We have often used schuster's method while performing any experiment on spectrometer. But I am not able to get the logic behind it. how does the turning of prism helps in focusing of collimator and telescope for "parrallel light" let me give the schusters method in brief: Focus the eye piece of the telescope on the cross wires and do not disturb this adjustment here after.Illuminate the slit of the collimator with sodium light.Adjust the width of the slit so that it is as narrow as possible.Place the prism on the table at its center with its refracting edge vertical and grounded face roughly parallel to the length of the collimator.The prism table is slowly rotated until the refracted image of the slit is in the minimum deviation position. Rotate the prism table slowly so that the refracting edge of the prism of refracting angle "A" moves towards the telescope so that the telescope receives the refracted rays at an angle greater than the angle of minimum deviation for the ray.The telescope is then focused to make the image of the slit sharp. The prism table is now rotated so that the refracting angle of the prism moves towards the collimator.The collimator is now adjusted to make the the image of the slit very sharp. The above two steps are continued until the image of the slit is sharp and without parallax with the vertical cross wire. does this thing has anything to do with the fact that at minimum deviation the light rays inside the prism are parallel to the prism base?