astronomy

Siemens star

Siemens star is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Siemens star rather than just read about it. In short: A Siemens star, or spoke target, is a device used to test the resolution of optical instruments, printers, and displays. It consists of a pattern of bright "spokes" on a dark background that radiate from a common center and become wider as they get further from it.

Key takeaways

Siemens star belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Siemens star to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Siemens star from memory before moving on to harder problems.

Reference excerpt

A Siemens star, or spoke target, is a device used to test the resolution of optical instruments, printers, and displays. It consists of a pattern of bright "spokes" on a dark background that radiate from a common center and become wider as they get further from it. In concept, the spokes only meet at the exact center of the star – the spokes, and the gaps between them, become narrower the closer to the center one looks, but they never touch except at the center. When printed or displayed on a device with limited resolution, however, the spokes touch at some distance from the center. The smallest gap visible is limited by the smallest dot of ink the printer can produce, making the Siemens star a useful tool for comparing two printers' resolutions (DPI). Similarly, it can be applied to a camera's optical resolution by taking photographs of a Siemens star printed at high resolution and comparing photographs from different cameras, to see which retained the center detail the closest.

In the field of video production, where it is often called a back focus chart, the Siemens star is widely used to adjust the back focus of removable lenses. It is also used during film or video shoots to help setting the focus in special situations. Siemens stars are similar to the sunburst pattern used as a background in graphic design, as in the Japanese Naval Ensign, Russian Air Force flag and Jordanian Royal Standard. They are useful in drawing the eye to a point on the page.

Under optical blur from defocus, a Siemens star (like any periodic pattern) gives rise to the phenomenon of spurious resolution above the resolution limit, i.e. toward the center of the Siemens star. (Spurious resolution appears similar to aliasing, but it is a purely optical phenomenon, so it occurs without need of pixels.) This results in inverted polarity of the stripe pattern: black stripes appear in the place of white stripes and vice versa (and further polarity inversions occur further inward). (The illustration under Optical transfer function shows spurious resolution caused by blurring.) When looking at the Siemens star with slightly blurred vision, e.g., without spectacles or with defocus from staring, this is seen as a shimmering ring around the Siemens star's center that changes size with viewing distance. The star was developed by Siemens & Halske AG (today Siemens) in the 1930s to test the lenses of Siemens narrow-film cameras.

Illustrations

Worked examples

Example 1 — a first encounter with Siemens star

Start with the simplest possible case. Write down what Siemens star claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Siemens star before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Siemens star ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Siemens star

In research: Siemens star appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Siemens star in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Siemens star is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical illusions, Visual perception, so understanding it makes those chapters shorter.
In everyday life: Look for Siemens star outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Siemens star in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Siemens star means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Siemens star out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Siemens star in simple terms?

Why does Siemens star matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Siemens star?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Siemens star.