An object infinitely far from the optical system forms an image at the rear focal plane

It is asserted that an OpticalTelescope always has a single focalPlane:FocalPlane[1] associated with a single focalPoint:FocalPoint[1], whereas any OpticalElement may have one of each. There is also now a clear definition of FocalLength.

You might have noticed that the OpticalTelescope as an entire optical system is starting to share a lot of features with any OpticalElement. They won't be integrated yet by having OpticalTelescope indirectly extend OpticalElement, but it's tempting to think of it. For now it suffices to have constraints to assert that the focalPoint and focalPlane of the OpticalTelescope are the same as those of its objective (which is an OpticalElement).

The object:DistantObject has now effectively been equated with an 'object infinitely far from the optical system'. The image is as before «produced at» the focalPlane, and has a Connector to its corresponding object, which is in fact typed by the 1-to-1 Association from the previous diagram.

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The basic scheme of an optical telescope: Keplerian refractor

Notes

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Snippets (quotes/extracts)

The front and rear (or back) focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points.

If the medium surrounding the optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from the principal planes to their corresponding focal points is just the focal length of the system.

A principal focus or focal point is a special focus: For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused.

rays that enter the system parallel to the optical axis are focused such that they pass through the rear focal point

An object infinitely far from the optical system forms an image at the rear focal plane.