The above observations are only valid when the distance between the slit and the screen is large
Spectrograph
An instrument used to obtain and record an astronomical spectrum by splitting or dispersing the light from an object into its component wavelengths
Ways to split or disperse light into a spectrum
Using a prism
Using a diffraction grating
Most astronomical spectrographs use diffraction gratings rather than prisms
Diffraction gratings
Consist of thousands of narrow, closely spaced parallel slits
Due to interference, the intensity of the light travelling through the slits depends on the wavelength and direction of propagation
Diffraction gratings are more efficient than prisms, which absorb some of the light passing through them
Every incident photon is precious when trying to get a spectrum from a faint source, so scientists prefer using a diffraction grating
The response of a diffraction grating is linear, whereas a prism disperses the blue light much more than the red part of the spectrum
Main components of a modern slit spectrometer
Slit
Collimating mirror
Diffraction grating
Detector
The slit allows only a small portion of incident light required for the spectral analysis to enter the spectrograph, and helps to filter out any stray light
The collimator focuses the light onto the diffraction grating
The diffraction grating splits the light into its constituent colours
The spectrum is finally captured by a detector or a camera lens and fed into a computer for further analysis
Figure 9.32 shows the schematic diagram and the main components of a slit spectrograph used for spectral analysis of the light from the stars
