A cryogenic cooling system comprising: a mechanical refrigerator having a first cooled stage and a second cooled stage , the second cooled stage being configured to reach a lower base temperature than the first cooled stage; a heat pipe having a first part coupled thermally to the second cooled stage and a second part coupled thermally to a target assembly , the heat pipe being adapted to contain a condensable gaseous coolant when in use; a heat switch assembly comprising one or more gas gap heat switches (11-13, 11'-12'), the heat switch assembly having a first end coupled thermally to the second cooled stage and a second end coupled thermally to the target assembly ; wherein the cryogenic cooling system is adapted to be operated in a heat pipe cooling mode in which the temperature of the second cooled stage is lower than the first cooled stage and wherein the temperature of the target assembly causes the coolant within the second part of the heat pipe to be gaseous and the temperature of the second cooled stage causes the coolant in the first part of the heat pipe to condense, whereby the target assembly is cooled by the movement of the condensed liquid coolant from the first part of the heat pipe to the second part of the heat pipe; wherein the cryogenic cooling system is further adapted to be operated in a gas gap cooling mode in which the temperature of the second cooled stage causes freezing of the coolant; and wherein the heat switch assembly is adapted to provide cooling from the second cooled stage to the target assembly during the gas gap cooling mode via the one or more gas gap heat switches (11-13, 11'-12'). a mechanical refrigerator having a first cooled stage and a second cooled stage , the second cooled stage being configured to reach a lower base temperature than the first cooled stage; a heat pipe having a first part coupled thermally to the second cooled stage and a second part coupled thermally to a target assembly , the heat pipe being adapted to contain a condensable gaseous coolant when in use; a heat switch assembly comprising one or more gas gap heat switches (11-13, 11'-12'), the heat switch assembly having a first end coupled thermally to the second cooled stage and a second end coupled thermally to the target assembly ; wherein the cryogenic cooling system is adapted to be operated in a heat pipe cooling mode in which the temperature of the second cooled stage is lower than the first cooled stage and wherein the temperature of the target assembly causes the coolant within the second part of the heat pipe to be gaseous and the temperature of the second cooled stage causes the coolant in the first part of the heat pipe to condense, whereby the target assembly is cooled by the movement of the condensed liquid coolant from the first part of the heat pipe to the second part of the heat pipe; wherein the cryogenic cooling system is further adapted to be operated in a gas gap cooling mode in which the temperature of the second cooled stage causes freezing of the coolant; and wherein the heat switch assembly is adapted to provide cooling from the second cooled stage to the target assembly during the gas gap cooling mode via the one or more gas gap heat switches (11-13, 11'-12'). <EOS>