Sky and Telescope - April 2018 - 67

UNLESS OTHERWISE NOTED, A LL PHOTOS BY RICH A RD S. WRIG H T, JR.;
DIA PHR AG M: LE A H TISCIONE / SK Y & TELESCOPE

nce upon a time I had two hobbies: photography and
astronomy. I had a telescope for observing, and I had
a camera for taking photographs. These two interests
lived separately and grew for years before they came together,
mostly because I had good photographic gear long before I
had good astronomy gear. When the two merged, it didn't
seem to be something entirely new; it was just that now my
photography extended beyond birds, animals, and landscapes
to also include astronomical objects.
All those years ago, I thought you had to use a telescope
for astrophotography. But really all you need to get started
is an off-the-shelf DSLR camera, some quality lenses, and
the ability to have them track the sky. In fact, this avenue
of astrophotography is so compelling to me that, even with
high-quality astrographs, astronomical CCD cameras, and
exotic ﬁlters at my disposal, I still frequently return to my
DSLR and its lenses to capture certain targets.
When it comes to wide-ﬁeld astrophotography, wellcorrected astrographs with focal lengths less than about 400
mm are extremely rare and generally astronomically expensive. But one of the enduring appeals of the DSLR is the wide
range of interchangeable lenses available. It's much easier to
ﬁnd quality DSLR lenses ranging from 400 mm all the way
down to 8-mm ﬁsheye lenses covering a 180° ﬁeld of view.
There is a reason for this, of course. The shorter the focal
length (and wider the ﬁeld of view), the more difﬁcult it is to
control optical aberrations, especially away from the center
of the image. The camera-lens folks have got this ﬁgured out,
and highly sophisticated lens designs using multiple elements
abound. And due to the immense size of the photography
market, they can be had relatively cheaply. These lenses make
excellent optics for astrophotography as well!
There is a caveat or two when using a camera lens for
the night sky. Stars, being point sources of light, are an acid
test for lens quality and sharpness. While stars in the center

Lens iris diaphragm

f /1.4
Wide

f/2

f / 2.8

f/4

f / 5.6

f/8
Narrow

p INTERNAL STOPS The diaphragm in a camera lens is designed to
reduce the amount of light entering the lens, but it can produce diffraction spikes around bright stars.

of the ﬁeld of view might be round and sharp, those near
the edges or corners of the frame often appear as elongated
streaks, distorted into "seagull" shapes, or simply out-offocus, multicolored blobs. It takes a critical eye to spot this in
a daylight image, but in a star ﬁeld the aberrations stand out
like a sore thumb.
The typical mitigation for this is to stop the lens down. It's
well known in the photography crowd that, say, a 200-mm
f/2.8 lens doesn't perform at its peak sharpness until you
stop it down to f/4 or f/5.6. The exact adjustment varies from
lens to lens, and there are websites aplenty with test charts
for various lenses from different manufacturers.
Increasing the focal ratio ("stopping down the lens")
decreases the opening through which the light enters the
camera and restricts the amount of light that's focused onto
the image sensor. It does so by blocking the edges of the lens
using only the central region of the lens where the sharpness
and aberration control are at their best.
Typically the light gets reduced using a 6- or 8-bladed
diaphragm within the lens assembly, though some high-end
lenses use more. Usually the blades have straight edges, which
create spikes or rays (one for each blade) radiating from a

Masks
Lenses

Here's a novel way to make
your camera lens perform like
a high-quality refractor.

tp NO SPIKES Author Richard S. Wright, Jr. describes his novel
approach to creating lens masks that eliminate the diffraction spikes
caused by internal iris diaphragms. He used the Canon EOS Rebel
3Ti DSLR above with a 200-mm f/2.8 lens stopped down to f/4 using
a modified lens cap aperture mask to shoot the colorful Rho Ophiuchi
region in Scorpius on the facing page.
sk yandtelescope.com * A PR I L 2 018

http://www.skyandtelescope.com

Sky and Telescope - April 2018

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