  Microrefrigerators based on normal metal-insulator-superconductor (NIS)
junctions represent a very attractive alternative to cool the microbolometers
and calorimeters for astrophysical observations in space-borne experiments. The
performance in such measurements requires a good knowledge of the noise sources
in the detectors. In this paper we present detailed calculations of the thermal
fluctuations and of the noise equivalent power due to the heat transfer through
the NIS junctions or due to the thermal contact between different subsystems of
the detector. The influence of the background radiation will also be evaluated.
Analytical approximations, valid at low temperatures, are given.
