68 July 2014 sky & telescopeQuick Optical Evaluation
Slightly Out of Focus:
Star-Testing
Yo u r Te l e s c o p e
Alan French
Most of us have limited timeto
observe the night sky. As such we crave
the best views through our telescopes
when opportunity allows. Fortunately, the
stars that entice us can also help opti-
mize our telescopes. Star-testing reveals
atmospheric steadiness, how well our
optics are aligned, thermal issues, and
our instrument’s overall optical quality.
Acquiring skill as a star-tester lets you
see and handle problems that can detract
from your eyepiece time.
Under steady skies and using high
magnifi cation (25× or more per inch of
telescope aperture), we will see a star’s
image as a tiny disk (called the Airy disk)
surrounded by a bull’s-eye pattern of
faint diff raction rings. The rings grow
increasingly dimmer farther out from the
center, and you’ll probably see only one
or two. This image is a consequence of
the wave nature of light. (For an interest-
ing, readable discussion of light’s nature,
I recommend Richard Feynman’s 1985
bookQED: The Strange Theory of Light
and Matter.)Requiring only a high-power eyepiece and a clear
sky, a simple star-test can quickly reveal a wealth of
information about a telescope’s optical performance,
including if the optics are properly aligned and accli-
mated to the ambient temperature. A star-test also
lets you assess atmospheric seeing conditions. With
experience, star-testers can determine if a telescope’s
optics suff er from astigmatism, spherical aberration,
and stress caused by improper mounting.
ALL S&T IMAGES BY DENNIS DI CICCOThe stars that
entice us can also
help us optimize
our telescopes.