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The spectrum of an astronomical object can provide clues as to its composition.
Having obtained a spectrum, you then inspect the spectral lines that are present. The positions in the spectrum at which these lines are present allows you to identify their wavelengths and thus you can identify the chemical elements responsible. The strength of the lines will also give an indication as to the relative amounts of each element present.
To obtain a spectrum, you merely need to pass the light from the object through a prism or through a diffraction grating. A prism is cheaper, but a diffraction grating will produce a more linear dispersion of the spectrum and thus will make it easier to identify the individual spectral lines. A "spectrometer" is merely the combination of a prism or grating with a camera.
For meteor spectra, there is an obvious question. You don't know where (or when) the meteor is going to appear, so in which direction should you point your camera?
As you don't know where the meteor is going to appear, all that you can do is to follow the same guidelines as are recommended when attempting the imaging of meteors in general.
This advice is to point the camera, with its associated prism or diffraction grating at an area of sky about 40 degrees from the meteor shower radiant and at an altitude of about 50 degrees above the horizon. If no meteor shower is active, then you would still point the camera of around 50 degrees altitude but the compass direction to use would probably be based on the darkness of the sky around your observing site.
One example of a spectrum is shown in the image to the right. This was captured by Bill Ward on the night of Quadrantid maximum in 2015. The associated meteor (outside of the field of view) will have been moving roughly horizontally relative to the field of view and so the individual spectral lines are aligned parallel to that path.
Meteor spectra will typically consist of a number of bright emission lines. Although spectra sometimes shown an indication of a continuous background spectrum, there is debate as to whether this is a genuine thermal feature or merely the combined result of many low intensity unresolved spectral lines.
In the above case, most of the spectrum was captured within the field of view. However, this will not always be the case, as is the case for the Taurid fireball whose spectrum was captured by Bill Ward in October 2014 and is shown in this next image.
The bright diagonal line across the bottom right should be ignored as it is merely an image artefact caused by the presence of the moon close to the field of view. The bright line across the centre of the image is the zero order spectrum, essentially an image of the fireball itself - you can see one bright flare and one broader flare in brightness in the path of the fireball.
A diffraction grating produces a first order spectrum to each side of the zero order spectrum. You can see bright patches in these at the times at which the flares appeared in the fireball.
But, as can be seen, the big problem is ... that large parts of the first order spectra have fallen outside of the camera's field of view.
Capturing meteor spectra always involves a compromise. The more that you disperse the lines, the easier it is to differentiate between lines of different wavelengths. The dispersion from a 600 lines/mm grating will be greater than that from a 300 lines/mm grating. However, increasing the dispersion also makes the spectrum fainter and increases the risk that part or all of the spectrum may fall outside of the camera's field of view. Bill uses a special blazed grating in which the grooves have been cut so as to direct most of the light into the first order spectrum on one side of the image. Thus that first order spectrum appears brighter and gives a clearer image (the first order spectrum on the other side will consequently be fainter, but Bill considers that the benefit of getting a brighter spectrum 50% of the time makes this risk worthwhile).
Having captured the image of the spectrum, the next step is to rotate/flip and crop the image so that the image is horizontal with the blue end of the spectrum to the left. The intensity and positions of the lines are then be measured and they can then linked to those of likely constituent elements.
One example, for a Perseid whose spectrum was imaged by Bill in August 2014, is shown to the right. The shorter wavelength (blue) part of the spectrum fell outside of the field of view and this shows the spectrum from the green to the near infrared. There is a bright green emission line from magnesium, along with a yellow emission line from sodium. Many meteors also show an emission line in the red from calcium, but this is not clearly seen here. The lines in the infrared are mostly related to the atmospheric gases with which the meteoric particle collided - the presence of these atmospheric lines in all spectra helps calibrate the wavelengths in the remainder of the spectrum.
People sometimes express surprise that some bright meteors appear green to the naked eye. This is probably because they don't see green stars. However, whereas stars emit a continuous spectrum of light, the sum of which adds up to being close to white, meteors only emit light at specific wavelengths and thus if the spectrum is dominated by a strong green emission line from magnesium, this can cause the meteor show a strong green colour.
Bill describes his process for generating an intensity plot, such as the one above:
"For processing the meteor spectrum image is re-oriented to be horizontal and the meteor trail is aligned vertically using a graphics package (IRIS).