The prime purpose of the lens-to-mount interface is to hold the lens in its proper position and orientation within the optical instrument. This implies the application of mechanical constraints, i.e., external forces that limit lens motions when the temperature changes or mechanical disturbances occur. How these constraints should be applied is the first topic considered in this chapter. The advantages of surface contact over rim contact are explained, and some impacts of dimensional tolerances on part alignment are summarized. We then explore how the applied forces may cause stresses to build up or birefringence to develop within the lens. The generalizations given here are the subjects of more detailed, quantitative discussions in Chapters 5 and 6. Our attention then is drawn to the importance of sealing as a means of minimizing entry of moisture, dust, and other potential contaminants into the instrument. The most common sealing techniques are identified. Finally, the significant impacts of dimensional tolerances on component and total instrument costs and manufacturability are recognized.
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