We all have learned that light is made up 7 colors since childhood, more precisely it is a combination of various frequencies of light, each corresponding to different shades of color for human eye, in visible region. A spectrograph is a device that helps us view each frequency of a light source separately and help us study the composition of incoming light.
This study is important for stars as different sources of light have different spectra, for ex. the sodium vapor lamp which were earlier used as street lights, had only yellow color, your ordinary tube light has different spectra than a CFL and LED. All star light is different for all stars are not exactly the same they have different composition and mixtures of gases in them. Spectrography is used to determine these unknown gases in stars.
Unlike a Prism PARAS exploits another phenomenon of light to study its composition namely Diffraction.
Diffraction is the phenomenon bending of light around an sharp object or slits, it is the reason why we can hear sound in a room from a source outside even when we cannot see it. This is due to bending of sound waves, similar phenomenon is also seen in light but at a much smaller scale.
When light gets diffracted all the components of the light bend at a slightly different angle than the other, this enables us to observe all the components of the light as a continues spectrum like a rainbow. We can see if all colors and all shades of a particular color ( frequencies) are present.
The spectrum of light is obtained through diffraction using a diffraction grating. It is an optical instrument that has fine slits engraved in it which bends the light passing through it and makes a fine diffraction pattern.
PARAS has a special kind of Grating known as an Echelle grating. after the light is passed through a regular grating it is again passed through a special grating. This grating has lower no of grooves and light is incident on it at a much higher angle. This enables us to resolve the spectrum of light even further. Hence PARAS can be referred to as an Echelle spectograph.