Spectroscopy

While the goal of the AAVSO has been mainly concerned with magnitude determination of stars and star systems, spectroscopy is now on the threshold to add significant value to the AAVSO.

Photometry takes the pulse of a star or star system, where spectroscopy examines its soul.

Before the advent of CCD cameras, most observers would agree that spectroscopy was well beyond anything the amateur astronomer or small observatory could do. Besides being extremely expensive, even fairly bright stars required very large telescopes to get spectra. When CCD cameras became plentiful that all began to change. A few years ago professional quality spectrographs became available for a cost within the means of many observers. Coupled with CCD cameras, they showed that a great deal of important work could be done with very modest equipment.

There are basically two types of visible spectrum astronomical spectroscopy, low and high resolution. Of course there are multiple resolutions in between also.

Low Resolution Spectroscopy (Diffraction Gratings with ~100 lines/mm)

The big advantage of low resolution is that it provides a complete view of the visible spectrum of the star of interest. The visible spectrum is around 3,000 Å (Angstroms, 10 Å = 1 nanometer) wide.

High-end spectrographs, such as the Lhires III, have low resolution (150 lines/mm) gratings available (around $500 just for the grating). A much less expensive means is available using a 1 ¼” standard filter size diffraction grating spectrograph such as the Star Analyser for under $200.

The Star Analyser can be used with most any telescope and CCD camera. In fact it can be used with a DSLR camera just mounted on a tripod. It requires some experimentation to develop a good technique to get useable spectra, but it can be done.

High Resolution Spectroscopy (Diffraction Grating with ~2,400 lines/mm)

Where the low resolution spectrograph is like a low power handheld magnifying glass the high resolution spectrograph is like a high power microscope. It cannot see much of the spectrum, but can zoom in in great deal on small segments. A Lhires III with a 2,400 lines/mm grating and DSI Pro II CCD camera will show a spectrum window about 90 Å wide. That window can be adjusted to show any 90 Å section of the visible spectrum. A CCD camera with a larger chip will show a wider window.

The built-in neon calibrator of the Lhires III is very handy. It can be used for easy experimentation and wavelength calibration. To provide an even more accurate wavelength calibration a heliocentric correction to compensate for the Earth’s motion around the Sun and its rotation can be made.

The use of atmospheric lines in an image provides an even more accurate calibration. Once the line profile is calibrated precise spectral line wavelengths can be determined which allows both element determination and Doppler Shift (radial velocity) measurements. Line strengths using what is known as Equivalent Width (EW), VR (violet to red EW ratio of Doppler shifted lines) and other calculations can be determined with a high degree of accuracy. The continuum can be calibrated and the star’s temperature determined.

Spectrum Processing

Like the CCD image of photometry the spectrum image by itself is of little value. Data must be extracted from the image, calibrated and reduced to be useful. The spectrum image must be processed to create a line profile and then the line profile calibrated. The information hidden within it can then be extracted.

There are several programs available for processing spectra. Two of the most popular are freeware programs. One is called Iris and the other VSpec. While they are not perfect and require a fair amount of effort to master, they are very powerful and worth the effort of the challenge.