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About the Ronchi Test
Figure 8 illustrates the basic idea behind
using a Ronchi grating to identify the
position of the focal plane. The light from a
single bright star passes through the optical
system and converges to a point on the
focal plane. When the Ronchi grating is
placed in front of or behind the focal plane,
it will block some of the light in this light
cone. This will cast a series of shadows that
appear as dark, wide lines when the
defocused light is viewed through the
ocular.
As the grating moves closer to the focal plane, fewer lines are visible, and they become
wider. When the focal plane and the Ronchi grating are perfectly aligned, no lines (or
perhaps one very wide line) are visible. See Figure 9 for an example.
In order to set the primary-to-secondary distance, you will be moving the secondary mirror.
By adjusting secondary mirror distance, you will be moving the focal plane relative to the
fixed position of the Ronchi grating. The job here is to move the secondary mirror such that
the focal plane coincides with the Ronchi grating on the Ronchi Ocular. When you have
done this, you have accurately set the primary-to-secondary spacing.
With reasonable effort, the primary-to-secondary spacing can be set to well within its 1mm
tolerance.
Figure 8: The Ronchi grating casts fewer shadows as
it moves closer to the focal plane.
