An illustration of the 3-mirror 120-inch 'Shane' reflector's foci.  Taken at the <a href="//commons.wikimedia.org/wiki/Lick_Observatory" title="Lick Observatory">Lick Observatory</a>, <a href="//commons.wikimedia.org/wiki/California" title="California">California</a>
<a href="//commons.wikimedia.org/wiki/User:Introvert" title="User:Introvert">User:Introvert</a>
CC BY 2.5
<span class="int-own-work" lang="en">Own work</span>