Much of our modern knowledge of the Universe is derived from spectroscopy – the analysis of light. Astronomers continue to hanker after ever-larger telescopes, such as the Keck Telescope on Mauna Kea in Hawaii, to gather ever more feeble light from the edge of the Universe to satisfy their theories.
In so doing they largely ignore stars brighter than mag. +10 as these overload their cameras, which leaves amateurs free to grab a slice of the action and make what could be a significant contribution to science.
This needn’t be the main reason to get involved, though.
There’s a tremendous buzz in seeing a spectrum created by your own hands that perhaps nobody has witnessed before, which most amateurs see as just a point of starlight in the eyepiece.
Today, with the common use of digital cameras, webcams and CCDs, this has become possible.
If you want to record what you see, no matter which imaging device you use, you’ll need a prism of glass or a diffraction grating (which looks like a filter) to stretch the light source into a line of colour.
An online search will show you any nearby shops that sell prisms and gratings.
However, you can see the colourful spectrum of the Sun for free.
Believe it or not, all you need is a CD.
Tilt it in your hand so that sunlight just grazes across its surface, and then bring your eye close to the CD surface – you can see within the colourful solar spectrum faint dark lines caused by elements in the Sun’s atmosphere.
These include hydrogen (H), sodium (Na), iron (Fe), and magnesium (Mg).
It takes a little practice to see the darker lines but, once perfected, it’s an amusing party-piece to impress friends.
The cereal-box spectroscope is a variant of this: it uses a small piece of CD mounted inside a cereal box to capture the spectrum of sunlight, domestic and street lights.
The next step up involves using a camera on a fixed tripod to capture the spectra of distant stars.
It’s easy to picture trailed starfields by pointing your setup skywards and opening the shutter for about 30 seconds, while focusing on infinity.
If you place a glass prism before the camera lens’s hood, then the trailed stars will be drawn out into colourful spectra.
In doing so, you will have taken the first step to building an objective prism spectrograph.
Film and digital SLRs, or webcams with a 50mm focal-length lens are best for collecting starlight in this way.
You’ll have to experiment to find the optimum orientation for the prism, which is no problem with a digital SLR’s LCD screen.
Initially, you should set the prism so that the spectrum is captured vertically: as the star drifts across the field of view during the exposure, it’s drawn out into a wider rectangle.
The technique works best on white, A-type stars like Vega and Sirius that have dark hydrogen lines, and cool M-type stars like Betelgeuse with banded spectra.
To reveal a spectrum in greater detail, you need a higher dispersion of light to form a longer spectrum.
This requires a telescope of at least 4-inch aperture.
As a full-aperture prism set before the telescope would be prohibitively expensive, here’s another way to diffract the light.
By screwing a small diffraction or transmission grating – like the ones made by Rainbow Star Optics and Staranalyser – into your telescope’s eyepiece, you’ll transform your setup into a field spectroscope.
Although any transmission gratings will do, these highly efficient models make the most of feeble starlight.
Using them, the brighter stars will show you much better spectra.
When you replace the telescope’s eyepiece with a camera, the number of stars you can analyse runs into thousands – you have a powerful and practical stellar spectrograph in your possession.
Even an undriven telescope can produce colourful drift scan spectra in this mode.
During an exposure of 30 seconds, the drifting star is trailed out into a broad, colourful band.
If you have a telescope that can track the stars, then you’ll be able to target much fainter objects.
To progress further, you’ll need a precision spectroscope, plus additional optics and a slit to replace the telescope’s eyepiece.
Here, a regular camera lens serves the dual role of collimator and lens, imaging the spectrum onto a CCD. Such a spectroscope is capable of professional results with a huge field of potential targets.
By swapping either the grating, the camera lens, or both, a whole range of spectral resolutions becomes possible to match various targets. But do remember that as targets get fainter, the spectral resolution will be lower.
Few amateurs are blessed with light buckets like Keck – the ultimate feeder for a spectroscope.
Even so, useful observations are possible with a field spectrograph: newly discovered stars, presumed to be novae or supernovae, can’t be confirmed until a defining spectrum is taken… perhaps by your spectroscope!
What you’ll need to get started
Prisms – Simple spectroscopy can be free: a discarded CD works well, or check your local camera shop for prisms from old binoculars. You can also buy high-quality prisms from astronomical suppliers.
Mirror and needle – As a next step, you’ll need a plain mirror and a sewing needle or chrome eyepiece, as well as the lenses and prisms from an old pair of binoculars. The mirror redirects sunlight onto the needle or eyepiece, which provides a sliver of light to be split by the prisms.
Camera – To record your spectra you’ll need a digital or film camera with a lens of at least 20mm aperture, and a tripod. You can attach a prism to the lens hood.
Telescope – For greater detail, an equatorially mounted 4-inch telescope will feed starlight to your camera. This setup may comprise a simple transmission grating in front of a camera, or a full-blown spectrograph, like an LHIRES III coupled to a CCD camera.
Software – A PC or laptop running Paint Shop Pro or Photoshop, or more specialised software like Vspec, AstroArt or IRIS. This will capture and process spectra, calibrating them and extracting spectrum profiles and graphs.
Start by finding an old pair of binoculars that you can take apart to extract the prisms and the objective lens.
Arrange the prisms and the objective lens on a tabletop that is level with the room’s windowsill.
Move the table near to the window, and adjust the curtains so that only a shaft of sunlight is admitted into an otherwise darkened room.
All lenses and prisms must be level so that the beam of light will pass horizontally through the centres of each without obstruction.
Small components, like the prisms, may need to be raised to bring them into line.
A stack of old computer disks or unused CDs will serve this purpose.
Once level, hold them in place with something sticky like Blu-Tack.
Prefocus the binocular’s remaining eyepiece on infinity by viewing a distant building.
Then set the polished chrome eyepiece or sewing needle before the objective lens.
This will act as a reflective slit and provide the shaft of sunlight that will be split by the prisms.
Adjust its distance from the lens until the spectrum is sharp, as viewed through the monocular.
Arrange a small mirror on the windowsill to catch the sunlight, and tilt it to project the beam horizontally onto the polished eyepiece or needle.
This is the sample of sunlight that will become your spectrum.
As the Sun moves during the course of your experimentations, you’ll need to adjust the mirror in order to maintain the beam’s position on the eyepiece.
When you have mastered this visual technique to your satisfaction, the prisms can be replaced with a transmission grating, held upright with a piece of Blu-Tack, to split the light into its spectrum.
Next, you can experiment by replacing your old pair of binoculars with a camera, which will enable you to record stunning images of the solar spectrum.
Now that you’ve got this far, you may want to progress further by taking the spectra of stars on which you can conduct scientific observations, like analysing elements in the atmosphere or tracing relative brightness. F
For this, you’ll need a telescope of at least 10cm aperture, and a Rainbow or Staranalyser grating coupled to a digital camera.