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Primary mirror

Primary mirror is a physics 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 Primary mirror rather than just read about it. In short: A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished reflective metal (speculum metal up to the mid 19th century), or in later telescopes, glass or other material coated with a reflective layer.

Primary mirror — main illustration
Primary mirror — illustration

Key takeaways

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

Reference excerpt

A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.

Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished reflective metal (speculum metal up to the mid 19th century), or in later telescopes, glass or other material coated with a reflective layer. One of the first known reflecting telescopes, Newton's reflector of 1668, used a 3.3 cm polished metal primary mirror. The next major change was to use silver on glass rather than metal, in the 19th century such was with the Crossley reflector. This was changed to vacuum deposited aluminum on glass, used on the 200-inch Hale telescope. Solid primary mirrors have to sustain their own weight and not deform under gravity, which limits the maximum size for a single piece primary mirror. Segmented mirror configurations are used to get around the size limitation on single primary mirrors. For example, the Giant Magellan Telescope will have seven 8.4 meter primary mirrors, with the resolving power equivalent to a 24.5 m (80.4 ft) optical aperture.

Superlative primary mirrors The largest optical telescope in the world as of 2009 to use a non-segmented single-mirror as its primary mirror is the 8.2 m (27 ft) Subaru telescope of the National Astronomical Observatory of Japan, located in Mauna Kea Observatory on Hawaii since 1997; however, this is not the largest diameter single mirror in a telescope, the U.S./German/Italian Large Binocular Telescope has two 8.4 m (28 ft) mirrors (which can be used together for interferometric mode). Both of these are smaller than the 10 m segmented primary mirrors on the dual Keck telescope. The Hubble Space Telescope has a 2.4 metres (7 feet 10 inches) primary mirror. Radio and submillimeter telescopes use much larger dishes or antennae, which do not have to be made as precisely as the mirrors used in optical telescopes. The Arecibo Telescope used a 305 m dish, which was the world largest single-dish radio telescope fixed to the ground. The Green Bank Telescope has the world's largest steerable single radio dish with 100 m in diameter. There are larger radio arrays, composed of multiple dishes which have better image resolution but less sensitivity.

See also Active optics Honeycomb mirror Liquid-mirror telescope List of largest optical reflecting telescopes List of telescope parts and construction Mirror mount Mirror support cell Secondary mirror Silvering

References

Illustrations

Primary mirror: Six of the 18 primary mirrors of the James Webb Space Telescope being prepared for acceptance testing.
Six of the 18 primary mirrors of the James Webb Space Telescope being prepared for acceptance testing.
Primary mirror: The correctly ground backup primary mirror built by Eastman Kodak for the Hubble space telescope (the mirror was never coated with a reflective surface, hence its honeycomb support structure is visible). It now resides in the National Air and Space Museum in Washington, DC.[1]
The correctly ground backup primary mirror built by Eastman Kodak for the Hubble space telescope (the mirror was never coated with a reflective surface, hence its honeycomb support structure is visible). It now resides in the National Air and Space Museum in Washington, DC.[1]
Primary mirror: The largest non-segmented mirror in an optical telescope in 2009, one of the Large Binocular Telescope's two mirrors.
The largest non-segmented mirror in an optical telescope in 2009, one of the Large Binocular Telescope's two mirrors.

Worked examples

Example 1 — a first encounter with Primary mirror

Start with the simplest possible case. Write down what Primary mirror claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Primary mirror 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 Primary mirror 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 Primary mirror

In research
Primary mirror appears in physics 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 Primary mirror 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
Primary mirror is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mirrors, Optical telescope components, so understanding it makes those chapters shorter.
In everyday life
Look for Primary mirror 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 Primary mirror in 20 minutes

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

Frequently asked questions

What is Primary mirror in simple terms?

A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical, parabolic, or hyperbolic shaped disks of polished reflective metal (speculum metal up to the mid 19th century)…

Why does Primary mirror matter?

Because it connects several physics 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 Primary mirror?

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 Primary mirror.

Tags

  • Mirrors
  • Optical telescope components

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