Author Topic: Intro. to Spectroscopy for Amateur Astronomers  (Read 2925 times)


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Intro. to Spectroscopy for Amateur Astronomers
« on: September 30, 2014, 11:12:21 AM »

    Much of what we know about stars has been and still is being discovered by spectral analysis: radial velocities, rotation rates, chemical composition, pulsation, luminosity class/photospheric pressure, surface temperature, internal oscillations and sub-surface structure (of the Sun), etc.  Because of the difficulties of making precise gratings and the overwhelming need for light, stellar spectroscopy until recently was the domain of professional astronomers working with custom-built spectroscopes and very large telescopes. But improvements in the manufacture of reasonably priced gratings and the CCD imaging revolution have broken down barriers.  CCD’s have quantum efficiencies some 50 to 100 times that of photographic film, making “typical” 200mm-300mm (8”- 12”) aperture amateur instruments the effective equivalent, for spectroscopy, of 1.5 to 3 meter telescopes in the first half of  the 20th century. A great deal of real science was done then on those instruments, and that level of result is now possible for the dedicated amateur.
    Not into “heavy scientific” work?  Just want to enjoy the colorful rainbows of other stars, or see the fascinating absorption line detail in the solar spectrum? A number of options, some quite inexpensive, are available to display spectra for you.

    Roughly four types of devices are available for the amateur astronomer. You can begin with spectroscopy for about the cost of an eyepiece and then,  if that piques your interest,  go on to more sophisticated (and expensive) instruments. 

    A. Slitless eyepiece transmission gratings are suitable for a small amount of spectral detail for bright stars: molecular bands in M-class stars, hydrogen lines (Balmer series) in A- and B-class stars, relative amounts of blue vs. red in the spectra of hotter and cooler stars. They need only a reasonably-sized telescope (200 mm aperture or larger) to present a colorful spectrum at the eyepiece.  If you have a larger telescope (50 cm and up) and a steady sky, then you can see some metallic lines in the spectra of the brightest stars, e.g. Capella.
    Stellar spectra viewed at the eyepiece, even with only minimal detail, are a big hit at star parties. They are colorful (especially popular with children), and everyone readily relates to them as the “rainbow” of another star.  The presence of absorption lines is a good lead into explanations about how scientists learn about stars, and just the relative amounts of the different colors (e.g. Rigel vs. Betelgeuse) demonstrates the different temperatures of stars.
    Eyepiece gratings are threaded like a filter and screw into either an eyepiece or the front of a star diagonal.  They are easy to use, and the cost is reasonable at 100 to 250 US$.

    B. Handheld spectroscopes, either grating or prism, can be used to project a low resolution solar spectrum onto a white page (NEVER look through them directly at the Sun!), showing some of the more prominent absorption lines. They also show the emission spectra of fluorescent lamps, handy if you are showing all kinds of spectra (not just the Sun) to schoolchildren. They are very easy to use. Cost, 50 to 300 US$, but you will be better off avoiding the low end here.

    C. Relatively high-resolution stand-alone (no telescope required) spectroscopes display quite detailed solar spectra. The abundance of photons available from the Sun allows a much greater spreading out (dispersion) of the spectrum, thus separating and delineating literally hundreds of absorption lines in a colorful continuum background. Plan on having an dark-cloth observing hood when doing this kind of work; if the pupils of your eye are too contracted,  holding your head in proper position relative to the light beam exiting the instrument is somewhat difficult. Use is simple, all you need is a photographic tripod with some decent slo-mo controls. Because sunlight completely illuminates the slit of the spectrometer, such instruments are insensitive to seeing. Cost takes a big leap upward here; to the range of 1,000 to 3,000 US$.

