A variety of astronomical observations at optical and ultraviolet wavelengths require an efficient detector covering a wide wavelength range from the Lyman limit to the near infrared which can simultaneously provide the wavelength, time of arrival and location of every single photon falling upon the detector from the widest possible field. A detector which possesses the ability to measure the wavelength of individual optical photons has up until now not existed.
A superconductor is theoretically
capable of detecting individual optical/UV photons and also measuring the
photon energy (Wood & White 1969; Perryman et al. 1993).
This ability simply
arises from the very small energy gap 
, of order meV, in a
superconductor between the ground state, as represented by the bound Cooper
electron pairs, and the first excited state, containing broken Cooper pairs
or quasiparticles. The recent experimental demonstration of photon counting
at optical wavelengths, coupled to a crude wavelength discrimination with
niobium based superconductors (spectral resolution 
nm
at 
nm) by Peacock et al. (1996, 1997a)), has clearly
shown the validity of this concept. A theoretical evaluation of the limits
of 
for various superconductors other than niobium has
been made by Peacock et al. (1997b) and supported through recent
improved experimental measurements in niobium and tantalum
(Peacock et al. 1996c; Verhoeve et al. 1996a).
In this
paper we report on measurements from the ultraviolet to the near infrared
on a tantalum based superconducting tunnel junction. This is an efficient
high speed photon counting detector which has a far improved spectral
resolution and wavelength coverage compared to the
original niobium based device reported by Peacock et al. (1996, 1997a,b). The
resolution of this tantalum detector may well provide a contribution in
fields as diverse as fast spectro-photometry to deep field imaging. Indeed
provided these detectors can be packaged into arrays their existing
performance may already be such that they could allow the simultaneous
measurement of broad band low resolution spectra of very faint
extragalactic sources to be obtained for all objects in the field. Such
spectra containing emission line complexes or continuum absorption features
(the Lyman edge) in very faint extragalactic objects may allow the direct
determination of red shifts.