al.*]{} 1979, Strittmater, 1980, Beckers [*et al.*]{}, 1983, Foy, 1988a) our instrument presents the advantage of a versatility and a full remote control of all its operating modes. PISCO offers thus a wide range of possibilities with fast switching between modes (less than one minute), allowing an optimal use of the seeing conditions.

[*2.1. Description*]{}

100000 The general layout is shown in Fig. . The external mechanical structure is a rectangular box of 100$\times$40$\times$36 cm$^3$ which is mounted at the Cassegrain focus of the telescope (Fig. ).

The input image plane (I1) of the telescope is located 200 mm downstream from the front flange of PISCO (this value is easily adaptable for the 3.6 m CFH or ESO telescopes). The converging input beam is transformed into a parallel beam by the collimating lens (L2), then focused into the image plane (I2) by the lens (L3).

Lens (L4) magnifies this image and projects it to the detector faceplate. The focal length of (L4) can be selected with the wheel GR that bears a series of eyepieces and microscope objectives. A magnification of at least 20 mas/pixel is needed to obtain a good sampling for speckle observations at the TBL while a 