It is more steep for incident angles less than im (angle of incidence for which deviation is minimum). This asymmetrical nature is what makes Schuster's method work. If you increase the angle of incidence, ie, move to the right of im, you would notice for small change in i change in delta is small.

Why is Schuster's method necessary?

(vi) Focusing for Parallel rays by Schuster's method: This is the best method of focusing the telescope and the collimator for parallel rays within the space available in the dark room. In order to focus the telescope parallel light rays are required and this in turn requires a properly adjusted collimator.

Why do the telescope and collimator are adjusted for parallel rays?

The telescope is brought along the axial line with the collimator. The slit of the collimator is illuminated by a source of light. ... Since the telescope is already adjusted for parallel rays. a well-defined image of the slit can be formed, only when the light rays emerging from the collimator are parallel.