RONCHI GRID.

The following is copied from "Building a Low Cost Telescope" by Eric Witcombe and credited to E. Lumley of Sydney.

To Compute the Shape of the Shadow Bands.

Select a maximum spacing for the shadow bands in the centre. The larger it is the more sensetive the test. but the bigger the central area of the mirror which is not tested. A god compromise is 25 to 30 percent of the diameter.

Multiplying this distance "K"in inches by the number of wires per inch on the grating gives the magnifying ratio for that particular zone , whose radius is ½ K. Dividing the racius of curvature of the mirror by this ratio gives the distance "d" from the grating to focus of this zone. For a spherical mirror ail zones have the same focus, so "d" is constant for all zones and the magnifying ratio B/d is constant, so the distance apart of the shadow bands is constant for all zones and they are thus parallel straight lines.

For other curves, such as the paraboloid for example the focal Length increases for the outer zones, and thus "n" must be substituted for "d" in the ratio B/d above. Thus the magnifying ratio decreases for the outer zones, and so the shadow bands become loser at the ends when tested inside focus. (See above diagram)

The calculation for a typical paraboioid mirror follows
Mirror Diameter 10 inches. Radius of curvature 120 inches
Magnification of zone K/2 = 3 x 1 CO = 300
Distance "d" = 120/300 = .400 inch.

Prepare the following table.