The shutter comprises a first and a second curtain, 47, 48 in planes parallel come together, wound on at least one roller 6, 9 on either side of the ed ris tho of exposure, and each comprising an edge 55, 56 of a to define a slot of the shutter 5, between the said edges. The roll of the first screen is driven by a first pinion integral with the motor 31. According to the invention, the shutter comprises a second pinion 11 is connected in rotation to the first pinion and a third pinion 14, which is coaxial with the second pinion and results from this latter by a device for angular adjustment 59, which drives the roller 9 of the second screen; the closure member also comprises means of a voltage 49, 52, 53 which hold each curtain in its respective plane. One thus obtains a slot of closure of variable width with precision. / p & & p & a monitoring aerial or sea.
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