Cv 1

Astm 3

Coma 3

Sphe 3

Tri S

Astm S

3aS 23

From the curvature sag, O20, we may notice that:

^ The curvature of a segment decreases when the off-axis distance increases.

For large telescope mirrors, the linear segment number (N) and the global mirror f-ratio (Q) are fundamental design parameters. These two numbers must be set such as the optical modes Onm which would introduce too much complexity to generate are reduced to negligible quantities for the marginal segment. Let us consider for instance a 42-m aperture paraboloid at f/1, Q = 1, with segments of 2-m in diameter, i.e. a = 1m. For a marginal segment, the off-axis distance is dm = 20m so that um = dm/R = 1/4.2. From (7.33), the optical ptv sags are respectively

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