64 ASTRONOMY • JANUARY 2018L
ast summer, I received
an email from 13-year-
old Adriana Baniecki
in Chandler, Arizona.
She wrote, “I have just
started viewing the sky with my
new telescope, which has an
aperture of 114mm (4.5 inches)
and a focal length of 910mm.
I have 25mm, 12.5mm, and
4mm eyepieces, as well as a 3x
Barlow. I was wondering, given
my eyepieces, what magnifica-
tion would be needed to view
the Moon and planets.
“Also, in your January
Astronomy column, ‘January’s
top 10 targets,’ I noticed refer-
ences to both the angular size
of an object and the magnifica-
tion needed to view it. [Author’s
note: I had mentioned that the
Pleiades star cluster (M45),
which spans 2°, is best viewed
with low power.] Given the
angular size of any object,
could you simply calculate the
magnification needed to view
it? If so, how?”
I responded by suggesting
that she always start with the
25mm eyepiece, which yields
36x with a 910mm focal length
scope, because its large field of
view makes it easier to key in
on sky objects. I also recom-
mended this eyepiece for deep-
sky objects wider than 0.5°,
such as the Pleiades and the
Andromeda Galaxy (M31), add-
ing that the 12.5mm eyepiece
(73x) and 25mm eyepiece with
3x Barlow (109x) would work
fine on the Moon and planets.
What about the magnifica-
tion needed for an object of
given angular size? I pointed
out that an eyepiece’s true fieldOBSERVINGBASICS
BY GLENN CHAPLEWhat’s my true
field?
Try these three methods to
determine an eyepiece’s true
field of view.fraction of a Moon, I could fit in
the field. Because this is the
most subjective of the three
methods and I didn’t want to
bias my results, I used this
method first.
My tool for the next method
was our spinning planet. Because
Earth rotates once every 24
hours, covering 360° of sky, a
star near the celestial equator
will drift 1° every 4 minutes. If
you time how many minutes it
takes the star to enter the field of
view, cross the center, and exit
on the opposite side, then dividethat time by 4, you have the true
field of view in degrees. If the
transit time is 6 minutes, for
example, the true field is 1.5°. My
star of choice was 3rd-magnitude
Sadalmelik (Alpha [α] Aquarii),
located just 19' south of the celes-
tial equator. I ran several trials
for each eyepiece.
The final method can be done
mathematically while indoors.
An eyepiece’s true field of view
equals its apparent field of view
(AFOV) divided by the magnify-
ing power it yields with a given
scope. An eyepiece with a 60°
AFOV that magnifies 40x has aof view, not its magnification, is
the ultimate determining factor.
This caused me to do a little
soul-searching. During my
nearly five decades as an avid
backyard astronomer, I’ve
become familiar with the mag-
nifying power all my eyepieces
produce with my various tele-
scopes, but I knew next to noth-
ing about their true fields of
view. With my 10-inch f/5
ref lector, for example, I nor-
mally use three eyepieces: a
32mm (40x), a 16mm (79x), and
a 6mm (212x). Spurred to actionby Adriana’s email, I went out-
side with scope and eyepieces
and went to work determining
their true fields of view.True to your field
There are three basic ways to
figure out the true field of view
an eyepiece provides. One is to
use the Moon, which has an
apparent diameter of 0.5°, as a
measuring tool. So if an eye-
piece captures a chunk of sky
three Moon diameters across, it
has a true field of view of 1.5°.
With each eyepiece, I measured
how many Moons, or whattrue field of 1.5°. Calculator in
hand, I worked out the true
field for each eyepiece. The
results for all three methods
appear in the table below.
Some final thoughts: Because
the Moon’s angular size varies
with its distance from Earth
(0.49° when farthest away, 0.56°
when closest), it’s not an ideal
measuring tool. Nevertheless,
the ballpark figure you get is
better than nothing, as my
results show. Also, by the time
this issue reaches the news-
stands, Sadalmelik won’t be as
easy to use for star-drift tim-
ings. Work instead with 2nd-
magnitude Mintaka (Delta [δ]
Orionis), the northernmost and
westernmost of the three stars
in Orion’s Belt and just 18'
south of the celestial equator.
Finally, the AFOV of an eye-
piece depends on its design.
Here are approximate AFOVs
for traditional types: Huygens or
Ramsden (labeled “H” or “R” on
the barrel), 30°; Kellner, achro-
matized Ramsden, or modified
achromat (“K” or “Ke,” “AR,” or
“MA”), 40°; orthoscopic (“Or” or
“Ortho”), 45°; Plössl, 50°; and
Erfle (ER) or König, 60°. For
newer designs, especially super-
and ultrawide types, refer to
the manufacturer’s website
for specifics.
Questions, comments, or
suggestions? Email me at
[email protected]. Next
month: another 13-year-old and
her eclipse adventure.BROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.The Full Moon
has an apparent
diameter of
about 0.5°, a
convenient
reference for
calculating an
eyepiece’s true
field of view.
JOHN CHUMACKGlenn Chaple has been an
avid observer since a friend
showed him Saturn through a
small backyard scope in 1963.CALCULATING THE FIELD OF VIEW
Eyepiece Mag. Apparent TRUE FIELD
Field Moons Star-drift Calculation
32mm 40x 70° 1.7 ° 1.8° 1.8°
16mm 79x 82° 0.8° 1.0° 1.0°
6mm 212x 60° 0.3° 0.3° 0.3°
The author calculated the true field of view of three eyepieces when attached to
his 10-inch f/5 reflector. The three methods he used — estimating how many
Moons would fit in the field, star-drift timings, and mathematical calculations —
gave pretty consistent results.