A curved primary mirror is the reflector telescope's basic optical element

The target snippet (including some editorial annotations) is: It is supplemented by: It is not entirely clear whether by 'focal plane' the 1st snippet means that of the primary (if there were no secondary mirror) or of the telescope (primary and secondary if there is one). We'll clear that up more below.
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In addition to the primary mirror of a ReflectingMirror being curved, it is assumed that it is always a ConcaveMirror. The property primary is also tagged as being {aka = "objective"} and it {subsets mirror} and {redefines objective}.

This previously analysed snippet was not quite right, as a Newtonian reflector (as we'll see in more detail a bit later) has a flat secondary mirror:

So the property mirror[0..*] of OpticalTelescope has now been promoted to be of the more general type Mirror.

It turns out that not every reflector design uses a secondary mirror:

So the multiplicity of property secondary of type Mirror in the intermediate abstract base ReflectingTelescope is [0..1] (it can be redefined to be multiplicity [1] in more specialised reflector versions that are known to always have secondary).

When a reflector does have a secondary mirror, the focal length of the telescope is not just that of the primary mirror (objective), hence the editorial annotations for this snippet:

There a discussion of so-called "two mirror" reflectors on this nice amateur telescope optics site telescope-optics.net, which gives the following formula for the 'final system focal length' of a Gregorian or Cassegrain:

ƒ = (ƒ1 ƒ2)/(ƒ1 - ƒ2 - s)

where ƒ1 and ƒ2 are the focal length of the primary and secondary mirror, respectively, and s is the mirror separation.

One of the longer term goals of this tutorial is to be able to model the Giant Magellan Telescope, which is an aplanatic Gregorian with multi-segment primary (and multi-segment secondary mirror):

The abstract block Mirror is therefore not assumed to be simple or segmented, and specific reflector designs can "mix-in" the composite abstract block SegmentedMirror as required.

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