Specific scientific programs should require increased performance. We mention some improvements concerning either new equipment or new observing procedures and processing.

6.1 High spatial resolution: Destretching and speckle imaging

The first one is adaptive optics. This should be installed at THEMIS during the next few years. It should provide very high resolution inside the isoplanetism domain, that is, less than one minute of arc (with corrections in one pupil only).

$\begin{figure} \par\includegraphics[width=4.6cm,clip]{MS1392f5.ps} \end{figure}$

Figure 5: I- and V-profiles of NaD1 along the line ab of Fig. 4. Two spots of opposite polarities are crossed. ( Courtesy J.-M. Malherbe).

The second one consists of off-line corrections by destretching and/or speckle restoration, when short exposure times are available to freeze the telluric turbulence. We plan to take advantage of new fast CCDs to record bursts of images. By adding the signals in all the MSDP channels, it is possible to build low-noise images (equivalent to the white channel in the case of narrow-band filters). They can be used to determine the destretching parameters.

Such processing can improve the spatial resolution in large fields of view. It can be noted that, in the case of polarization analysis, the size of each instantaneous image created by the grid is close to isoplanetism domains (around $10\hbox{$^{\prime\prime}$ }\times 10\hbox{$^{\prime\prime}$ }$ ). This can be useful for destretching procedures.

The best equipment for high spatial resolution should consist of large fast cameras (at least $1500\times$ 1000 pixels, 2 to 5 Hz) and/or optical enlargement at the F2 focus (typically $\times 1.5$ ). In addition, a project using a very bright beamshifter is ready (18 channels, width-slits 12 pm, resolution 12 pm). It should allow very short exposure times, freezing the telluric turbulence ( $\sim$ 10 ms) for strong lines.

High resolution is crucial in spectro-polarimetry of flux tubes, fine structure of flares, or threads of filaments acting as magnetic tracers of the low corona.

6.2 High speed capabilities

Table 1 shows that the expected speeds in 2001 allow the instrument to scan a $120\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$ field of view within 120 s or less, in 2 Stokes parameters (optics B or C). With larger pixel sizes, for example 0.4 arcsec, the same time should apply to $120\hbox{$^{\prime\prime}$ }\times 240\hbox{$^{\prime\prime}$ }$ (240 $^{\prime\prime}$ is the maximum simultaneous field of THEMIS).

Fast evolutions of magnetic and velocity fields can be recorded over full active regions. Many programs are relevant: wave propagation, flares and mass ejections, photosphere-corona investigations from coordinated ground-space observations with large fields-of-view.

6.3 Large and fast scans using earth rotation

It is quite possible to scan the solar disk with the motion of earth rotation. Scan-steps of 7.5 arcsec imply only a rate of 2 exposures per second. Successful trials have been performed during 1999 with the small THEMIS CCDs, without polarization analysis. The exposure times were around 50 ms. For polarization analysis and larger exposure times, it would be necessary to compensate for the earth's velocity during the integration time. This might be done, either by the telescope transfer optics, or by a rotating plate at the F2 focus. It can be noted that the use of earth rotation allows any scanning length, while the scanning by the telescope transfer optics alone cannot exceed 4 min of arc.

As an example of a relevant program, we can mention proper motions of magnetic tracers, which require a very uniform scanning velocity.

6.4 Improved spectral resolution

It could be possible to increase the spectral resolution and the number of channels by a factor of 2 with beamsplitters slightly inclined in front of the optics A, B, C or D. In Table 1, the spectral resolutions would become respectively 12, 4 and 2.5 pm. The field-of-view would be reduced by a factor of 2, but the number of channels would be increased by the same factor (respectively 18, 32, 32 and 28).

This would be especially interesting for investigations of photospheric lines with optics B, C and D.

6.5 Multi-line imaging-spectropolarimetry

Multi-line observations are a specific advantage of spectrographs, unlike narrow-band filters. The plane mirrors feeding the MSDP beamshifters allow the simultaneous observation of 2 lines. To exchange lines, the adjustments are still difficult. New devices could reduce the times, and extend the capabilities to 3 or 4 lines.

All investigations concerning the stratification of the solar atmosphere should take advantage of such improvements.

6.6 Improved polarimetric accuracy

The accuracy of polarization measurements depends on the possible integration over time and space. The 2D-character of the MSDP can be used to increase the integration by 2D-smoothing. This does not degrade the spatial resolution, because exposure times are generally long for weak polarization observations, except in the case of very efficient adaptive optics. The smoothing leading to the best compromise can be optimized off-line with MSDP data.

6.7 Compatibilities MSDP/MTR and MSDP/IPM

The output corresponding to optics A and B coincides with the standard focus of the MTR mode. If the focus F2 was equipped with an automatic exchange of slits and field-stops, fast successive observations could be possible with MTR and MSDP. It must be noted also that the CCD used for the MSDP can be useful in performing slit-spectroscopy in Additive Double Pass, with the two echelle gratings. The dispersion is almost twice the usual MTR dispersion. The spectral range remains large because of the large CCD.

The MSDP and IPM modes are already compatible, in the case of observations without polarization analysis. Polarimetry should be operational in both cases in the future. Two new CCDs are required; MSDP and IPM cannot use the same detectors at the same time.