Rainbow Optics Star Spectroscope
A Review
by Barry D. Malpas
United Astronomy Clubs of New Jersey
Observatory Chairman

From 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.

Spectroscope Types

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.

Eye Sensitivity

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.

Design Comments

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.

Grating Testing

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 throughout
Antares (M1.5) - TiO and G bands prominent. Many weak metal lines in green through orange

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 were observable).

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 astronomical research.

Maintained by BDM