The basic scheme of an optical telescope: Keplerian refractor

These snippets in the Wikipedia page are offered as representations all optical telescopes (not all of which are inverting), but are in fact only good descriptions of a Keplerian Refractor.

Click on the image to view it full size

The diagram also includes an embedded figure with a caption:

Wikipedia: 'Schematic of a Keplerian refracting telescope. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere'

Image author: Szőcs Tamás; Copyright licence: Creative Commons CC BY-SA 3.0.

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):

Wikipedia: 'There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as erecting telescopes.'

Such contradictions can be tracked using this technique:

Webel Parsing Analysis: Relationships between «snippet» items can be tracked using additional tagged values such as 'contradictedBy' and 'contradicts'. This is robust, but it can be verbose when Snippets have long names if the tagged values are displayed.

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.

Up next

IBD of a SimpleOpticalTelescope in context with some relationships

Notes

[CAPABILITY, FEATURE, TOOL]{INFORMATIVE} MagicDraw/Cameo supports embedding images in diagrams

[DISPLAY, STYLE, TIP]{SUGGESTED} Webel Parsing Analysis: If the dashed-line "anchors" from your «snippet» ElementGroup symbols to elicited elements make a «pa» Parsing Analysis Diagram (PAD) too hard to read, try making them light grey using symbol properties.

[POLICY, STYLE]{STRICT} Webel «pa» Parsing Analysis Diagrams (PADs) are "scratchpads" used to elicit model elements traceably from text Snippets (extracts from source Documents). While BDDs are a good initial choice, most types of SysML diagram can be used as a «pa» diagram.

[POLICY, STYLE]{STRICT} Webel Parsing Analysis «pa» diagrams are NOT intended as final presentation diagrams! They serve merely to traceably elicit model elements, which may then be shown in other (typically much tidier) presentation diagrams elsewhere.

[POLICY]{STRONG} About the Single Source of Truth (SSOT) and Don't Repeat Yourself (DRY) principles

[PROPOSAL]{PROPOSED} Webel: WISHLIST for SysML: explicit support for showing "secondary" stereotype keywords on Connectors typed by Associations with stereotypes applied.

Snippets (quotes/extracts)

The basic scheme is that the primary light-gathering element, the objective (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object to a focal plane where it forms a real image.