Method for athermalisation of an optoelectronic system comprising: - an optical device that is able to observe at least one portion of the environment and which has a focal plane; - a detector that engages with said optical device and which comprises a detection matrix that has a stable orientation with respect to said focal plane; and - means of mechanical connection between said optical device and said detector, method according to which: a) at least one first expansion law of said optical device in a first reference standard is determined, and at least one second expansion law of said detector in a second reference standard, an expansion law that indicates the displacement due to the expansion according to the temperature in the corresponding reference standard in at least one direction of space; b) from said first and second expansion laws, at least one relative expansion law between said optical device and said detector in a third reference standard is determined; c) means of mechanical connection are determined that have expansion properties that make it possible to offset said relative expansion law in a predetermined range of temperatures and which comprise at least one element made of composite material, at least by adapting said composite material to obtain the expansion properties sought, by adapting the properties of a fibrous preform of said composite material, by adapting at least some of the following parameters of the preform: the type of layers of the fibres, the orientation between the layers, the number and the positioning thereof, and by adapting the material of the fibres of the preform, said means of mechanical connection being determined by implementing a method comprising a prior theoretical study that makes it possible to determine the general features of the elements of the means of mechanical connection, followed by tests and/or simulations intended to refine these features so as to obtain the expansion properties, said means of mechanical connection comprising at least one element made of composite material and at least one metal element; and d) the means of mechanical connection thus determined are arranged on said optoelectronic system. - an optical device that is able to observe at least one portion of the environment and which has a focal plane; - a detector that engages with said optical device and which comprises a detection matrix that has a stable orientation with respect to said focal plane; and - means of mechanical connection between said optical device and said detector, a) at least one first expansion law of said optical device in a first reference standard is determined, and at least one second expansion law of said detector in a second reference standard, an expansion law that indicates the displacement due to the expansion according to the temperature in the corresponding reference standard in at least one direction of space; b) from said first and second expansion laws, at least one relative expansion law between said optical device and said detector in a third reference standard is determined; c) means of mechanical connection are determined that have expansion properties that make it possible to offset said relative expansion law in a predetermined range of temperatures and which comprise at least one element made of composite material, at least by adapting said composite material to obtain the expansion properties sought, by adapting the properties of a fibrous preform of said composite material, by adapting at least some of the following parameters of the preform: the type of layers of the fibres, the orientation between the layers, the number and the positioning thereof, and by adapting the material of the fibres of the preform, said means of mechanical connection being determined by implementing a method comprising a prior theoretical study that makes it possible to determine the general features of the elements of the means of mechanical connection, followed by tests and/or simulations intended to refine these features so as to obtain the expansion properties, said means of mechanical connection comprising at least one element made of composite material and at least one metal element; and d) the means of mechanical connection thus determined are arranged on said optoelectronic system.