An apparatus (100) for thermally treating a body (101, 102, 103) to be thermally treated, in particular for thermally connecting a first partial body (101) to a second partial body (102) to form a composite body (103) at an interface (104) between the partial bodies. The apparatus also includes a jacket (105), a temperature-controllable space (108) within a temperature-control unit (106) and a heating element (109), with the jacket contactlessly surrounding the body to be thermally treated before, during and after the thermal treatment. Also disclosed are a method for thermally treating a body to be thermally treated, e.g., for high-temperature bonding of a first partial body to a second partial body (102) to form a composite body (103), an optical element (609), e.g., a reflective optical element, e.g., one (609) which is temperature-controlled with a channel (208) through which media flow.