A spectrometer with a diffraction grating with a ruling of 600 lines mm⁻¹ is used in second order. The grating is illuminated at an angle of 330 and the dispersed spectrum emerges along the direction of the normal to the grating. Calculate the reciprocal linear dispersion (RLD) when a camera of 0·5 m focal length is used. If the grating is 70 mm × 70 mm square, calculate the theoretical spectral resolving power of the instrument. If the system is used to record stellar spectra under atmospheric conditions such that the seeing disc is ∼10 arc sec, estimate the wavelength spread of each resolved spectral element if all the light from the star image is allowed to pass through the system. How does this compare with the wavelength spread of the theoretical resolved element say at 500 nm?