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

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:

Wikipedia: 'A telescope is an optical instrument using lenses, curved mirrors [DISPUTED], or a combination of both to observe distant objects ...'

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:

Wikipedia: 'In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary 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):

Wikipedia: 'A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. ... They are used as objectives for large reflecting telescopes.'

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.

Up next

The Gregorian telescope employs a concave secondary mirror that reflects the image back through a hole in the primary mirror.

Notes

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Snippets (quotes/extracts)

A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane [OF THE PRIMARY IF THERE IS NO SECONDARY]. The distance from the mirror to the focal plane is called the focal length.

A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. ... They are used as objectives for large reflecting telescopes.

In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror.

Film or a digital sensor may be located here to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation.

A telescope is an optical instrument using lenses, curved mirrors [DISPUTED], or a combination of both to observe distant objects ...