Home » What is “parallax”? Why does it make paleo astronomers think that the earth will not move? ——Distance in Astronomy (2)-PanSci

What is “parallax”? Why does it make paleo astronomers think that the earth will not move? ——Distance in Astronomy (2)-PanSci

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  • Author/Chen Zixiang|Department of Earth Sciences, Normal University|Founder of EASY Astronomy and Geosciences Team

As the latest generation of NASA’s flagship space telescope, James. The Webb telescope must of course be the best in space telescopes in terms of performance. However, how do engineers and scientists design the Webb telescope so that it has powerful observation capabilities? This question is quite complicated to study, but the general direction is unexpectedly simple, that is: “The bigger the better.”

How to measure the observation ability of a telescope

Before explaining why the larger the telescope, the better, let us first think about how to measure the observation performance of a telescope? Generally speaking, the two most important performance indicators of a telescope are its “resolving power” and “light-gathering power.”

Resolution can be said to represent the “sight” of the telescope. A telescope with a better resolution can capture more details of celestial bodies, or a telescope with a poor resolution can not distinguish between two very close stars. It’s like doing a vision check. When you can’t see exactly where the “E” in a row on the vision check sheet is pointing, it actually means that the “limit resolution” of your eyes cannot resolve the “E” in that row. “Hello! And astronomers, of course, would hope that the “sight” of the telescope is super good!

The light-gathering power can measure the efficiency of the telescope to collect starlight (electromagnetic waves from celestial bodies). When we usually take pictures with our mobile phones, we usually only need a few hundredths of a second to take a picture of the surrounding scenes in our daily lives. However, because the celestial bodies in the universe are often very bleak, to collect data on these celestial bodies, the exposure time of an astronomer who conducts academic research to a target often takes several hours. Sometimes it even takes more than a week of exposure time!

It can be imagined that under such circumstances, the light collection efficiency of a telescope is a very important thing. If your telescope has four times the light-gathering power of others, it will take a month for others to capture the target, you only need one week to complete. The extra time can be used to shoot more targets, or to shoot the same target for a longer period of time to study more dim details.

Hubble is extremely deep, and the exposure time is approximately 11.3 days. Photo/NASA, ESA, and S. Beckwith (STScI) and the HUDF Team

Big caliber, really fragrant!


Resolving power and light-gathering power are the most important performance indicators of a telescope, and they are all closely related to the same factor, which is the “aperture” of the telescope, that is, the diameter of the primary mirror of the telescope.

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If it is assumed that the primary mirror of the telescope is a complete circle, the resolving power is proportional to the aperture, and the light-gathering power is proportional to the square of the aperture. For example, a telescope with a diameter of two meters, compared to other conditions are the same, but a telescope with a diameter of only one meter, its ultimate resolution will be twice as high, and its light-gathering power will be four times higher. Speaking of this, I believe you should be able to understand why astronomical telescopes are basically “the larger the caliber, the better.”

The caliber is up to 6.5 meters, so can it fit into the rocket?

Since the large-diameter is so great, it seems reasonable for Webb, as the latest flagship space telescope, to go straight to the largest-diameter in the history of space telescopes!

The aperture of the Webb Telescope is 6.5 meters, which is more than 2.5 times larger than the 2.4 meters of the predecessor Hubble Space Telescope. The main reason why the Hubble telescope’s lens diameter was designed to be 2.4 meters was that if it had a larger diameter, it would not fit into the cargo compartment of the space shuttle. So the question is, the size of the Webb Space Telescope can be upgraded so much at a time. Is it because the rocket that launched Webb is much larger than the space shuttle at the time?

The size of the primary mirror of the Hubble telescope and the Webb telescope is compared. Photo/NASA

the answer is negative. In fact, there is currently no rocket in the world that can fit a lens with a diameter of 6.5 meters! Moreover, if a new rocket is specifically designed for launching Webb, the budget and schedule of the project will definitely be greatly improved and delayed, and the gains outweigh the losses. However, the mountains don’t turn around, and the road doesn’t turn and people turn around. Maybe the rocket cannot be changed for the telescope, but maybe we can think about it from another angle and let the telescope adapt to the rocket!

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Imagine if you have a sum of money and want to buy a bicycle, so that you can drive and travel with a bicycle in the future, but you find that the trunk of your car cannot fit an ordinary bicycle. What would you do? I believe that after this time, it would be a more reasonable option to choose a folding bicycle than to buy a new big bike directly. The engineers who designed JWST also adopted this strategy, designing the entire Webb telescope into a “foldable” type. The primary mirror, the secondary mirror bracket, the sunshade and other mechanisms can all be stowed and reduced in size, allowing the Webb telescope to be able to It is stuffed into a rocket fairing with quite limited space, and after launching into space, it will automatically unfold into an operational state step by step.

Figure: The Webb telescope folded into the fairing of the Arion 5 rocket. Photo/ArianeSpace, NASA, ESA
Webb telescope deployment process

Special design of Webb telescope

The most obvious feature of the Webb telescope is a huge main reflector with a diameter of 6.5 meters composed of 18 hexagonal golden lenses. Unlike the Hubble telescope, it does not have a barrel structure, but directly exposes the primary and secondary mirrors of the telescope to maintain the structure in the form of a bracket. And such designs are actually quite common in large observatories on Earth.

The primary mirror of the Webb telescope in the clean room, at this time the secondary mirror is folded. Photo/NASA

Light from celestial bodies in the universe will reflect through the primary and secondary mirrors and enter the black cone-shaped structure in the center of the primary mirror. There are some lens groups in this structure, which will further guide the light to the camera and spectrometer behind. The Webb Telescope is equipped with a number of different cameras and spectrometers, and each has a suitable observation target, providing important research data for astronomers in various fields.

And under the huge main mirror, the layer of silver “canvas” is the shading sail of the Webb telescope. It can block the light and heat radiation from the sun, the earth, and the moon for the telescope, so that the telescope can be in a dark and low-temperature excellent observation environment.

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According to scientists’ estimates, when the Webb telescope is operating in space, the surface-light side temperature of its sunshade sail can reach about 110 degrees Celsius, but the backlit surface where the telescope is located can maintain a low temperature of about minus 210 degrees Celsius. The lower the temperature, the less the effect of thermal noise on the observation instrument. Such a low temperature environment is very important for infrared telescopes.

The shading sail of the Webb Telescope divides the telescope into two parts: the surface side and the backlit side, and the body of the telescope will be on the dark and low-temperature backlit side for a long time. Photo/ NASA

The backlit side of the sunshade sail provides the dark and low temperature operating environment required by the telescope and camera, but not all equipment needs such conditions. For example, solar panels that provide electricity require plenty of sunlight to operate. At the same time, there are also some devices that generate heat by themselves, such as small rocket engines and fuel used to maintain orbit stability, reaction wheels that control the pointing of the telescope, and so on. These devices are also set on the light-facing side of the sunshade sail, so that the sunshade sail can also insulate the heat generated by these devices by the way and avoid interfering with the observation of the telescope. The placement of different devices on the Webb telescope can be said to be based on what is needed, which is quite ingenious.

On the surface side of the Webb telescope, there are solar panels, communication antennas, rocket engines and other equipment. Photo/ NASA

For example, the first part of this series of articles: “Why does NASA send it to space even if it throws a big coin?” According to the introduction, sending infrared telescopes into space can bring many benefits and research potentials. However, designing and building such scientific instruments is by no means an easy task. James. The Webb Space Telescope can be said to be a collection of top science, engineering and technology, as well as the crystallization of the joint efforts of many people, and it is also expected that it will bring abundant observational data and results in the future.

Further reading

Reference

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