Cosmic Rainbows: The Revival of Amateur Spectroscopy.
From its beginnings in the early 19th century, spectroscopy - the analysis of starlight by wavelength - has unlocked a treasure-trove of astronomical information. Most of what we know about a star's chemical composition, temperature, axial spin, magnetic field, and motion through space is deduced by studying its spectrum. Although light is greatly diluted when spread into a long spectrum, pioneers such as Angelo Secchi of the Vatican Observatory did remarkable work with visual spectroscopes. Furthermore, some bright stars present truly spectacular spectra to the eye. One example is the Wolf-Rayet star Gamma Velorum, about which the Scottish astronomer Ralph Copeland commented in 1883, "An intensely bright line in the blue, and the gorgeous group of three bright lines in the yellow and orange, render the spectrum . . . incomparably the most brilliant and striking in the whole heavens."
Even at that time William Huggins in England was using photography to record spectra of targets too faint to be studied visually. Many of spectroscopy's pioneers were amateur astronomers, but as the 19th century drew to a close, amateur interest was on the wane. By then spectroscopy had been adopted by professional observatories in North America and Europe, which had the large-aperture telescopes needed to exploit this powerful tool.
This year we mark the 70th anniversary of Edwin Hubble's announcement that the spectra of distant galaxies exhibit a redshift, which we now interpret as due to the expansion of the universe. Even though he used the 100-inch telescope at Mount Wilson Observatory, Hubble required exposures up to 20 hours spread over several nights to record the spectra of some galaxies. Today the inefficient photographic plate has been superseded by electronic CCD detectors, and professional astronomers probe the depths of the universe with huge telescopes like the 10-meter Keck reflectors in Hawaii.
The remarkably sensitive CCD camera has also become a tool for amateur astronomers, and it has reawakened their interest in spectroscopy. Now a modest backyard telescope can study the planetary atmospheres of Venus, Titan, Uranus, and Neptune, or record the redshifts of quasars at the edge of the universe. The power of the Internet makes it possible to post an observation worldwide almost instantly, invoking a vital and stimulating exchange of news and ideas.
Early amateur spectroscopists did not have to contend with the modern scourge of light pollution. Fortunately, most of the faint targets of interest to amateurs are starlike point sources. Placing a slit at the entrance of the spectroscope will help mask degrading skylight. It is also possible to take a series of short CCD exposures and add them together for a long effective exposure. Both techniques are used successfully by amateur spectroscopists observing from suburban locations.
There is always a tradeoff between spectral resolution (the detail visible within a spectrum) and the brightness of the target. Spreading light into a long spectrum improves the resolution but greatly reduces the intensity. The trick is to match the spectrograph to the task at hand, since no single instrument will cope with every application.
In the 1980s I built a solar spectrograph that produced a spectrum 40 inches long and could resolve detail as fine as 0.04 angstrom on film. Segments of this colorful spectrum were captured in snapshots of less than 11/410 second. Recently I recorded the spectra of distant quasars several billion times fainter than the Sun using a simple CCD spectrograph and exposures less than an hour. The complete spectrum was just 2 millimeters long with a dispersion of 40 angstroms per pixel on the camera's CCD chip. Nevertheless, this low resolution was still adequate for detecting the quasar's strong hydrogen emission lines shifted toward the red end of the spectrum.
Getting precious photons onto the CCD is the prime objective of stellar spectroscopy. Light can be dispersed into a spectrum with a prism or diffraction grating, and each method has pros and cons. The grating can be either a reflection or transmission type. While a prism disperses light into a single spectrum, a grating creates multiple spectra of varying lengths and intensities (normally only the brightest one is used, so the remaining light is lost). A grating, however, conveniently produces a spectrum with an essentially linear dispersion, while a prism compresses the spectrum at red wavelengths.
When one is working with small dispersions and very low resolutions, a target's real image (the so-called zero-order spectrum) produced by a grating can be recorded together with its spectrum on the CCD chip. The separation between this image and the spectrum is fixed and serves as a convenient reference when one is measuring wavelengths. This is a novel application, since it eliminates the need to have a reference spectrum (usually formed by feeding light from an emission lamp into the spectrograph, and this is not possible when the spectrograph has no slit).
