The Hubble Space Telescope suffers from significant spherical aberration. 
It does not give the expected diffraction limited images. A maximum of about
15% of the light from a point source is concentrated in a 0.1 arcsecond
radius. The images consist of this core, surrounded by a 4 arcsecond diameter
halo caused by portions of the primary mirror that are not focussing correctly. 
The halo has a complex structure related to obscurations in the telescope pupil.
Ground test results uncovered by the Allen commission agree with results
derived from studies of the on orbit imagery.

The pointing performance is also degraded, with guide stars for fine lock
presently limited to 13th magnitude, compared to an expected limit of 14.5 
magnitude. Some areas of the sky are therefore not accessible in fine lock, at
high galactic latitudes. In addition, the spacecraft undergoes severe pointing
disturbances during portions of the orbit, caused by thermal shocks.  

Nevertheless, HST represents an unique resource for high resolution imaging of
low contrast bright objects through deconvolution techniques. Such techniques 
rely on the detailed information about the PSF that is given here.
Furthermore, it can split higher contrast fields into components when
photometric accuracy is not important. Unfortunately, there is a loss of about
4 magnitudes in limiting magnitude for point sources.