Abstract: A small size e.g. 12 mm diameter by 40 mm long cryostatic device with photodetectors. A hollow finger having the photodetectors at one end is adapted to be brought to a very low temperature by a cold fluid circulating in the finger. A wall or housing surrounds finger and provides a vaccuum space surrounding the finger. A window made from a material transparent to the radiation to be detected forms part of the wall and faces the detector. The cold finger is made from a ceramic material which is either a tight ceramic or a sealed ceramic which has low thermal conductivity.