The basic scheme of an optical telescope: Keplerian refractor

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The diagram also includes an embedded figure with a caption:

The Wikipedia page on optical telescopes has chosen the Keplerian refactor as representative, whereas this trail is ultimately focused (oops pun) on reflectors, so it won't be modelling in great depth.

A block KeplerianRefactor has isInverting redefined to be 'true'.

Concerning the real image formed by the objective we are told 'This image may be recorded ...' - which we touched on earlier with Photograph and ImageSensor - or 'viewed through an eyepiece, which acts like a magnifying glass'. The comparison with magnifying glass is not very useful here; the block EyePiece, which already extends Lens, has been set to additionally extend a block MagnifyingGlass that is otherwise considered out-of-scope.

We have to be careful interpreting this bit: 'The eye then sees an inverted magnified virtual image of the object'. This clearly contradicts an earlier snippet (i.e. not all optical telescopes have an inverted virtual image):

Such contradictions can be tracked using this technique:

Also, caution has to be exercised with the word 'sees'. It is not so much that the 'eye then sees ... a magnified virtual image of the object'; rather, the eye "looks at" or "observes" the virtual image of the object then "sees" a real image produced on the retina by the lens and combined optics of the eye (which the optical nerve system and brain then process).

There is a general «observes» Association from Eye to OpticalImage, which has now also been named 'Observes', and similarly the «observes» Association from Telescope to DistantObject has been named 'Observes'. This is so that when the Association is used to type Connectors in IBDs the name of the Association will be clear, as we'll see next .

If you are thinking to yourself this is WET and breaks the DRY principle you are dead right, this is a weakness in UML-2.5.1/SysML-1.6. It would be nice to have explicit support for showing "secondary" stereotype keywords on Connectors typed by Associations.

Later in this trail the Block EyePiece is renamed Eyepiece.
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