  Superconducting tunnel junctions are being developed for application as
photon detectors in astronomy. We present the latest results on the development
of very high quality, very low critical temperature junctions, fabricated out
of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic
demagnetisation refrigerator. The contacts to the top and base electrodes of
these junctions are fabricated either out of Nb or Ta, which has strong
implications on the loss time of the quasiparticles. The Nb contacted junctions
show quasiparticle loss times varying between 5 and 80 usec, depending on the
device size. The bias range of the Nb-contacted junctions is limited to the
range 0-100 uV, because of the set-in of strong non-equilibrium quasiparticle
multiplication currents at higher bias voltages. The Ta-contacted junctions, on
the other hand, show quasiparticle loss times in excess of 200 usec. These long
loss times lead to very strong quasiparticle multiplication, which prevents the
stable biasing of the junctions even at very low bias voltages. Junction
fabrication and characterisation are described, as well as the response of the
detectors to monochromatic light with wavelengths varying from 250 to 1000 nm.
The energy resolution of the detectors is discussed.
