  Axion helioscopes aim at the detection of solar axions through their
conversion into x-rays in laboratory magnetic fields. The use of low background
x-ray detectors is an essential component contributing to the sensitivity of
these searches. Here we review the recent advances on Micromegas detectors used
in the CERN Axion Solar Telescope (CAST) and proposed for the future
International Axion Observatory (IAXO). The most recent Micromegas setups in
CAST have achieved background levels of 1.5$\times10^{-6}$\ckcs, a factor of
more than 100 lower than the ones obtained by the first generation of CAST
detectors. This improvement is due to the development of active and passive
shielding techniques, offline discrimination techniques allowed by highly
granular readout patterns, as well as the use of radiopure detector components.
The status of the intensive R&D to reduce the background levels will be
described, including the operation of replica detectors in test benches and the
detailed Geant4 simulation of the detector setup and the detector response,
which has allowed the progressive understanding of background origins. The best
levels currently achieved in a test setup operating in the Canfranc Underground
Laboratory (LSC) are as low as $\sim10^{-7}$\ckcs, showing the good prospects
of this technology for application in the future IAXO.
