STAR SPECTROSCOPE: Visual/Photo/CCD
A RAINBOW OFTICS Star Spectroscope consists of two components, a
grating Cell and a lens cell, which attach to your 1.25" eyepiece. Both
components are made of black-anodized aluminum.
The grating cell, which screws into the bottom of your eyepiece,
contains a glass, blazed, diffraction grating. Its delicate grating
surface is protected by a second glass disk. This grating's performance
is far superior to that of common, non-blazed, cheap plastic gratings.
And unlike a prism, which distorts a spectrum by spreading out blue
light more than red, this grating creates an undistorted, constant
dispersion across the entire visible spectrum.
The lens cell, providing spectrum-widening auxiliary optics, is
secured to the top of your eyepiece by nylon thumbs crews.
There are some important
advantages to this new spectroscope design. The most obvious
is that its cost is much lower
than the discontinued $400 star
spectroscope by Edmund Scientific.
Another advantage is that the
width and length of a star's
spectrum can be adjusted through
the observer's choice of eyepiece
focal length and eyepiece-tograting cell distance. Complete
instructions concerning these
adjustments are contained in the
owner's manual.
This spectroscope is very versatile and is compatible with a
variety of telescope types and setup configurations. I find that it
works well with medium-power eyepieces in the 12mm-18mm range. However,
each telescope has its own ideal setup. For example, my favorite setup
for examining the spectra of bright stars with an 8-inch SchmidtCassegrain is to use a 26mm Plossl eyepiece and then screw the grating
cell into the back end of the star diagonal. With this setup, Sirius'
spectrum is breathtakingly beautiful and Betelgeuse's spectrum reveals a
wealth of detail in its banded structure.
SPECTROSCOPY PRIMER
A good introduction to stellar spectroscopy appeared in the
October 1996 issue of "Sky and Telescope" magazine on pages 48-51.
The article includes an excellent set of reference spectra.
WHAT YOU'LL SEE
When a star is brought into the field of view and the spectroscope
is properly focused and adjusted, you will see a beautiful spectrum with
the colors of the rainbow spread out along its length. Depending on the
spectral type and luminosity class of the star, and your particular
setup, you may see hydrogen lines cutting perpendicular across the
spectrum, or many fine lines of metals, or wide absorption bands of molecules. These spectral features are easy to see in some classes of stars
and more difficult to see in others
The visibility of spectral features is dependent not only on the
type of star being examined but also on the variables that most amateur
astronomers are already familiar with: stellar magnitude, telescope
aperture, seeing, and the experience of the observer
I GUARANTEE that you'll see hydrogen absorption lines in the
spectra of Vega and Sirius and molecular bands in the spectra of
Betelgeuse and Antares with any small telescope set up for 1.25"
eyepieces
OTHER POSSIBILITIES
Besides using this instrument visually, you can use the grating
cell to photograph spectra through a telescope or with telephoto lenses.
Also, CCD cameras can be used, as described in the Fall 1995 "CCD
Astronomy" and the June 1999 "S&T" page 14.
The grating cell can be used as a planetary nebula-seeking filter.
Some small planetaries are star-like and can be difficult to distinguish
from the stars in a field-of-view. When the grating cell is screwed into
a 1.25" eyepiece or hand-held above the eyepiece, it transforms stars
into continuous, streak-like spectra while any small planetary nebula
becomes conspicuous because it remains a point of light. A similar
technique is described in the "Webb Society Deep-Sky Observer's
Handbook, Volume 2, Planetary and Gaseous Nebulae".
The spectroscope can be used to test the wavelength response of
color filters and nebula filters. Similar tests were done in an article
in the July 1995 issue of "Sky & Telescope" magazine.
PUBLIC STARGAZING PROGRAMS
If you have ever conducted a public stargazing session, you know
that when discussing the properties of stars, someone inevitably asks,
"How do they know that?" Now, instead of being limited to replying,
"Astronomers use special instruments attached to their telescopes”, you
can actually demonstrate one of those instruments! Children especially
enjoy the vivid colors seen through this spectroscope
SPECTRA OBSERVER'S NEWSLETTER
Since spectroscopy is a new field for most amateur astronomers, I
am publishing a newsletter containing short articles, comments, and a
list of stars that have been observed with this instrument. Included
with each spectroscope is an observer's form which can be filled out and
submitted to receive a free newsletter.
PRODUCT LIST
Star Spectroscope: Visual/Photo/CCD, $229. This is the spectroscope
described in the preceding pages. The following accessories are for use
with this instrument
Spacer, $10. This is an empty grating cell. It can be placed
between the grating cell and the eyepiece, to increase the spectrum
length (dispersion). Multiple spacers can be stacked. These spacers
are most useful with Newtonian telescopes, which usually do not
have enough focuser travel to use the 1.25-inch threaded adapter
described below. But I recommend that you do not buy any of these
until you have used the spectroscope a while and have read the
Owner’s Manual. You can remove the filters from any 1.25-inch color
filter cells you may already have and use them as spacers.
1.25-inch Threaded Adapter, $25. Page 7 of the spectroscope Owner's
Manual describes an optional technique of screwing the grating cell
into the star diagonal of an F/10 Schmidt-Cassegrain. This setup is
also useful with slow focal-ratio retractors. But if your brand of
star diagonal is not threaded for 1.25-inch filters, you can use
this adapter. With the grating cell screwed into the adapter, it
can be used in either of two positions. When placed between the
diagonal and the telescope, it creates a long dispersion. When
placed between the eyepiece and the diagonal, the spectrum
dispersion is shorter
Star Spectroscope: Visual, $179. This instrument is for visual spectroscopy with 1.25-inch and .96-inch eyepieces. It fits eyepieces with an
eye-end diameter of 1.5-inch or less. This is typical of most Plossls,
Kellners, and orthoscopics. This spectroscope looks just like the lens
cell of the two-piece spectroscope described in the preceding pages
but it contains both optical elements: a spectrum-widening lens and a
high-performance, glass, blazed diffraction grating. The grating is
different than the one used in the grating cell of the two-piece spectroscope because it is positioned on top or the eyepiece instead of
below. Spectrum length is fixed and spectrum width is adjustable. This
spectroscope is a bit easier to set up than the two-piece model.