Short description of the proposed MASS instrument

**Figure 1:** Four concentric apertures A, B, C, D (left) and the associated weighting functions for normal (bottom left) and differential (bottom right) scintillation indices.
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**Figure 2:** Simplified optical layout of the MASS detector box. Only two of the 4 channels are shown. Light enters into the box at the level of focal plane F, where a field-limiting aperture 1 is located. A Fabry lens 2 (followed by a glass filter 3) forms pupil image (plane P) on the splitting system 4. Then, segments of light beam are reflected by annular mirrors in separate directions. The lenses 5 serve to re-image corresponding annular segments of entrance aperture on the photo cathodes 6 of respective PMTs. The viewer 6 and 7 is provided for initial instrument adjustment and star image control.
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**Figure 3:** Reduction of the MASS data.
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The MASS instrument consists of the feeding telescope and of the detector box. The feeding telescope must have a clear (non-obstructed) aperture of at least 15 cm diameter. Four concentric zones are isolated in the aperture (Fig. 1), and stellar fluxes in these zones are measured by 4 PMTs with a time resolution of 1 ms. The photon counts are used to compute the 4 scintillation indices (one per aperture) and the 6 differential scintillation indices for all pairwise aperture combinations.

Both normal and differential indices are equal to the ${\rm C^2_n}(h)$ profile multiplied by some weighting function ${\rm W}(h)$ and integrated over altitude. The weighting functions for the MASS apertures are shown in Fig. 1. It is possible to extract the profile with $\approx 6$ independent points from this set of indices.

As can be noted in Fig. 1, all weighting functions go to zero at telescope pupil. To sense the turbulent layers near the ground, the 2 smallest MASS apertures can be conjugated to a negative altitude of

km. This permits full measurement of the profile and seeing, and also the measurement of the atmospheric time constant (by analyzing the temporal behavior of the scintillation, see [4]).

A simplified optical layout of the detector box is shown in Fig. 2. More detailed information can be found in [3]. The schematic data flow during acquisition and analysis is shown in Fig. 3. MASS wil be piloted by a PC computer under Linux using the RS 485 interface via a standard board.