Barlow lens<br><p>A: ocular<br>
B: Barlow lens<br>
red: beam without Barlow lens<br></p>
green: beam with Barlow lens
<a href="//commons.wikimedia.org/wiki/User_talk:Andreas_06" title="User talk:Andreas 06">Andreas 06</a>
<span class="int-own-work" lang="en">Own work</span>