What do you mean by pure spectrum?

A specttrum in which there is no overlapping of different colours is called a pure spectrum.

English: Triangular prism causing the dispersi...

How can we produce a pure spectrum?

For the production of pure spectrum, it is necessary that the slit should be narrow and the incident beam of light should be parallel. Then the emergent rays will be parallel to each other and will be focussed at a point by the telescope. Also, the rays of different wavelengths will be focussed at different points.