ASTRONOMER’S WORKBENCH by Jerry Oltion
Two Simple Spectrographs
Homebrew science for a sunny day
JUST ABOUT EVERYTHING we know
about stars comes from observing
the light they emit. For centuries, our
observations were limited to examin-
ing the brightness and color of those
distant suns, but that changed in 1814
when Joseph von Fraunhofer split the
incoming light into a wide spectrum
and noticed dark gaps within it. Later,
Gustav Kirchhoff and Robert Bunsen
realized that those lines corresponded
to the emission/absorption spectrum
of specifi c chemical elements, and
with that discovery we were able to
determine the chemical composition of
stars. A new element, helium, was even
discovered spectroscopically in the SunpDavid Britz’s spectrograph uses a polished metal rod in place of the
more traditional slit. Light enters on the left, bounces off the rod in back,
and illuminates the diffraction grating in front right. The camera (not
mounted here) slides left and right to capture the entire spectrum. Dif-
ferent diameter rods, visible on the right, provide different resolutions.pThe slitless spectrograph produces a sharp, detailed solar spectrum.pJoseph Gerencher’s spectrograph uses a traditional slit but a decid-
edly non-traditional CD or DVD for a diffraction grating. Light enters on
the left, bounces off the grating at the junction on the right, and goes
through a lens at the white coupling partway up. The resulting spec-
trum reaches the camera body (without lens) on top.pA CD or DVD produces a curved spectrum due to the spiral lines
that create the diffraction.SLITLESS SPECTROGRAPH AND SPECTRUM: DAVID BRITZ; CD/DVD SPECTROGRAPH AND SPECTRUM: JOSEPH GERENCHER32 FEBRUARY 2020 • SKY & TELESCOPE
before it was found here on Earth.
Later still, we learned how to mea-
sure a star’s velocity by studying the
Doppler shift of those elemental lines.
The thickness of the lines can tell us
things about the strength of the star’s
magnetic fi eld and its rotational speed.
We know the age of the universe and
its rate of expansion largely through
spectrographic studies. And so on.
Because this column is about ama-
teur telescope making, you probably
already suspect where this is going: Yes,
you can indeed build your own spec-
trograph and measure your own stellar
spectra. Here are two different designs
for you to consider.First a word on nomenclature: A spec-
troscope is something you look through
or which projects an image on a screen
that you observe by eye. A spectrograph
is a device that allows you to record the
spectrum. We’ll be looking at spectro-
graphs here, in part because the easiest
star to obtain a spectrum of is the Sun.
It’s bright enough that you don’t need
a long exposure, so you don’t need to
worry about tracking, but you really
don’t want to be looking directly into
it without a specially designed spectro-
scope that’s beyond the purview of this
column. So these spectro-gadgets use
cameras — hence they’re spectrographs.
A spectrograph consists of three