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Rainbow Optics Star Spectroscope.

Visual star colors occupy a venerable niche in sky lore. Although marveled at for millenniums, the cause of stellar color was still a mystery in 1844 when W. H. Smyth's Bedford Catalogue appeared, chock full of references to telescopic star tints. Limited to what their eyes could tell them, Smyth and his contemporaries created a chaotic color nomenclature. Pretty descriptions like "ashen," "silvery white," "pale lilac," and their tongue-twisting Latin equivalents crowded the star lists. Color judgments were doubtlessly further confounded by eyesight differences and the highly variable color corrections of the refractors then in use by many observers.

Methods developed in midcentury eventually showed that the spectral signatures of stellar chemical elements were related to their temperatures and colors. The romantic but subjective color descriptions in catalogs were superseded by the dry but accurate alphanumeric spectral types that prevail today.

In the process something basic was lost to amateur astronomy. The analysis of starlight demands specialized equipment and knowledge, so amateur observers turned to other things. Even the term "stargazer" may be in jeopardy, since single stars are often viewed chiefly as guideposts to deep-sky objects.

THE STAR SPECTROSCOPE

Enter the Star Spectroscope. This product fills a void left by several manufacturers' discontinued models. Jim Badura, owner of Rainbow Optics of Hayward, California, calls his spectroscope an improvement on a tried-and-true concept. Badura says he worked on the design over several years to make the device as effective and affordable as possible.

The instrument consists of two basic units. Spectral dispersion is provided by a transmission grating mounted in a standard 1 1/4-inch threaded eyepiece filter cell with a clear aperture of 25 millimeters. The grating is made by a proprietary method of replication. Its finely grooved surface is formed on a clear optical glass disk, protected by a cover glass of the same material.

Looking through the grating at a light source with the unaided eye gives a 1x preview of its effect on starlight in a telescope. Three images are seen. The direct, undispersed view is flanked by a pair of first-order spectral images, 180 [degrees] apart. The grating deviates the light of these first-order spectra away from the optical axis at an angle of about 7 [degrees]. The spectra are of unequal brightness because Rainbow's grating is "blazed" to throw most of the dispersed light into one of the images. An unblazed grating would share the light equally between the two spectra. According to the manufacturer, 75 percent of the light is directed into the brighter spectrum.

A second cell of 1 5/8-inch diameter fits over the top of most 1 1/4-inch eyepieces. It contains a cylindrically ground lens of 50-mm focal length centered behind a 12-mm aperture. The lens, which magnifies the spectrum in one direction for viewing, is held in the cell with a plate retained by small Allen screws. The unit is machined aluminum, brushed and black anodized. Knurled 10-24 nylon bolts run in cleanly tapped holes, holding the unit snugly on the eyepiece without marring it. In all, it's a neat piece of work.

ACCESSORIES

Accessories offered include an 18-mm Kellner-type eyepiece ($49.00), threaded spacer rings ($15.00 each), and a universal 1 1/4-inch filter adapter ($25.00) that allows eyepieces with nonthreaded barrels to be used with the spectroscope.

Rainbow Optics makes no claims for the Kellner eyepiece - which is a standard imported unit offered to observers who may not have a 1 1/4-inch ocular in their kits. The accessory spacer rings can be piggybacked on the grating cell to increase the distance between the grating and the eyepiece field stop. Badura frankly admits that his spacers are merely empty filter cells, which observers could obtain by taking the glass filter disks out of their own filter cells to try the effect. The 1 1/4-inch filter adapter is a standard type available elsewhere; it consists of an anodized aluminum cylinder with internal eyepiece-barrel-type threads in the base and a knurled brass screw at the top to hold the eyepiece.

FIELD TESTING

Directions for using the spectroscope are straightforward and well covered in the short manual provided. The observer points the telescope at a star, focuses, and then removes the eyepiece to screw the grating cell into the eyepiece barrel. When the eyepiece is replaced, a pair of colorful spectra should be seen extending to either side of the star. After the brighter spectrum is brought into the center of the eyepiece field, the lens cell is fitted over the eyepiece head and centered to a loose fit with the nylon screws. Turning the lens cell rotates the axis of the cylindrical lens and changes the width of the spectrum. The manual explains that it is now easy to focus the stellar spectrum sharply.

With the idea that this spectroscope was designed for educational use, I chose a 4-inch (100-mm) f/10 refractor for testing. This common aperture is about the smallest that would be routinely used by a modern amateur and is well within the size readily available to science classes.

Rainbow's accessory eyepiece and adapter were ordered along with the star spectroscope, and these were also checked out. The 18-mm eyepiece yielded 55x and showed an actual field about 50 arcminutes wide. The view was somewhat better than I expected. In fact, the Kellner ocular performed quite well with this f/10 system. (Are generic Kellners better than they used to be, or have eyepieces advanced less than we might think?)

