An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum

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Let's start with 'An optical telescope is a telescope that gathers and focuses light'. We already have «focuses» and «collects», but now «collects» is documented as being the same as 'gathers' concerning light.

Next, consider 'create a magnified image for direct view'. Presumably magnification can apply to any image, not just an optical image, so the very abstract Image gets value properties magnification with ValueType Magnification and isMagnified:Boolean.

The snippet implies that the 'magnified image for direct view' is not necessarily the same as the image captured by a photograph or by an image sensor. If you know a little about telescopes you might already know that in fact when an eyepiece is used a real image gets magnified by the eyepiece to create a virtual image that the eye can in turn view (as a further real image on the retina), as we'll explore in more detail later in the trail. For now it suffices to introduce a distinct reference property viewedImage:OpticalImage (in addition to image:OpticalImage) with a constraint {isMagnified}.

For 'make a photograph' a block Photograph (owned by OpticalTelescope) is introduced, which extends an abstract block Photograph with the same name but a different owner. It has a reference property image:OpticalImage[1] - although in reality astronomers and astrophotographers sometimes perform multiple exposures in one photograph, and sometimes even of different images. A Constraint indicates that the 'image' of a photograph of a telescope is the same as the image formed by the telescope:

photograph.image = image

Presumably there is a camera somewhere too, but it's not mentioned yet so for now it's left out.

Finally, 'An optical telescope ... can collect data through electronic image sensors'. An «electronic» OpticalImageSensor has data:OpticalImageData[0..*], where any OpticalImageData is assumed to be of image:OpticalImage. An OpticalTelescope may have zero or more imageSensor with Constraints:


imageSensor.image = image
imageSensor.data.image = image

These assert that the image of the sensor and of any image data set correspond to the focal plane image of the telescope. We'll see later than in fact some reflector telescopes can exploit both a prime focus focal plane image and a secondary focal plane image (the result of both a primary and secondary mirror), and might even have modes for image sensors at either, but for now we'll assume there is a single telescope image of interest.

There is a "redefinition ladder" for sensors, images, and image data as shown next .

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