  ASTEP (Antarctica Search for Transiting ExoPlanets) is a pilot project that
aims at searching and characterizing transiting exoplanets from Dome C in
Antarctica and to qualify this site for photometry in the visible. Two
instruments were installed at Dome C and ran for six winters in total. The
analysis of the collected data is nearly complete. We present the operation of
the instruments, and the technical challenges, limitations, and possible
solutions in light of the data quality. The instruments performed continuous
observations during the winters. Human interventions are required mainly for
regular inspection and ice dust removal. A defrosting system is efficient at
preventing and removing ice on the mirrors. The PSF FWHM is 4.5 arcsec on
average which is 2.5 times larger than the specification, and is highly
variable; the causes are the poor ground-level seeing, the turbulent plumes
generated by the heating system, and to a lower extent the imperfect optical
alignment and focusing, and some astigmatism. We propose solutions for each of
these aspects that would largely increase the PSF stability. The astrometric
and guiding precisions are satisfactory and would deserve only minor
improvements. Major issues are encountered with the camera shutter which did
not close properly after two winters; we minimized this issue by heating the
shutter and by developing specific image calibration algorithms. Finally, we
summarize the site testing and science results obtained with ASTEP. Overall,
the ASTEP experiment will serve as a basis to design and operate future optical
and near-infrared telescopes in Antarctica.
