Parabolic mirrors offer excellent correction of spherical aberration. Parabolic mirrors can either be on-axis or off-axis. On-axis parabolic mirrors produces collimated reflected light. Off-axis parabolic mirrors focus reflected light to a focal point off-axis. An important dimension to consider is the diameter. The diameter of the mirror if viewed straight on can also be thought of as the height. Another important parameter is the focal length. Focal length is the distance from the mirror the light converges.

Materials

Parabolic mirrors can be supplied in a variety of materials. Mirror material refers to the classification of the material used to make the mirror. The material used influences the reflectivity characteristics of the mirror. These materials include BK7 glass, copper, fused silica, nickel, optical crown glass, and UV grade fused silica. BK7 is boro-crown glass (borosilicate glass). Copper is used typically in high power applications because of its high thermal conductivity. Fused silica has a very low coefficient of thermal expansion and is ideal for use with moderately powered lasers or changing environmental conditions. Nickel is considerably more durable than glass substrates, both to thermal and physical damage. Optical crown glass is typically used in non-imaging applications, including light gathering or conventional beam manipulation tasks. Crown glass is generally used when thermal stability is not a critical factor. UV grade fused silica is of higher quality than fused silica.

Coatings

Mirror coatings are used to enhance the reflectivity of a mirror. Common coatings for parabolic mirrors include none (uncoated), bar aluminum, enhanced aluminum, protected aluminum, dielectric, bare gold, protected gold, rhodium, and silver. Surface quality ratings for parabolic mirrors include 10-5 scratch / dig, 20-10 scratch / dig, 40-20 scratch / dig, 60-40 scratch / dig, and 80-50 scratch / dig. A dig is a defect on a polished optical surface that is nearly equal in terms of its length and width. A scratch is a defect on a polished optical surface whose length is many times its width. 10 / 5 indicates the average diameter of the digs to be .05 mm and the average length of a scratch is .10 mm. 20 / 10 indicates the average diameter of the digs to be .10 mm and the average length of a scratch is .20 mm. 40 / 20 indicates the average diameter of the digs to be .20 mm and the average length of a scratch is .40 mm. 60 / 40 indicates the average diameter of the digs to be .40 mm and the average length of a scratch is .60 mm. 80 / 50 indicates the average diameter of the digs to be .50 mm and the average length of a scratch is .80 mm.

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