The advent of the HST astronomy, which makes available observational data acquired through filters different from those employed in ground-based photometry, requires the correct understanding of how the observations of familiar objects like young and old stars in fields and clusters, stellar populations of different ages and metallicity, etc. would look like in the still poorly known photometric systems of HST. This is particularly true for the UV pass-bands for which the serious problem of the visible/red-leak is known to exist (Nota et al. 1994).
There have been several studies aimed at providing useful calibrations of
the HST photometric system.
In brief, Harris et al. (1988) have made observations of globular and
open clusters, using a camera similar to those of the WFPC and
a larger number of filters, many of which matched those used in the
WFPC. They also derived transformation equations between their magnitudes
and the UBVRI magnitudes and colours. Paltoglou & Bell (1991) and
Edvardsson & Bell (1989) presented
large grids
of synthetic surface brightness magnitudes for 21 of the WFC
pass-bands. The magnitudes were calculated from a set of
synthetic spectra for 
's in the range 4000 to 7250 K,
surface gravities typical of dwarfs and sub-giants, and for metallicities
from solar to one thousandth of the solar metallicity. Furthermore, they
derived the absolute magnitudes in these 21 pass-bands for a set of
globular cluster isochrones with different helium abundances, metallicities,
oxygen abundances, and ages. The isochrones were taken from VandenBerg
& Bell (1985), Bell & VandenBerg (1987), Hesser et al.
(1987), and
McClure et al. (1987). Finally, they presented useful transformations
from the 21 WFC pass-bands to various classical photometric systems, like
the Johnson UBV, Cousins VRI, Stromgren ubvy, and Thuan-Gunn uvgr.
Rocca-Volmerange & Guiderdoni (1988) derived the integrated magnitudes and
colours of model galaxies together with their cosmological evolution at high
red-shifts in various UV pass-bands of the HST WFC.
Bertola et al. (1995) in order to interpret the UV data for the
stars in M 31 acquired with the FOC combined their own observations in
the pass-band
F150W+F130LP with those of King et al. (1992) in the pass-band F175W
and presented synthetic magnitudes and colours that were the
preliminary version of the present study.
Finally,
Yi et al. (1995) presented colour calibrations for the FOC UV
filters F140W, F220W and F342W together with isochrones with composition
typical of globular clusters.
In this study, first we present large grids of isochrones and integrated magnitudes and colours of single stellar populations (SSP) for several pass-bands of the WFPC2 of the refurbished HST and a few of the old FOC, second with the aid of these results we address the question of galaxy ages. The plan of the paper is as follows.
Section 2 contains the definition of the HST photometric system for the
WFPC2 pass-bands
F170W (17W), F218W (21W), F300W (30W), F336W (33W), F439W
(43W), F450W (45W), F555W (55W),
F606W (60W), F702W
(70W), F814W (81W), and F850LP (85W),
and for the old FOC pass-bands
F150W+F130LP (15F), F175W (17F),
F220W (22F), and F342W (34F).
Thereinafter all pass-bands are indicated either by 
or
the abbreviation in brackets.
Section 3 presents the basic colour-temperature relations for the above
pass-bands.
All magnitudes and colours are obtained using the large library of
stellar spectra calculated by Kurucz (1992) however implemented as
described by Bressan et al. (1994), Bressan et al. (1996), and
Tantalo et al. (1996).
The main characteristics of the spectral
library in usage are shortly described below.
Since we intend that the results of this study can have as wide applicability
as possible, independently of our choice for the
stellar models to construct the isochrones, we
present also tables of conversions as a function of the metallicity,
, and gravity that allow to transform the bolometric
luminosity of a star into magnitudes of the HST system.
The goal is
achieved calculating grids of Bolometric Corrections
to be defined below.
Section 4 presents the grids of isochrones in the HST photometry. The ages go from those typical of old globular to those suited to very young clusters and associations. Each isochrone extends from the zero age main sequence (ZAMS) till the end of the asymptotic giant branch (AGB) phase and down to the White Dwarf stage or the core carbon ignition as appropriate for the initial value of the star mass.
Section 5 presents the integrated magnitude and colours of SSPs over the same age range as for the isochrones.
For the sake of brevity, the presentation of the theoretical results
in Sects. 4 and 5 is limited to the
solar composition [Y=0.28, Z=0.020], even if other compositions are
occasionally mentioned.
The considered chemical compositions are
[Y=0.23, Z=0.0004], [Y=0.232, Z=0.001], [Y=0.24, Z=0.004], [Y=0.25, Z=0.008],
[Y=0.28, Z=0.020], [Y=0.352, Z=0.050], [Y=0.475, Z=0.10]. The helium and metal
contents vary according to the enrichment law 
(cf.
Pagel et al. 1993).
Section 6 presents the CMD of a composite population containing both young and old stars with the aim of illustrating how the morphology of a CMD varies with the pass-bands in use, calling the reader attention on unexpected peculiarities of the CMD in extreme UV pass-bands.
Section 7 addresses the question of galaxy age and analyzes in some detail
the colour-redshift relation of model galaxies in order to check whether
suitable observations in the UV can provide useful tools to determine the age of
galaxies and/or to discriminate among different models of the Universe, i.e.
constrain the parameters 
and 
.
Finally, Sect. 8 contains some concluding remarks.