Slit changing device for monochromator

Claim 1

A slit positioning system for a monochromator including a diffraction grating comprising: (a) slit disc means having a plurality of spaced pairs of slits, each pair including a light-incident slit for directing light onto said grating and a light-ejection slit for receiving light diffracted by said grating, the slits of each pair being of the same width and being diametrically disposed on the slit disc means, the slit widths of the respective pairs being different from each other, each slit pair having a slit number corresponding to a predetermined address on said slit disc means with respect to a positive direction of rotation of the slit disc means; (b) stop apertures provided in said slit disc means for each slit pair having aperture widths inversely proportional to the slit widths of the associated slit pairs; (c) means for directing light passing through the light-ejection slits back through the associated stop aperture; (d) photodetector means for measuring the intensity of light passed back through said stop aperture; (e) pulse motor means coupled to said slit disc means for rotating said slit disc means in either said positive direction or a negative direction to position a slit pair of a selected slit number into optical alignment with the diffraction grating; (f) means for determining the slit number of the present slit pair in optical alignment with the diffraction grating; (g) keyboard means for selecting a slit number of a new slit pair to be positioned by said pulse motor means in optical alignment with said diffraction grating; (h) means for comparing the new slit number with the present slit number and generating a positive drive signal when the new slit number is greater than the present slit number and a negative drive signal when the new slit number is smaller than the present slit number; and (i) control means for driving said pulse motor means in a positive direction in response to said positive drive signal and in a negative direction followed by said positive direction in response to said negative drive signal to position said new slit pair in optical alignment with said diffraction grating; whereby the final direction of rotation which achieves the positioning of said new slit pair is always in said positive direction.