by Barry D. Malpas
United Astronomy Clubs of New Jersey
The Practical Observer Magazine
Volume 7, Issue 2 - 1996
Before reviewing the Rainbow Optics
Spectroscope, since most amateur astronomers are
novice to this very interesting, but seldom explored frontier of astronomy, I feel
obligated to provide a few basics of what a spectroscope does, something about
spectroscopy, a few notes about the different types and designs of instruments,
as well as some comments on expectations by the user of any spectroscope.
Spectroscopy Background Basics
All spectral instruments do basically one thing. They break light (electromagnetic
radiation) up into its constituent components. In the visible, white light will be
dispersed into the colors Red, Orange, Yellow, Green, Blue and Violet. (The near
infra-red, Ultra-violet and other regions will also be dispersed, however, they
are, by definition, undetected by the human eye.) The visible colors ranges in
wavelength from 4000 to 7000 Angstroms.
Spectra and Spectroscopy
All gases when heated to high temperatures will give off light at particular
wavelengths characteristic to the number of electrons in the atomic orbits of
the material. Every element, as well as ion or molecule, has its own 'finger-print'.
Therefore when the light from the sun and stars are observed, elements in the
atmospheres of stars are detectable as characteristic wavelengths (colors) which
enable the astronomer to identify that material, as well as the temperature at
which it exists, and a host of other physical characteristics. The device used
to separate the varying wavelengths to detect these chemical and physical properties
is known as a spectral instrument. The science of doing so is spectroscopy.
The difference between the terms spectroscope, spectrometer, or spectrophotometer,
often referred to in magazine articles, has little to do with any differences in
the basic spectral functionality of these instruments. Rather the differing
suffixes refer to the method of detection: -scope for visual use; -meter implies
photography; and -photometer uses some electronic means. The first instrument to
be used, of course, was the spectroscope which was developed in the early 1800's.
As with telescopes, there are two basic types of spectroscope. The first one
developed incorporates a prism, using the property of refraction, to disperse the
light into the constituent colors. The second type has a grating, a series of very
finely ruled, evenly spaced, lines that uses the optical property of diffraction
to disperse the colors.
The main differences between the types of spectra that are produced is: the
diffraction grating produces many spectra, each of which is linear (the distance
between wavelengths are of equal separation no matter what part of the spectrum
you look at) ; while a prism produces a single spectrum which is not linear
(the red end of the spectrum is foreshortened, while the blue end is broadened).
The primary advantage of the diffraction grating is its linearity, which is of
particular importance when trying to measure the wavelengths of lines. Hence,
diffraction gratings are used more extensively in research today. (The diffraction
grating also has the advantage of having a zeroth order which can be used, when
adapted for photography, as a reference point for the line measurements.)
The prism's advantage is in the vast amount of published material available
(including all stellar classification catalogues) that was done in the early
part of this century using prisms. However, the broadening of the blue end of
the spectrum, and the blue sensitivity of the early photographic emulsions have
produced comparison spectra that usually range from 3500 to 5000 angstroms, which
is a problem when making comparisons visually.
All eyes receive slightly varying wavelength ranges, but usually detect from violet
through red. But the spectral sensitivity of the human eye is not evenly distributed
over the full color range. Rather, for the average observer, it is most sensitive at
about 5500 Angstroms (green), dropping off on either side in what is most commonly
described as a bell shaped curve diminishing in the ultraviolet at about 4000 A, and
in the red at about 7000 A.
The intensity of the object also comes into play. When dealing with the meager light
sources of stars, the trailing 'wings' of the bell curve will often be cut shorter,
on both sides, by some 200-300 Angstroms, - i.e. the weaker the object, the lesser
the color range visually detectable. This can cause some difficulty with trying to
observe stars in the moderate temperature ranges (F, G and K stars) that do not have
broad or distinctive lines in the most sensitive range (4500 - 6500 A). However, the
Hydrogen Alpha and Beta lines of the O, B and A stars, the many fine absorption lines
of the heavier elements, as well as the broad molecular lines, such as Titanium
Oxide (TiO), in the M class stars, and the multitude of dark lines in Carbon stars
are well within their range.
Grating Types and Blaze
There are also two types of gratings, transmission and reflection. The former is made
on a glass or plastic substrate where the light passes through the optic diffracting
the light on the opposite side. The latter has its lines ruled on glass and is then
aluminized, just as a telescope mirror is, and diffracts and reflects the light energy
at the same time. Reflection gratings are used more in industry because of the better
control of energy distribution and less light loss, while transmission gratings are
more usually used in educational laboratories since they are decisively less expensive.
There are also two methods of manufacturing gratings. The first, and historically
conventional, method is with a device called a ruling engine which scribes one line
at a time. More recent technology has developed the laser holographic method which
now produces gratings far superior to the earlier ruled variety.
Since a diffraction grating produces many spectra on both sides of the impinging
light source which are referred to as 1st, 2nd, etc. orders (The non-diffracted
light is referred to as 0th order.) The groves are beveled in such a way as to
reflect or refract most of the energy in the direction of the preferred part of
some particular order. This is known as the Blaze. For example, the most common
lab research gratings are of the reflection type, with 1200 groves/mm, blazed at
5000 Angstroms, in the first order.
Star Spectroscopes and Spectra
In a spectral instrument the length of the spectrum (from red to violet) is produced
by the physical characteristics of the grating or prism (dispersion angle, order, etc.)
