Sky & Telescope - September 2016 - 32

Eye and Scope

GEEK3 / WIKIMEDIA COMMONS / CC BY-SA 3.0

THE IDEAL This is
the computer-generated
diﬀraction pattern of an
equal double star seen
precisely at the Dawes
limit. The two central
Airy disks overlap, but a
slight notch appears at
their edges.

refractors resolve somewhat more sharply (both 6.0″/a)
than a 4-inch Schmidt-Cassegrain that I also use (6.4″/a).
In the other direction, on a night of almost rocksteady seeing, S&T's Alan MacRobert detected the
duplicity of an 0.32″ pair of 7th-magnitude stars at 800×
in his 12.5-inch f/6 reﬂector. It has a mirror whose ﬁgure
is ﬁne-tunable by a knob at the eyepiece that applies electric warmth to the center of the mirror's back (S&T: Feb.
1996, p. 76). He estimated the position angle of the pair
as seen in the eyepiece, and this proved to be correct.
Those numbers yield a Dawes parameter of 4.0″/a.
Several factors further confound the idea of a standard
resolution limit. Few telescopes are optically perfect. Seeing aﬀects large apertures more than small ones. A scope
with a central obstruction shows a slightly larger Airy
disk (the bright center of a star's diﬀraction pattern) than
an unobstructed aperture. And what exactly did Dawes
mean by "moderately favorable" atmospheric steadiness?

Star-Test Your Telescope
If you're going to be focusing stars at
very high power, you'll be star-testing your
telescope's optics whether you know it or
not. So here's what to look for. The star
test is so simple, yet so stringent, that
every telescope user should know it.
Give your scope a good hour to cool to
the surrounding temperature. If you have
astigmatism, wear your glasses. Center a

S&T: DENNIS DI CICCO

Refractor, inside focus

Refractor, outside focus

Moreover, Dawes deﬁned a double star as being
resolved if he could discern a "notch" between the overlapping Airy disks of the two components, as in the computed simulation at left. But it's possible to detect that a
star is double even if the Airy disk is merely elongated a
trace, the way MacRobert described his 0.32″ pair, rather
than notched to show the beginnings of a ﬁgure-8.
Lastly, what exactly did Dawes mean by "two stars of
the sixth magnitude"?
To create my grand list of test stars on the facing
page, I had to be speciﬁc. The table lists doubles in
which the primary component is between visual magnitude 5.7 and 6.9 and the secondary is 0.0 to 0.4 magnitude fainter than the primary. I think this deﬁnition is
reasonable, but it's somewhat arbitrary.
Close pairs this similar are indeed unusual. In searching the primary double-star sources - the Washington
Double Star Catalog (WDS) and the Sixth Catalog of
Orbits (CO), for instance - I found only 49 such pairs
visible from my location (latitude 41° N). I may have
missed a pair or two, but my searches were careful and
earnest. I did my own searches of the catalogs and also
used T. V. Bryant's online search tool for the WDS (available at http://is.gd/WDSsearch).
Have at it! ✦
Phil Kane's increasing Dawes constant over the years has
induced a slow panic, but he still counts his blessings for
dark ranch skies and plenty of doubles to observe. He welcomes mail at icycomet1944@gmail.com.

moderately bright star, then switch to your
highest power.
1. Turn the focuser slightly to one side of
best focus, then slightly to the other side.
As the star moves out of focus, it will swell
to a little disk with subtle rings and maybe
a dark spot at or near the center.
This slightly out-of-focus disk should
look the same on both sides of best focus.
This test is so powerful that few tele-

Reﬂector, inside focus

Reﬂector, outside focus

TWO WINNERS These are the kind of star tests that telescope owners yearn to see. For
each scope, the diﬀraction patterns are nearly identical on either side of sharp focus. These
are photos of an artiﬁcial star shot through a high-quality 85-mm refractor and a 90-mm
Maksutov telescope in an indoor test setup.

September 2016 sky & telescope

scopes pass it perfectly. A little diﬀerence
in how the star disk looks on either side
of focus is acceptable. But a really obvious
diﬀerence means the scope is a reject.
2. Next, on a night of very steady seeing, focus the star as sharply as you can.
You should see it as a tiny bright dot surrounded by a few thin little rings. This is
the diﬀraction pattern caused by the wave
nature of light. When the air settles way
down, it should be tight, crisp, and highcontrast - not messy, spiky, or vague.
3. Still at very high power, move the
focus slightly in and out again. Look for
what telescope users call "snap." This
means that the star, or anything else,
should "snap" into sharp focus at a fairly
precise point. What you don't want to see
is the star "mush" through focus, with
some distortions and aberrations growing
into view even before others disappear.

http://www.is.gd/WDSsearch

Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016