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Optical Telescope Element

Optical Telescope Element 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 Optical Telescope Element rather than just read about it. In short: Optical Telescope Element (OTE) is one of three major sections of the James Webb Space Telescope, a large infrared space telescope launched on 25 December 2021, consisting of its main mirror, secondary mirrors, the framework and controls to support the mirrors, and various thermal and other systems. The OTE collects the infrared light and directs it to the science instruments in Webb's second major section, the Inte…

Optical Telescope Element — main illustration
Optical Telescope Element — illustration

Key takeaways

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

Reference excerpt

Optical Telescope Element (OTE) is one of three major sections of the James Webb Space Telescope, a large infrared space telescope launched on 25 December 2021, consisting of its main mirror, secondary mirrors, the framework and controls to support the mirrors, and various thermal and other systems. The OTE collects the infrared light and directs it to the science instruments in Webb's second major section, the Integrated Science Instrument Module (ISIM). The OTE has been compared to being the "eye" of the telescope and the backplane of it to being the "spine". The third major section of the JWST is the Spacecraft Element (SE), which includes the spacecraft bus and sunshield. The OTE has a three-mirror anastigmat (TMA) design, with an effective f/20 focal ratio and focal length of 131.4 meters (431 ft). The primary mirror is a tiled assembly of 18 hexagonal elements, each 1.32 meters (4.3 ft) from flat to flat. When properly aligned, this combination yields an effective aperture of 6.5 meters (21 ft) and a total collecting surface of 25.4 square meters (273 ft2). The secondary mirror is a convex circular mirror with a diameter of 0.74 meters (2.4 ft), and it feeds into the Aft Optics Subsystem (part of OTE), which contains the tertiary mirror (fixed) and the Fine Steering Mirror (FSM, movable). All mirrors are made of gold-plated beryllium, due to its low weight, structural stability at low temperatures and high infrared reflectivity. The principal subcontractor for the JWST optics was Ball Aerospace, and the mirror development team included Brush Wellman (forming), Axsys Technologies (shaping) and L-3 Communications SSG-Tinsley (polishing). The components of OTE were integrated by L3Harris Technologies to form the final system.

Overview

The OTE combines a large amount of the optics and structural components of the James Webb Space Telescope, including the Main mirror. It also has the fine steering mirror, which provides that final precise pointing, and it works in conjunction with the fine guidance sensor and other controls systems and sensors in the spacecraft bus. The main mirror segments are aligned roughly using a coarse phasing algorithm. Then for finer alignment, special optical devices inside NIRCam are used to conduct a phase retrieval technique, to achieve designed wavefront error of less than 150 nm. To function as focusing mirror correctly the 18 main mirror segments need to be aligned very closely to perform as one. This needs to be done in outer space, so extensive testing on Earth is required to ensure that it will work properly. To align each mirror segment, it is mounted to six actuators that can adjust that segment in 5 nm steps. One reason the mirror was divided into segments is that it cuts down on weight, because a mirror's weight is related to its size, which is also one of the reasons beryllium was chosen as the mirror material because of its low weight. Although in the essentially weightless environment of space the mirror will weigh hardly anything, it needs to be very stiff to maintain its shape. The Wavefront sensing and control sub-system is designed to make the 18 segment primary mirror behave as a monolithic (single-piece) mirror, and it does this in part by actively sensing and correcting for errors. There are nine distance alignment processes that the telescope goes through to achieve this. Another important aspect to the adjustments is that the primary mirror backplane assembly is steady. The backplane assembly is made of graphite composite, invar, and titanium. The ADIR, Aft Deployable Infrared Radiator is a radiator behind the main mirror, that helps keep the telescope cool. There are two ADIR's and they are made of high-purity aluminum. There is a special black coating on the radiators that helps them emit heat into space.

Some major parts of the OTE according to NASA:

Primary mirror (18 segments) Secondary mirror (74 cm (29 in) diameter) Tertiary mirror (3rd) (in Aft Optics Subsystem) Fine Steering Mirror (in Aft Optics Subsystem) Telescope structure primary mirror backplane assembly main backplane support fixture (BSF) secondary mirror support structure deployable tower array Thermal Management Subsystem Aft Deployable ISIM Radiator (ADIR) Wavefront sensing and control The Aft Optics Subsystem includes the Tertiary mirror and the Fine Steering Mirror. One of the tasks for the Fine steering mirror is image stabilization. The metal beryllium was chosen for a number of reasons including weight, but also for its low-temperature coefficient of thermal expansion compared to glass. Furthermore beryllium is not magnetic and a good conductor of electricity and heat. Other infrared telescopes that have used beryllium mirrors include IRAS, COBE, and Spitzer. The Subscale Beryllium Model Demonstrator (SBMD) was successfully tested at cryogenic temperatures, and one of the concerns was surface roughness at low kelvin numbers. The beryllium mirrors are coated with a very fine layer of gold to reflect infrared light. There are 18 hexagonal segments that are grouped together to create a single mirror with an overall diameter of 6.5 meters (21 ft).

DTA

… excerpt ends here. Continue reading the full article.

Illustrations

Optical Telescope Element: The mirror assembly from the front with primary mirrors attached, November 2016
The mirror assembly from the front with primary mirrors attached, November 2016
Optical Telescope Element: A test mirror being cleaned with carbon dioxide snow
A test mirror being cleaned with carbon dioxide snow
Optical Telescope Element: JWST major components: spacecraft bus, sunshield, Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM)
JWST major components: spacecraft bus, sunshield, Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM)
Optical Telescope Element: Testing of the Aft Optic Subsytem in 2011, which contains the Tertiary (3rd) mirror and Fine Steering Mirror
Testing of the Aft Optic Subsytem in 2011, which contains the Tertiary (3rd) mirror and Fine Steering Mirror
Optical Telescope Element: The Deployable Tower Assembly (DTA) is where OTE connects with the rest of telescope such as the spacecraft bus. During stowage there is another attachment point for the folded sunshield higher up on the OTE
The Deployable Tower Assembly (DTA) is where OTE connects with the rest of telescope such as the spacecraft bus. During stowage there is another attachment point for the folded sunshield higher up on the OTE

Worked examples

Example 1 — a first encounter with Optical Telescope Element

Start with the simplest possible case. Write down what Optical Telescope Element 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 Optical Telescope Element 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 Optical Telescope Element 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 Optical Telescope Element

In research
Optical Telescope Element 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 Optical Telescope Element 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
Optical Telescope Element is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gold objects, James Webb Space Telescope, Mirrors, so understanding it makes those chapters shorter.
In everyday life
Look for Optical Telescope Element 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 Optical Telescope Element in 20 minutes

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

Frequently asked questions

What is Optical Telescope Element in simple terms?

Optical Telescope Element (OTE) is one of three major sections of the James Webb Space Telescope, a large infrared space telescope launched on 25 December 2021, consisting of its main mirror, secondary mirrors, the framework and controls to support the mirrors, and various thermal and other systems…

Why does Optical Telescope Element 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 Optical Telescope Element?

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 Optical Telescope Element.

Tags

  • Gold objects
  • James Webb Space Telescope
  • Mirrors

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