Sky & Telescope - May 2021 - 68

Magnification Musings

visual resolution to about 1 arcsecond on even good nights.
But for the sake of argument, let's take a more-demanding
resolution of ½ arcsecond, which the Dawes limit states
can be resolved with a 10-inch telescope if the atmosphere
cooperates. People with excellent eyesight can resolve about
1 arcminute with their unaided eyes, which means they
could resolve that ½-arcsecond angle if it were magniﬁed just
120×. But let's make it even easier on our eyes and double
the magniﬁcation to 240×. Most of us don't think of that as
extremely high magniﬁcation for a telescope, but it should be
more than enough to show us all the detail that can be seen
in a 10-inch or larger telescope, and certainly for smaller
instruments and even the best " average " seeing conditions.
That 240× works out to a magniﬁcation of just 24× per
inch of aperture for the 10-inch telescope or 40× per inch for
a 6-inch instrument. These values ﬁt well with the 25× to 50×
per inch of aperture often suggested by experienced planetary
observers. Furthermore, many of the great planetary observers
in the late 19th and early 20th centuries rarely found an
advantage to using maximum magniﬁcations greater than
400× to 500× with even with the largest professional telescopes. Last October I was among the legions of observers
viewing Mars. There was never a night when magniﬁcations
above 300× offered a better look at the planet with telescopes
between 6-inch and 18-inch aperture, and most of the time

my best views were at magniﬁcations between 155× and 220×
with a 6-inch refractor, which experienced acceptable seeing
conditions far more often than the larger instruments.

Final Thoughts
Armed with the above information, this is what I consider
as a good set of eyepieces for a new telescope. At the " lowpower " end of the spectrum I'd aim for an eyepiece giving
the highest magniﬁcation and still providing the true ﬁeld of
view I want. And this would be the one that most warrants
the investment necessary for one of today's eyepieces with a
truly wide apparent ﬁeld of view. My choice for a high-power
eyepiece would be one delivering around 200×. And rounding
out a basic set would be a third that delivers a magniﬁcation between these two. Rather than another eyepiece, my
next addition would be a quality 2× or 3× Barlow that, when
coupled with the three eyepieces, gives different magniﬁcations than what's available with any of the eyepieces alone.
The result would be a range of six magniﬁcations that covers
virtually everything I'd ever want for a typical night under
the stars. Your mileage may vary.

DENNIS DI CICCO says old habits die hard, and he still occasionally catches himself reaching for a lower-power eyepiece
rather than one that offers a wider field of view.

ON THE TEST BENCH Using equipment from his shop, the author set up a system for accurately measuring an eyepiece's apparent field of view. It works by using a paper target to position the exit pupil of an
eyepiece attached to a refractor directly over the axis of a rotary table, and measuring the angle needed
to sweep the crosshairs of a small finderscope (which provides a magnified look at the eyepiece's field
of view) from one side of the field stop to the other. Of the dozens of eyepieces examined over the years,
the measured fields rarely deviated by more than 1% or 2% from that specified by the manufacturer, and
more often than not were exact.

M AY 2 0 2 1 * S K Y & T E L E S C O P E

Sky & Telescope - May 2021

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