Spectroscopy

Since October 2021 I'm also working on stellar spectroscopy. To this end I have purchased the blaze grating Star Analyser 200, which I mostly use in connection with the Skywatcher Dobson telescope and the ZWO ASI 178MC. To image the spectrum I use FireCapture, taking several frames of the stellar spectrum. Calibration and reduction of the spectral data is accomplished with the software package RSpec, with which I have made very good experience. It is easiest to start with stars of spectral type A as this readily shows the Balmer lines of hydrogen. These allow an easy wavelength calibration.

It is a lot more difficult to calibrate stars of spectral type F, G, or M, since in this case the Balmer lines are suppressed or even missing. Instead the spectrum is dominated by heavy elements (O, N, C, Fe, TiO,...).

The blaze grating Star Analyser 200 is a high-quality grating with 200 lines/mm in a standard 1,25" filter socket. It is quite thin (5.2mm above the thread). MC-coated glas shields the grating. The spectral direction is indicated on the frame, hence its adjustment is rather easy. Both, the star and the spectrum can be imaged together, thus simplifying the identification and calibation. The grating can also be used visually with an eyepiece.

The raw spectrum of the star Alhena (γ Gem) is ready to be worked on with RSpec.

Spectrum of the star Alhena (γ Gem). It is calibrated, and instrumental effects (e.g. spectral response of the sensor) have been eliminated. The first four Balmer lines, Hα, Hβ, Hγ and Hδ are visible. The next followiing line, Hε, is also already indicated at the short-wavelength end. Spectral class A0 IV.

Spectrum of the star Betelgeuse (α Ori). Because of the absence of Balmer lines, calibration is a lot more difficult here. The spectrum is dominated by heavy elements (Na, Fe, Ti, O), which the star has produced via nuclear fusion in the course of its evolution. Spectral class M1-2 Ia - Iab.

Spectrum of the star Capella (α Aur). Because of the absence of Balmer lines, calibration is also rather difficult here. This spectrum, too, is dominated by heavy elements (Na, Fe, Mg, Mn, Ca, Cr), which the star has produced via nuclear fusion in the course of its evolution. Spectral class G8 III G0 III.

Spectrum of the star Rigel (β Ori). The first five Balmer lines, Hα, Hβ, Hγ, Hδ and Hε are visible. Spectral class B8 I B9 V.

Field around the planetary nebula M57, obtained with Maksutov/Newton (D = 190 mm, f = 1000 mm), Canon 1100Da and Star Analyzer 200. Exposure 3.1 hrs with ISO 800. In this image, each object is spectrally dispersed. Hence, the stars exhibit the colours of the rainbow spectrum, with dark orthogonal absorption lines in each of them. These are characteristic for the chemical elements contained by the stars. The planetary nebula M57, however, discloses its chemical elements in emission. Therefore, it appears (apart from its main image, the so-called zero-order) more or less bright at different locations with different colours, which correspond to the wavelengths at which they are emitted by the corresponding chemical element.

The annotation of the above image - a cutout of the total image - indicates the chemical elements, with the roman numbers designating their ionization states (I: neutral, i.e. not ionised; II: singly ionised, III: doubly ionised). In this way, one can detect the chemical elements of the hot gas in a single image.