Equations Explained

1. Extension Factor (m)

How much the system focal length is extended by the secondary mirror.

2. Secondary to Primary Focus (A)

Distance from the secondary mirror to the primary focus point.

3. System Focal Plane to Secondary (a)

Distance from the secondary mirror to the image plane.

4. Mirror Separation (B)

Physical separation between primary and secondary mirrors.

5. Radii of Curvature

Radii of curvature for the primary and secondary mirrors.

6. Conic Constants

Describes the conic shape of the primary and secondary mirrors.

7. Coma Coefficient

For the true Cassegrain Coma = 0.5, i.e. exactly the same as for a paraboloid of the same diameter .

8. Astigmatism Coefficient

This coefficient quantifies how much astigmatism is introduced by the geometry of the mirror system.

9. Petzval Curvature

Petzval curvature represents the mean field curvature from both mirrors and affects how a flat image plane curves in space.

10. Field Curvature (COF)

The total field curvature, taking into account both Petzval and astigmatic effects. A positive curvature indicates a focal surface that is concave towards the incident light.

11. Coma and Astigmatism (Angular)

Conversion to arcseconds based on system geometry and field of view, θ (in radians).

12. Radius of Curvature of Field

How much the image plane curves due to field curvature aberration.

13. Secondary Mirror Axial Diameter

The physical diamter of the secondary mirror required to capture all the light reflected from the full aperature of the primary.

13. Secondary Mirror Optical Diameter

The diameter of the light cone from an off-axis object. Uses field angle θ (in radians).

14. Epsilon (Wavefront Error)

The maximum surface deviation from the sphere is given by the above equation.

15. Edge Slope of Mirrors

The slope at the edge relative to the sphere is given by the derivative of Eq. 14.