The height of a star's spectrum is a problem since stars are point sources. If
photography is used to capture the image, star trailing may be used to make the
image taller. However, since the eye cannot accumulate light as does photographic
film, a cylindrical lens is used to stretch the spectrum perpendicular to the
dispersion giving the spectrum height.
Two types of lines are detectable in a star's spectrum, emission and absorption.
Emission lines appear bright against a dark background, while absorption lines are
dark against a bright rainbow background. The latter is the most common, and are
what constitutes the greater part of the O through M star types.
The Rainbow Optics Spectroscope
Evaluating any instrument, when it is the only one on the market, can be rather
subjective. However, since I have been involved with spectroscopy for over two dozen
years, I have become familiar with several home-made instruments, as well as the Goto
prism star spectroscope that was available from Edmund Scientific from the 1960s
through the mid-1980s and sold for about $400. The Goto instrument will be used in this
article only for general comparison since it is no longer available on the market.
The general design used is of the transmission diffraction grating type. One of similar
grating-lens optical configuration was written about in Scientific American's The
Amateur Scientist, in December 1952. The unit consists of a diffraction grating
mounted at the tube-end of an eyepiece, preceding the eyepiece optics, with a
cylindrical lens mounted between the ocular lens and the eye. Unlike previous
designs, the ROS is manufactured as two separate pieces (grating and cylindrical
lens sections) to be used in conjunction with ones own eyepiece.
The diffraction grating is manufactured by holographic laser method, and mounted on a
glass substrate. It is fitted in a filter style cell with a protective glass disk
which allows the user to thread the device into any standard 1 1/4 inch eyepiece.
The quality of the grating is far superior than those found in educational supply
houses, has about one fourth the dispersion (better for eyepiece application), and,
unlike common transmission gratings, is blazed in the first order. All the black
anodized machined parts are of professional quality.
The general optical arrangement is normally not the best design for a spectral
instrument since the diffraction of the starlight by the grating is done in the
convergence cone of the telescope. Normally the grating should be placed in a
parallel beam (i.e. the light should first be collimated). The resulting effect
of this design slightly warps the linear spectrum produced by the grating (the
steeper the cone, the greater the warp). This choice of arrangement was obviously
made as a cost trade-off since to design the latter arrangement would have required
more machining and possibly more optics. The end result of the warping, however, is
trivial for a visual instrument and would only be noticeable in photographic
analysis where accurate line positional measurements would come into play.
The ROS comes with a 12 page booklet that describes the instruments care, use,
adjustments, observing methodology as well as other useful ideas and suggestions.
It is written in an easy to understand style which makes it easy to use.
A simple test was performed by passing a laser beam at a normal incidence through
the grating to view the diffracted beams. The light intensity of the 1st order and
0th order beams were compared to the brightness of the incident beam. The results
showed that the blazed first order beam contained about 75% of the energy, while the
zero order passed about 15%, leaving only about 10% in all other orders. This is
extremely good for a transmission grating.
For comparison, the same test was performed on a common holographic lab grating from
Learning Technologies. The diffraction angle of this grating was about four times
that of the ROS grating and was unblazed. It showed that less than 25% of the light
passed through the 0th order, less than 20% for each of the two first orders, with
the remaining energy distributed over higher orders. This type of grating would be a
far distant second for this application.
Observing with a Spectroscope
The telescope selected for this review was a Meade 10-inch Cassegrain with a 26mm
Televue Plossel eyepiece. To get a feel for the spectroscope, five bright well known
stars of varying type were chosen to observe for spectral detail. Several stars of
lessening magnitude (down to 8th) were then selected to determine the range of
brightness the instrument was capable of. The observations were also made with the
Goto prism star spectroscope for comparison purposes.
A spectra work sheet was created to record the data having a range from 4000 to 7000
Angsstroms, with the standard divisions between colors (violet, blue...red) clearly
marked. When observing, the absorption lines would then be drawn in the positions observed
relative to the color delineation markers. The data was later compared with prominent
known lines for element identification.
Spica (B2) - Hydrogen Beta weak other Hydrogen lines not visible
Vega (A1) - H-Alpha and H-Beta both very strong
Altair (A7) - H-Alpha and H-Beta strong (but not as much as Vega)
Arcturus (K1) - Moderate Na and Fe lines along with many very weak metal lines
Antares (M1.5) - TiO and G bands prominent. Many weak metal lines in green
Magnitude 6 and slightly dimmer were comfortably the brightest stars observable
with reasonable detail. The blue to red range lessened as the magnitude decreased,
as was expected. Using the spectroscope in a 17-inch Dobsonian 8th magnitude stars
The Goto prism spectroscope performed about the same.
Final Comments and Summation
The ROS performed admirably for such an inexpensive device. It is obvious that
Jim Badura of Rainbow Optics has spent a lot of time designing, testing and
perfecting not only his spectroscope, but developing the techniques and procedures
to use it which are described in the manual which comes with it. The spectroscope
is an excellent mixture of good optical quality, while trading off features that
tend to add more cost than productivity.
The ROS is the first affordable, easy to use, well manufactured and documented
instrument that introduces the amateur astronomer, first hand, to stellar spectroscopy.
I highly recommend it to anyone who wishes to broaden their regular observing program,
to educators who encourage their students to learn using a hands-on approach, and
especially to those who would like to get their feet wet in this 'new' area of amateur