  Distant supernovae have been observed to be fainter than what is expected in
a matter dominated universe. The most likely explanation is that the universe
is dominated by an energy component with negative pressure -- dark energy.
However, there are several astrophysical processes that could, in principle,
affect the measurements and in order to be able to take advantage of the
growing supernova statistics, the control of systematic effects is crucial. We
discuss two of these; extinction due to intergalactic grey dust and dimming due
to photon-axion oscillations and show how their effect on supernova
observations can be constrained using observed quasar colours and spectra. For
a wide range of intergalactic dust models, we are able to rule out any dimming
larger than 0.2 magnitudes for a type Ia supernova at z=1. The corresponding
limit for intergalactic Milky Way type dust is 0.03 mag. For the more
speculative model of photons mixing with axions, we find that the effect is
independent of photon energy for certain combinations of parameter values and a
dimming as large as 0.6 magnitudes cannot be ruled out. These effects can have
profound implications for the possibility of constraining dark energy
properties using supernova observations.
