  The SPHERE (spectro-photometric exoplanet research) extreme-AO planet hunter
saw first light at the VLT observatory on Mount Paranal in May 2014 after ten
years of development. Great efforts were put into modelling its performance,
particularly in terms of achievable contrast, and to budgeting instrumental
features such as wave front errors and optical transmission to each of the
instrument's three focal planes, the near infrared dual imaging camera IRDIS,
the near infrared integral field spectrograph IFS and the visible polarimetric
camera ZIMPOL. In this paper we aim at comparing predicted performance with
measured performance. In addition to comparing on-sky contrast curves and
calibrated transmission measurements, we also compare the PSD-based wave front
error budget with in-situ wave front maps obtained thanks to a Zernike phase
mask, ZELDA, implemented in the infrared coronagraph wheel. One of the most
critical elements of the SPHERE system is its high-order deformable mirror, a
prototype 40x40 actuator piezo stack design developed in parallel with the
instrument itself. The development was a success, as witnessed by the
instrument performance, in spite of some bad surprises discovered on the way.
The devastating effects of operating without taking properly into account the
loss of several actuators and the thermally and temporally induced variations
in the DM shape will be analysed, and the actions taken to mitigate these
defects through the introduction of specially designed Lyot stops and
activation of one of the mirrors in the optical train will be described.