    D. Spectroscopes used with a telescope and CCD detector can record detailed spectra of all kinds.  Planetary observers, do you want to measure the rotation rates of Saturn or Jupiter, while recording absorption lines related to their atmospheres?  Place the slit of the spectroscope along the planets’ equators and record the tilt in the spectral lines due to rotational velocity Doppler shifts.  Double star observers can sneer at Dawes’ Limit as they record the separation and merging of spectral lines of spectroscopic binaries (also a Doppler effect, due to the stars’ orbital rotation velocities). Variable star observers can track not just brightness variations, but record the spectral changes that many variables  undergo. For general observers, spectra of vast numbers of stars can be recorded; each “just another bright dot” will acquire individual traits and, often, peculiarities. 
     Units capable of these kinds of results do come at a pretty steep cost, from 3,000 US$  upwards to 20,000 US$. Some of these have the ability to accommodate comparison to calibration lamp spectra, and can be used for serious and precise investigations.

     Almost all of the instruments form their spectra by using diffraction gratings, essentially very precisely spaced parallel lines on glass (transmission grating) or a mirror (reflection grating). Both work, but you should be aware of a couple of things that will affect what you see.
     Generally, the more lines/mm on the grating, the higher the theoretical resolution and the more detail is possible. Spectral lines separated by only a few Angstroms in wavelength can be seen as distinct from one another.  However, the detector (your retina, or the pixels of a CCD) also have limits.  To see all the theoretical detail, it is necessary spread out the spectrum adequately.  If the wavelengths of several spectral lines all fall on the same pixel, for example, then they will of course be indistinguishable; the physical length of the spectrum across the retina or CCD chip (i.e. its dispersion, the spread of wavelengths, in Angstroms, per detector width, in millimeters) is important.  However, the more you spread out a spectrum to get detail,  the dimmer the spectrum becomes as the same amount of light is covering a wider area. This is why so much light is needed for detailed spectral work.

    The eye does not collect light over a long time, sending an image to the brain roughly every 1/20 of a second (varies with brightness level and the age of the observer).  You should not, if using the eye as your detector, expect to see extremely detailed stellar spectra unless you have a huge telescope.  As you increase the dispersion, the spectra start to dim, lose color (blues and reds go first, as that is where the eye is least sensitive), become ghostly gray (though you can still see some details) and eventually fade away.  Therefore, relatively low dispersion is fine for direct viewing. The exception here is the solar spectrum, where the extravagant supply of photons allows very high dispersions and a huge amount of detail, hundreds to thousands of spectral lines, can be seen.
    CCD detectors can collect light over much longer time periods, allowing much fainter objects to be analyzed. But this requires accurate guiding to keep the object’s image on the spectroscope’s entrance slit; long exposure times entail the same level of observational care as is necessary for astrophotography: accurate tracking mount, guiding either by hand or guide scope, periodic error correction, etc. The mount itself may need to be heavier duty as the spectroscope plus camera is more of an instrument load than a camera alone. Also like astrophotography, computer post-processing of the recorded data is necessary.

    Some vendors are listed below (with types available: A = slitless eyepiece transmission gratings,    B = handheld spectroscopes, either grating or prism,  C = stand-alone spectroscopes, D = instruments aimed mainly for use with CCD cameras). All of  these vendors have websites, alluring places that will tempt you to spend much money:

      Rainbow Optics  (A)
      Shelyak Instruments (ABCD)
      Baader Planetarium (ACD)
      SBig Instruments (D*)
      Edmund Scientifics (B)

               (*main use: correcting for flat field and color balance in astrophotography,
                           not nearly as versatile as the D’s from the other vendors.)

 Other vendors, e.g. DFM Engineering, will design and build you a spectroscope, but the cost will be, well, astronomical.


