Astronomy

66 ASTRONOMY • DECEMBER 2015

O

btaining a sharp

focus is critical to

many aspects of

amateur astron-

omy. Ironically,

however, placing the object of

attention purposefully out of

focus can help us make better

sense or more accurate obser-

vations of what we see. Let’s

look at some examples.

Star color

The surfaces of stars radiate at

different temperatures, which

to our eyes appear as different

colors: Blue stars are hot; red

stars are cool; yellow-white

stars (like our Sun) are in

between. If you view a star in

focus, its color can be difficult

to pin down. It can change in a

matter of milliseconds because

of the refractive effects of our

ever-moving atmosphere.

When it does, it displays an

array of prismatic colors as it

scintillates (twinkles) like a

diamond under light.

Viewing the same star at

high power and slightly out of

focus averages out the scintilla-

tion, leaving us with a good

representation of the star’s true

color. This method helps espe-

cially in double star observing;

just be sure not to rack the

stars so far out of focus that the

disks overlap.

Variable star

brightness

Observers often try to estimate

the brightnesses of variable

SECRETSKY

Out-of-focus

observing

Sometimes, a slightly fuzzy view can help you.

stars (those that change in

brightness). To do this, they

use comparison stars that lie

nearby (or better yet, in the

same field of view). Sometimes,

it’s hard to judge the brightness

of faint variables.

One way to improve your

accuracy is to keep racking the

variable out of focus until

either it or a similarly bright

comparison star disappears.

The first one to vanish from

view is the fainter of the two; if,

however, they disappear simul-

taneously, you won’t have to

repeat the process with another

comparison star.

Comet brightness

Amateur astronomers use two

different acceptable methods

to accurately determine the

brightness of a comet that

sports a significant coma. The

first is the Vsekhsvyatskij-

Steavenson-Sidgwick method,

where you compare a memo-

rized in-focus image of the

comet to a defocused image of

a comparison star.

The second is the O’Meara-

Morris method, where you

defocus the stars more than the

comet. This yields a more cor-

rect surface brightness for the

images of both the comet and

star. See http://www.icq.eps.

harvard.edu/ICQMM.html for

more details.

Cluster patterns

Do you enjoy drawing deep-

sky objects, especially fanciful

open star clusters? One way to

enhance patterns of stars with-

in them is to slightly defocus

the view. Doing so blends the

light of individual points and

reveals the most pronounced

stellar groupings and interven-

ing voids more distinctly.

Dark nebulae

Although they can look inky

black and dramatic in images,

dark nebulae can be difficult

to detect visually if they don’t

appear against a bright region

of Milky Way. So, if the nebula

winds through star fields inter-

laced with regions devoid of

bright stars, you might lose the

dark nebula among the web-

like confusion.

The remedy is to slightly

defocus the Milky Way until it

appears as a smooth back-

ground glow. Against it, the

size and shape of the dark neb-

ula will stand out.

It also helps to focus your

attention on the darkness

rather than the light. You may

think it odd, but “telling” your

eyes what to see can help you

focus on your target in the out-

of-focus field.

As always, please send your

thoughts related to my column

to [email protected].

BY STEPHEN JAMES O’MEARA

BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.

To estimate the brightness of an object like Comet PANSTARRS (C/2012 K1), ignore its

tails and memorize the size and brightness of the greenish coma. Then, compare the

comet’s in-focus image with a similar size out-of-focus star image. When the star’s fuzzy

image equals the comet’s sharp one, their magnitudes will be the same. GERALD RHEMANN

The International

Astronomical

Union (IAU)

announces new

publicly chosen

nicknames for

dozens of exo-

planets. Of course,

by IAU rules none

of the planets are

actually planets

since they don’t

“orbit the Sun.”

What’s in a

name?

COSMIC WORLD

A look at the best and the worst that astronomy and

space science have to offer. by Eric Betz

Cold as

space

Supernova

hot

The Smithsonian

raises $700,000

to reboot Neil

Armstrong’s suit

for display after

Apollo 11’s 50th

anniversary in

2019. As a
      stretch goal,
      they rehab a
      lunar module
      and reboot the
      space program.

Reboot the suit

NASA astronauts

Scott Kelly and

Kjell Lindgren

eat the first food

ever grown on

the International

Space Station —

lettuce. Up next:

potatoes, which

every astronaut

knows are vital

for surviving

on Mars.

Space salad

Stephen Hawking

throws support to

the Breakthrough

Initiatives’ E.T.

hunt, despite his

belief advanced

aliens could crush

us like humans

stomping on an

anthill. Out of a

billion Earths,

surely we’re not

the only jerks.

Alien ant farm

ESO/M. KORNMESSER/N. RISINGER (SKYSURVEY.ORG) (WHAT’S IN A NAME?); NRAO/AUI (ALIEN ANT FARM); NASA/GIOIA MASSA (SPACE SALAD); NASA (REBOOT THE SUIT)