (Fig. 7). If TESOS is operating in default mode, two images are on the camera and it is sufficient to read out only one half of the chip. The camera used in TESOS is part of a joint project between the Kiepenheuer-Institut and the National Solar Observatory (NSO), Sunspot, New Mexico. It consists of a Thomson TH79KA95 evaluation kit, an EDT-SDV digital video interface and a double processor SUN SPARC20 workstation (see Fig. 9). The software development of the camera driver was done at NSO.
The camera chip has [FORMULA] pixels with a pixel size of 19 µm and a digital resolution of 10 bit. The pixel scale is 0.15 arcsec/pixel within the 50 arcsec field and 0.3 arcsec/pixel in the 100 arcsec field. The readout time for a full frame is 300 msec. Writing to disk needs another 200 - 300 msec, depending on the storage media used (Exabyte, Hard Disk or Magneto-optical disk).

4.4. Photon statistics

The signal-to-noise ratio is calculated using the relevant solar data, the transmission figures for the telescope and the instrument. Fig. 8 displays the integration time of the CCD as a function of wavelength for different signal-to noise ratios (SNR) and for the different fields of view available. The parameters used for the computation are listed in Table 3. The SNR curves refer to the spectral continuum. In the cores of strong lines the intensity may be more than ten times lower, requiring longer integration times for a given SNR.


[TABLE]

Table 3. Transmission values used for the SNR-computations.


[FIGURE] Fig. 8. Exposure time as a function of wavelength for different signal-to-noise ratios. The left (right) panel corresponds to the small (large) field of view.

4.5. Spectrometer control

TESOS is controlled by a double processor SUN SPARC 20 workstation. The user operates the whole system via a graphical user interface (written in C, libsx) without manual manipulations inside the instrument. Fig. 9 shows a sketch of the control system. All moving parts within the instrument are motorized. The DC drives, the laser, PIN-diode and photomultiplier (PM) are connected via a CAN-BUS (Etschberger 1994) to the workstation. Most of the necessary tasks for the etalon adjustment and observing setup are working automatically or interactively. In particular, the following procedures are (more or less) automatic:

  • Setting parallelism of the FPIs.

  • Locking both FPIs on a solar line.

  • Centering of the prefilter passband.

  • Performing flat-field and dark sequences.

[FIGURE] Fig. 9. Scheme of the TESOS remote control system. All devices are controlled by a SUN SPARC 20 workstation and are operated via a graphical user interface.

Complete observing setups can be saved for later use, including all motor positions, camera settings and etalon parameters.

The TESOS workstation is part of a local network and also connected to the outside world. Remote control from other workstations allows and greatly simplifies support and troubleshooting from outside the telescope site. Ethernet connections are used to interact with other instruments, e.g. the correlation tracker.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998
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