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Null corrector

Null corrector 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 Null corrector rather than just read about it. In short: A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses.

Null corrector — main illustration
Null corrector — illustration

Key takeaways

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

Reference excerpt

A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses. One method of doing this using a Shack cube is shown at the right, and many other setups are possible. An interferometer test such as this one generates a contour map of the deviation of the surface from a perfect sphere, with the contours in units of half the wavelength used. This is called a null test because when the mirror is perfect, the result is null (no contours at all). If the result is not null, then the mirror is not perfect, and the pattern shows where the optician should polish the mirror to improve it.

However, the mirrors used in modern telescopes are not spherical – they are rotations of parabolas or hyperbolas, since these more complex shapes reduce optical aberrations and give a larger field of view. (See, for example, Ritchey-Chrétien telescope, or three-mirror anastigmats such as LSST.) Non-spherical mirrors such as these will not give a null result when tested as above, and tests that give null results are strongly preferred (they require little interpretation, and the results translate directly to polishing requirements). One solution is to introduce a null corrector. This consists of one or more lenses and/or mirrors introduced into the optical path that make the desired mirror look like a perfectly spherical mirror. Using this device, the measured contour map now shows the difference from the desired shape instead of the difference from a sphere. Now measurement and polishing can proceed just as in the spherical case. This method is used in the manufacture of almost all large mirrors for modern telescopes. Since the mirror will be ground to what the null corrector reports as the right prescription, it is critical that the null corrector be itself correct. An error in building the null corrector led to the mirror in the Hubble Space Telescope being ground to the wrong shape. Less famously, this has happened in other cases as well, such as the New Technology Telescope. Originally, there was no easy way to test a null corrector, so mirror fabricators needed to take extra care that the lenses were correct and spaced correctly (this second part, spacing, was the source of the Hubble null corrector failure). With the advent of computer-generated holograms, it is now possible to create a hologram with the phase response of an arbitrary mirror. Such a hologram can be made to analytically duplicate the phase response of the desired mirror, then be tested with the null corrector just as the real mirror would be tested. If the combination looks like a spherical mirror to the interferometer, then both the null corrector and the hologram are correct with high probability, since the null corrector and the hologram are constructed independently by different procedures. This procedure was used to test (and find an error in) the null corrector used for the MMT Observatory single-mirror retrofit.

References

Illustrations

Null corrector: Testing a spherical mirror using an interferometer. All surfaces in the tester are either flat or spherical, so the tester itself is fairly easy to fabricate and test. This setup can test a spherical mirror of any size – since the wavefront is spherical, the mirror can be small and close up, or many meters across and further away. The test only requires that the pinhole be located at the center of sphere defined by the mirror's surface.
Testing a spherical mirror using an interferometer. All surfaces in the tester are either flat or spherical, so the tester itself is fairly easy to fabricate and test. This setup can test a spherical mirror of any size – since the wavefront is spherical, the mirror can be small and close up, or many meters across and further away. The test only requires that the pinhole be located at the center of sphere defined by the mirror's surface.
Null corrector: Adding a null corrector so the interferometer test can measure an aspheric mirror. The null corrector cancels the non-spherical portion of the mirror figure, so when viewed from point A, the combination looks precisely spherical if the mirror under test has the correct figure. Diagram is not to scale – the null corrector is much smaller than shown here.
Adding a null corrector so the interferometer test can measure an aspheric mirror. The null corrector cancels the non-spherical portion of the mirror figure, so when viewed from point A, the combination looks precisely spherical if the mirror under test has the correct figure. Diagram is not to scale – the null corrector is much smaller than shown here.

Worked examples

Example 1 — a first encounter with Null corrector

Start with the simplest possible case. Write down what Null corrector 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 Null corrector 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 Null corrector 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 Null corrector

In research
Null corrector 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 Null corrector 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
Null corrector is common in secondary-school and first-year university syllabi. It links to neighbouring topics Microscopy, Mirrors, Optical devices, so understanding it makes those chapters shorter.
In everyday life
Look for Null corrector 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 Null corrector in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Null corrector 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 Null corrector out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Null corrector in simple terms?

A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses.

Why does Null corrector 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 Null corrector?

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 Null corrector.

Tags

  • Microscopy
  • Mirrors
  • Optical devices

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