A CCD with high quantum efficiency and broad spectral response is desirable but often quite expensive. Nevertheless, the chips in today's popular CCD cameras are more than adequate for spectroscopy, and they offer a huge improvement over conventional photographic film. CCDs also have extended red sensitivity compared with film. For example, my Starlight Xpress camera with its Sony camcorder chip has fairly even response between 3970 angstroms in the violet and 9300 angstroms in the near-infrared.
There is a small downside to the CCD's extended sensitivity. Some spectrograph designs use a conventional lens from a 35-millimeter camera to focus the spectrum onto the CCD. The focus of these lenses will usually need to be adjusted when you are recording the infrared region of the spectrum, as few lenses yield sharp images at visible and infrared wavelengths using the same setting.
A low-resolution spectrograph for point-source targets can be made by placing a prism or grating just ahead of a CCD at a telescope's focal plane. Because of its thickness, however, a prism must be placed in a collimated beam of light and the resulting spectrum focused onto the CCD with an additional lens. The results are very similar to those obtained by using a costly objective prism in front of the telescope's aperture. A single exposure records the spectra of all stars in the field of view. However, just as when using the telescope for astrophotography, exposure durations are limited by skyglow.
Such a system works well with telescopes of 2-meter focal length and less. Because the resulting spectrum is only a few pixels high (regardless of its length), the spectrum is kept relatively bright and requires the minimum exposure. The narrow spectrum can be electronically stretched with image-processing software to show as a traditional rectangle, which easily reveals any bright or dark lines.
High spectral resolutions with dispersions of, say, 4 angstroms per pixel, typically require advanced equipment designs that incorporate a slit at the spectrograph's entrance. The tracking accuracy needed to keep a target on the narrow slit is a daunting task for a typical amateur telescope.
There are, however, three ways to solve this problem. The slit can be aligned east-west in the telescope's focal plane since most periodic drive errors are in this direction. As such, errors simply broaden the spectrum while the detail remains sharp. Another technique is to have some form of autoguider to keep the target on the slit. The last method is to use a fiber-optic cable to pipe light from a circular area in the focal plane to a "slit" formed by positioning the cable's individual fibers in a line at the spectrograph's entrance.
The autoguider method is the one chosen for the new commercial spectrograph developed by Santa Barbara Instrument Group (SBIG). The fiber-optic approach is used in the Nu-VIEW spectrograph sold by Sivo Scientific and based on a prototype described in this magazine last February, page 134. You can find out more about these new spectrographs at www.sbig.com and www.sivo.com.
The recent introduction of two spectrographs aimed at amateur astronomers is a promising sign. Nevertheless, because the current demand is relatively small the market is not overwhelmed with choice. As a result, many amateurs build their own equipment. In addition to keeping costs down, this approach is both rewarding and instructive.
Just about anyone with a CCD camera can make an almost instant introduction to astronomical spectroscopy with the purchase a transmission grating, such as the one sold by Rainbow Optics (1593 E St., Hayward, CA 94541, phone: 510-581-8266). Placed an inch or so in front of the CCD chip and attached to a telescope (preferably f/6 to f/10), it yields satisfactory low-resolution spectra that will have any would-be spectroscopist up and running after an evening or two.
How faint a star will a spectrograph record? This depends on many factors, including the telescope aperture, spectrograph design and dispersion, and the CCD. There is, however, a simple way to estimate what exposure is required to capture a star of given magnitude. Start by finding the proper exposure necessary to record the spectrum of a bright star. Then for each whole-magnitude drop in brightness, you multiply the exposure duration by 211/42 times.
With my 12-inch Meade Schmidt-Cassegrain telescope and a Rainbow Optics transmission grating arranged to yield a low-resolution spectrum having a dispersion of 40 angstroms per pixel, I can record the spectrum of zero-magnitude Vega with a 11/4100-second exposure. A 6th-magnitude star requires 1 or 2 seconds, and a 15th-magnitude source needs about an hour-long exposure. This holds quite well to my "rule." For higher resolutions, say 4 angstroms per pixel (which is enough to detect the radial velocities of stars in the solar neighborhood), amateur equipment is usually confined to naked-eye stars.