     #1. “Practical Amateur Spectroscopy,"  Tonkin (ed), covers use of various instruments, from simple to sophisticated, plus some introductory theory. This is a good book for getting started, and in many ways this brief article is a tickler to get you to read one of the introductory books (#1, #2, #14, #15).
     #2. “Stars and Their Spectra,"  Kaler, is a non-mathematical introduction to the application of spectroscopy to stellar astrophysics. With lots of detail and minimal math, Kaler does a good job of keeping the fascination factor high. The same author has a second book, “Extreme Stars” which covers a lot of the same ground but has an approach based on  “biggest, hottest, coolest, smallest” etc.
     #3. “Astronomical Spectroscopy," Tennyson, is heavy on theory starting from the quantum mechanics, and goes pretty deeply into the details of both atomic and molecular spectral lines, and requires that the reader knows something about quantum theory.
     #4. “A Spectroscopic Atlas of Bright Stars," Martin. gives an idea of what kind of stellar spectra one obtains with good amateur equipment.  As a bright star atlas, though, it has some unfortunate omissions, such as Canopus, Alpha Centauri, and Arcturus.
     #5. “Optical Astronomical Spectroscopy," Kitchin, has theory and practice (mostly at the professional observatory level) of spectroscopy. Intermediate between Refs. #2 and #3 on the theory.
     #6. “An Introduction to Modern Astrophysics," Carroll & Ostlie, is an excellent textbook on many aspects of astrophysics, including spectroscopy. Of course, an upper level college textbook uses a lot of math, but it is more approachable than comparable volumes.
     #7. Kitt Peak National Observatory "Atlas of the Solar Spectrum," has great detail on the absorption lines of our local G2V star. Also available to download from Kitt Peak,  as 145 postscript files, is an extremely detailed solar spectrum (shows 20,000+ lines), plus much other useful stuff.
     #8. “The Sun from Space," Lang, presents results from many of the orbiting solar observatories, including work on helioseismography enabled by almost unbelievably precise (velocity differences on the order of 1 to 2 meters/second can be distinguished) solar spectroscopy. Fascinating reading.
     #9. “Optics,"  Ghatak, is a textbook: lots of theory, lots of math.  It is one of many such available optics texts, the one this author happens to own. 
   #10. “Stellar Spectral Classification," R. Gray and Corbelli,  takes a very detailed look at the art and science of classifying stars from the information contained in their spectra, including myriad stellar peculiarities, methodological difficulties, and what unfinished business remains in the field. It is not too mathematical most of the time (has its moments), but still a technical work.
   #11. “Observation and Analysis of Stellar Photospheres," D. Gray, is a text aimed at roughly the same level as #6, but with much greater detail in its subject matter. Since stellar photospheres are where spectra form, everything about them is covered very deeply. The author describes this book as “introductory”, making one wonder about the density of the advanced treatments.
   #12.  “Astronomy, Methods” (1st vol) and “Astrophysics, Processes” (2nd vol.) , Bradt, are a text that is somewhat simpler that #11 and comparable to #6; the author covers a lot of ground within the discipline of astronomy and skips much of the heavy math and derivations, but does a good job with explaining concepts.
    13. “Introduction to Optical Spectroscopy," Appenzeller, is a reasonably easy to read paperback about how spectroscopy is done at major observatories.  It has stuff in it (e.g. volume phase gratings, atmospheric dispersion compensators) that is not covered in any of the other references listed here, and is interesting reading. The first 3 chapters are the most useful for amateur purposes.
   14. “Astronomical Spectroscopy for Amateurs,” Rozelot and Neiner, eds.  This book is a series of lectures given at a conference on amateur spectroscopy, on a variety of topics: physics of spectral line formation, spectrograph design, spectrum data reduction, spectra of: Be stars, nebulae, comets and the Sun. Some parts get technical but most is readily understandable.  The translation into English (from the French) is uneven, mostly all right but obscure in meaning here and there.
   15. "Astronomical Spectroscopy for Amateurs," Harrison.  This is a good introductory book, similar to #1 but more comprehensive and better organized, and a bit less sophisticated than #14.  It does have a few boo-boo's in it, such as the Baader DADOS spectrograph being incorrectly described (p. 73) as a Littrow design.
   16. "Spectrograph Design Fundamentals," James.  If you wish to build your own instrument, this book is a good place to start.  It goes over both basic theory and a lot of practical aspects such as materials, calibration, and alignment.  It is not for the beginner.
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