  We present a HST study of the metal-rich globular cluster NGC6553 using
WFPC2, NICMOS and STIS. Our primary motivation is to calibrate the STIS
broad-band LP magnitude against $V_{555}$ and $I_{814}$ 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 NGC6553 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 NGC6553 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.