Observing time for most professional telescopes is booked months in advance. The flexible schedules of amateurs, however, give them a clear advantage when it comes to observing transient events such as new comets, novae, or supernovae. There is also a growing trend for professional cooperation with amateurs, especially in areas of long- term monitoring. There is no reason why, within the limits of amateur instrumentation, this should not now include amateur spectroscopic observations.
What can the amateur spectroscopist successfully observe, whether for science or satisfaction? The solar system, dominated by bright targets, is a good starting place. The atmosphere of Venus contains carbon dioxide that has been detected with amateur spectrographs. The gas giants - Jupiter, Saturn, Uranus, and Neptune - have strong methane absorption lines in their spectra, which are relatively easy to record. Saturn's satellite Titan also shows methane lines in its atmosphere. Complex molecular lines in cometary spectra are within reach of the backyard spectroscopist.
Beyond the solar system there are hundreds of stars worthy of spectroscopic attention, with variables leading the list. Mira-type long-period variables and many irregular variables are cool giant stars with spectra that vary along with their brightness. Since some of the fading is caused by a shift of light from visual to infrared wavelengths, these variables are ideal targets for the extended red sensitivity of a CCD spectrograph. This sensitivity also gives amateurs an opportunity to explore the coolest stars (spectral classifications M, N, R, and carbon stars). These spectra are easily recognized by their "string-of-beads" appearance caused by absorption bands due to gases like titanium oxide and other complex molecules.
At the opposite end of the temperature scale are the hot O and B stars and those with an e suffix added to their spectral classification, indicating they have emission-line spectra. Again some notable variable stars are included, such as Gamma Cassiopeiae and the eclipsing binary Beta Lyrae, which shows variation in its spectrum that tracks the light cycle. Many interesting targets can be culled from the spectral classification published in Sky Catalogue 2000.0 and other similar listings.
Stellar emission lines are generally easier to record than dark absorption lines when one is using a low-resolution spectrograph. Wolf- Rayet stars can show variations in their emission-line spectrum during a short time scale. These stars are extremely hot (up to 100,000[degree sign] Kelvin) and have their own WR spectral classification. Some show spectacular emission lines due to highly ionized elements like carbon and nitrogen.
The outburst of a nova or bright supernova is perfect for the amateur spectroscopist (see the following article). The Blaze Star, T Coronae Borealis, is a recurrent nova that last flared from 10th to 3rd magnitude in 1946 and bears monitoring. Other stars worthy of amateur attention are nearby red dwarfs. Among them are flare stars like UV Ceti or Wolf 359 in Leo, which exhibit unpredictable bursts of brightness amounting to several magnitudes, accompanied by enhanced emission lines in their spectra.
One of the most powerful aspects of spectroscopy is determining the velocity an object moves toward or away from the observer. This motion is recorded as a Doppler shift of the spectral lines. Objects approaching the observer show a shift toward the blue, while those receding are redshifted. Extremely subtle spectral shifts are what professional astronomers currently use to detect planets around stars. This is currently beyond the amateur realm, but higher velocity measurements are not.
The Sun's rotation is just detectable in a powerful amateur spectrograph when one compares light from the east (approaching) and west (receding) limbs. Numerous spectral lines due to oxygen and water vapor in Earth's atmosphere are superposed on the solar spectrum and serve as static markers for determining the Doppler shift. Amateurs have even measured the subtle radial velocities of bright stars with a high-resolution spectrograph - something that would have been virtually impossible before the advent of sensitive CCD cameras.
At the opposite velocity extreme are highly redshifted quasars. Since they are remarkably bright for their distance and are receding at a sizable fraction of the speed of light itself, they are detectable with very-low-resolution equipment. They offer the amateur a tantalizing glimpse into the early history of the universe by looking back through cosmological time toward the Big Bang.
We have come a long way since Angelo Secchi looked at stars for the first time through his spectroscope more than a century ago. CCDs give amateurs an opportunity to relive the excitement he experienced in those pioneering days. The adventure can begin afresh for the inquiring amateur.
Retired architect and former president of the British Astronomical Association, Maurice Gavin records astronomical spectra from his home observatory 12 miles from the heart of London.
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|Publication:||Sky & Telescope|
|Date:||Aug 1, 1999|
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