detector for high resolution imaging is still to be desired. Some technical developments are under way in our team to contribute to this problem. A prototype of a new photon-counting camera that would allow a high photon rate and a direct numerisation of photon coordinates is beeing tested (DELTA camera, Koechlin & Morel, 1998).

[**5. Performances and limitations**]{}

10000 [*5.1. Physical limitations*]{}

10000 The effects of seeing on speckle observations is a strong reduction of the limiting magnitude for bad seeing conditions, whereas the angular resolution attainable in the image restoration process degrades more slowly from the theoretical limit of $\lambda/D$, where $\lambda$ is the wavelength and $D$ is the telescope diameter (Roddier, 1981).

As solar observations have demonstrated, the Pic du Midi site sometimes features slow seeing variations and extended isoplanetic patch, which indicates that the TBL is potentially well suited to speckle and adaptive optics observations at short wavelengths, despite its modest size. With a diffraction limit at 0.06$\,$ in V, the TBL can provide high quality data from which many astrophysical programs could benefit.

Due to the necessary short exposure times photon noise is the most severe limiting factor in speckle imaging (Dainty and Greenaway, 1979, Beletic 