The maker suggests using the spectroscope on a few bright stars first, to learn the technique. Since Gemini was high in the sky, I pointed the telescope at 1.6-magnitude Castor, the fainter of the "twin" stars at the head of the constellation. Castor is a multiple star, with two visual components of spectral class A0. Their combined color was certainly "silvery white," just as in the classic catalogues. Castor's doubling showed in the 4-inch, but disappeared when the grating was inserted. Two strips of spectral light appeared, one on either side of the stellar image, extending out of the field of view. It was a simple matter to center the brighter strip and install the cell containing the cylindrical lens on the eyepiece. I rotated the cell until the spectrum attained maximum width.

The cylindrical lens had stretched the stellar image between the spectra into a short, bright line. Bringing the linear star image to the edge of the field served as a useful guide in fine focusing and for rotating the lens to attain maximum spectral width. Since everything had gone so well, I was not surprised to see absorption lines pop into view, spaced at intervals across the blue end of a small, clear, and very colorful spectrum of Castor.

Most dark lines seen visually in the spectra of such hot, white stars are due to absorption by hydrogen at different temperatures and ionization states in the stellar atmosphere. The effect was first discovered in the spectrum of the Sun by Joseph Fraunhofer in the early 19th century.

Pollux was close at hand, a 1.2-magnitude yellowish type K0 giant star. The spectrum of this cooler star contrasted noticeably with Castor's, showing several fine absorption bands in the dim, violet end of the spectrum. These were near the position of calcium II on spectral wavelength charts, just below 4000 angstroms. A darker line glimpsed in the nearby blue region corresponded in position to the Fraunhofer G band. Indications of threadlike darkenings also appeared near the blue-green border. These may have been due to lines of iron and magnesium around wavelengths of 5000 to 5100 angstroms.

The maker suggests varying the eye-piece-to-grating distance to suit the magnitude of the star and the telescope used. This tactic changes the brightness and length (effectively the dispersion) of the spectrum seen. The ability to do this is a useful innovation over other eyepiece spectroscopes that have fixed grating distances and restrict users to the eyepiece the grating is mounted in. Placing the transmission grating in the accessory adapter added an inch of distance, stretching the spectrum of Pollux and making the lines more easily visible.

Is more dispersion always better? Various eyepiece and extension combinations were tried, including a 26-mm Plossl and roughly 3-inch extension suggested in the manual. Such a distance is easily achieved by threading the grating into the far end of a 1 1/4-inch star diagonal. The change was effective for Castor and Pollux. But, when viewing fainter Eta Geminorum in mediocre seeing, the added dispersion dimmed its stretched-out spectrum to illegibility.

To view this 3.3-magnitude red giant I removed the diagonal to shorten the spectrum. An absorption band in the violet, apparently from calcium I at 4227 angstroms, appeared. A thicker one nearby, possibly from titanium oxide and magnesium in the blue-green near 5200 angstroms also became faintly visible. Both features are strong in this class of star. It was apparent that better sky conditions would allow clear visibility of strong spectral features in 3rd-magnitude stars using a 4-inch telescope. Sky condition is critical because the spectrograph has no slit and relies on good seeing to bring out fine details in a spectrum.

Daytime tests using a sunlit ball bearing as an artificial star revealed an added possibility for educational demonstrations. The solar spectrum, complete with strong Fraunhofer absorption lines, was very apparent. Unfortunately, it also appeared centered in a haze of color. I found I could clear this up quite a bit by placing a dark background behind the point source. Brilliant continuous spectra were also produced by a high intensity lamp shining through a pinhole in foil about 30 feet from the telescope.

Rainbow's Badura is an avid observer who puts out a newsletter for amateur spectroscopists. Owners of his device are encouraged to send in the results of their observations and techniques. One ingenious correspondent, for example, had put together a list of stars with details on their spectral types for demonstration at an upcoming star party. Another had employed a chromed radio antenna to obtain an elongated solar reflection, mimicking the effect of a slit for solar spectrum viewing.

SUMMARY

The manufacturer states that the Star Spectroscope is not a research instrument. It doesn't have to be, since it fills its intended niche well. The manual and other literature supplied give a host of tips for the beginner, lists of candidate stars, and suggestions for projects.

The field test showed that the Star Spectroscope does what it claims to do and more. It is well made, durable, and certainly capable of extended use. In addition to the fun of trying something new, astronomically savvy teachers with telescopes might find that Rainbow's device offers a welcome change from textbooks and the indoor laboratory routine. The ability to see stellar spectra in the night sky might even turn some amateur astronomers back into stargazers!
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Author:Pepin, M. Barlow
Publication:Sky & Telescope
Date:Oct 1, 1995
Words:1819
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