We present a HST study of the metal-rich globular cluster NGC 6553
using WFPC2, NICMOS and STIS. Our primary motivation is to calibrate
the STIS broad-band LP magnitude against V555 and I814
magnitudes for stars of known metallicity and absolute (visual)
magnitude, for application to our study of LMC globular clusters.
NGC6553 has been shown in earlier studies to have a very unusual
colour-magnitude diagram, so we also use our data to investigate
the reddening, distance, luminosity function and structure of this
cluster. We deduce a higher metallicity and smaller distance
modulus than did some previous studies, but emphasise that very
large patchy extinction on small angular scales prohibits accurate
determination of the parameters of this cluster. The horizontal
branch of NGC 6553 in (V,V-I) is tilted at an angle close to that
of the reddening vector. We show that extinction does not, however,
explain the tilt, which is presumably a metallicity effect. The
colour-magnitude diagram shows an apparent second turnoff some
1.5 magnitudes fainter than that of the cluster. We show that
this is most likely the background Galactic bulge: however, in that
case, the colour-magnitude diagram of NGC 6553 is not a good match to
that of the field bulge population. The cluster is probably more
metal-rich than is the mean field bulge star.