In conclusion we have shown that a tantalum superconducting tunnel junction
should have considerable potential as a photon counting broad band low
resolution spectrometer in optical and ultraviolet astronomy. Already the
spectroscopic resolution has nearly reached the theoretical limit for
symmetrical tunnel junctions, of below 10 nm at wavelengths shorter than
nm. The key features can therefore be summarised as follows:
a) Photon counting with an inherent spectroscopic capability from the
ultraviolet 
nm) to the near infra-red (
m)
with 
nm
at 550 nm. Simple signal to noise calculations show that single photon
detection is still possible up to wavelengths of order 
m.
b) A theoretical quantum efficiency ranging from 
over the
waveband 200 nm to 
m respectively peaking at 
for
nm.
It should also be possible to
extend the short wavelength limit to 110 nm
through the replacement of the sapphire substrate with magnesium fluoride.
Future developments must involve the practical astronomical application of such devices through the development of arrays. The first of these have already been fabricated and are already under evaluation (Jansen et al. 1996).
Acknowledgements
The authors acknowledge the technical support of A. van Dordrecht (ESTEC), R .Venn of Cambridge Microfab Ltd. UK and D. Goldie of Oxford Instruments Ltd. UK. The many fruitful discussions with M.A.C. Perryman, and P. Jakobsen of the Space Science Department ESA on the potential applications of such detectors are gratefully acknowledged.