pThe glint of sunlight off a polished rod is
squeezed down to a line. Just as with a tradi-
tional slit, that line is only as long as the Sun’s
POLISHED ROD: DAVID BRITZ; CD HOLDER: JOSEPH GERENCHER angular diameter — about half a degree.
skyandtelescope.com • FEBRUARY 2020 33
simple parts: something to split the
incoming light into a spectrum, some-
thing to narrow the light source enough
to prevent blurring of that spectrum,
and something to record that spectrum
with. Let’s look at each element in turn.
Splitting light into its component
colors was fi rst done with a prism.
When light passes through a wedge-
shaped piece of glass, the shorter
wavelengths (blue) bend more than
the longer wavelengths (red), and the
light beam disperses. You can still use
a prism and get a decent spectrum,
but a simpler, lighter, and often better
choice is a diffraction grating. Diffrac-
tion gratings have thousands of fi ne
lines etched into them, which causes
light that passes through or bounces
off them to interfere with itself in such
a way that it spreads out just as if it
had passed through a prism. Diffrac-
tion gratings used to be very diffi cult to
make, but nowadays they’re everywhere.
You know them as CDs and DVDs.
That’s right, a simple CD or DVD
has enough lines etched into it, at close
enough spacing, to act as a diffraction
grating. And that’s exactly what Joseph
Gerencher (Moravian College profes-
sor of Earth Science, emeritus) uses in
his spectrograph. David Britz (research
consultant for AT&T Labs), on the other
hand, uses a commercially available dif-
fraction grating. The CD/DVD version
is called a refl ection grating, producing
spectra by refl ecting light, while the
more traditional one is a transmission
grating, which light passes through.
Both do the job admirably.
When you’re pointing your spec-
trograph at an extended object like the
Sun, which is ½° across, the spectrum
that the diffraction grating produces
is smeared out. Each spot on the Sun
produces its own spectrum, and those
multiple spectra blend together to pro-
duce a wide smear. Your spectrograph
needs something to narrow down the
width of the incoming light beam so the
sample size is thin enough to not blur
the absorption lines. The usual way to
do this is by making the sunlight pass
through a slit before reaching the dif-
fraction grating, and that’s what Joe hasdone, using two razor blades
to create a sharp-edged,
narrow opening.
Dave came up with
another solution. Recogniz-
ing that a shiny ball bearing
refl ects the Sun as a tiny
point, he reasoned that a shiny cylinder
would refl ect sunlight as a long, slender
line. He uses a highly polished metal
rod instead of a slit, and its cylindri-
cal shape produces a perfectly straight
and narrow refl ection line. And there’s
an advantage to this method: The thin
refl ection samples the entire face of
the Sun, compressing it down into a
line, rather than simply selecting a slice
of the Sun. That makes for a brighter
image. And you can use different
diameter rods to get wider or narrower
refl ections, which in turn affects the
resolution of the fi nal image. (Dave
says, “I will never look at chrome strip-
ping the same way again.”)
The difference between a CD and
a DVD is much the same. A DVD has
much fi ner line spacing so produces a
much more detailed spectrum... at the
cost of spreading it out about 3× as far
(requiring more overlapping photos to
assemble a full spectrum).So the incoming sunlight passes
through the slit or bounces off the rod,
hits the diffraction grating/CD/DVD,
and scatters into a spectrum. If the light
goes through the grating, then you place
your sensor (a digital camera) behind
that grating; if it bounces forward, then
you place your sensor in front of it.
Tilting the CD or DVD will sweep dif-
ferent sections of the spectrum across
your sensor, but tilting a transmissive
diffraction grating doesn’t do that. You
need to move the camera’s aim instead.
Joe took the lens off his camera and
uses one part of an inexpensive close-up
lens set mounted inside the spectro-
graph to focus the image directly onto
the camera’s sensor, but you can still
use the camera’s normal lens and focus-
ing system if you want. Dave’s system
does just that.
Both of these spectrographs are set
up for solar observing. To photograph
stellar spectra, you don’t need a slit or
a polished rod, since stars are already
effectively point sources. You do need
tracking, however, and a very sensitive
astronomical camera. There’s also some
question of whether a CD or a DVD
would provide enough signal for distant
stars. But for solar work, these two
designs work beautifully!
For more information on these two
spectrographs, and on a star spectro-
graph that Joe also built, visit Joe’s
website at https://is.gd/CDspectro, and
contact Dave at [email protected].■ Contributing Editor JERRY OLTION
keeps a diffraction grating in his eye-
piece case to examine bright stars with,
and another to play soft music with while
he observes.uJoe’s CD diffraction grating can
be tilted to sweep the full spec-
trum across the camera’s sensor.
At right you can see the slit made
from two opposing razor blades.