Eli5 James Webb telescope


If we can use it to see farther than we have ever seen before why haven’t they got a really good close up of the nearest Solar system outside of our own?

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Because the reason that Webb can see further is more down to the type of light it picks up, not how big it is. Hubble for example, sees mainly in visible light. If we wanted to get a better picture of a nearby star we’d need a telescope with a mirror much bigger than Hubble’s. While Webb’s mirror is bigger, its big difference is that it can see light that is way down in the infrared range. (this is why its instruments need to be kept so cold) To way oversimplify, light coming from distant galaxies is “red-shifted” meaning its wavelength moves further into the infrared range the longer it’s been traveling, so Webb was specifically designed to look at this redshifted light.

It’s not just a camera like your phone, it detects a certain type of light, which is coming from things far away. It can’t zoom in to show a rock on the moon or something

Space is *really* big, and objects (even planets) are *really* small in comparison.

JWST has an angular resolution (how fine of a detail it can see) of 0.1 arcseconds. At 4.2 light-years away, Proxima B, the nearest exoplanet, takes up 0.0001 arcseconds of image. Both Proxima B and the star it orbits would easily fit in just a single pixel of JWST.

There are three big things to consider when it comes to astronomy:

* Resolution

* Intensity

* Wavelength/frequently

For resolution, this is dependent on both the telescope and the camera. The bigger the telescope, the better the resolution, but only if the camera can keep up. A better resolution means you can see things with smaller *angular* size. To give an example of angular sizes, stars are much bigger than the planets in our solar system, but they are further away and end up appearing smaller.

The intensity is about how much light there is, and this is important so that we can distinguish it from noise. Having a bigger mirror means you collect more light and so get a better signal to noise ratio.

Then there is the wavelength of light, telescopes are made to observe a certain range of light.

As the universe expands, light traveling through it gets redshifted to a longer wavelength, making it easier to observe distant things in longer wavelengths. The amount of light also decreases with distance, as the light is getting more spread out. This means that the JWST is good at observing distant galaxies because it observers in a longer wavelength than Hubble and has a big mirror.

To get details of other star systems though requires an even bigger mirror, because they have a very small angular size.

There’s 2 things that affect what you can make out with a telescope just by the nature of optics. The size of the telescope D, and the wavelength of light you are using λ. These limit how far apart in your field of view you can differentiate 2 objects θ (which is an angular measurement in radians). For reference, the full moon takes up about half a degree of space in the night sky.

A better way to explain θ would be tan(θ)=size of target/distance to target. Since we are working with very small angles, we can approximate that to be θ=size/distance

The formula is sin(θ)=1.22λ/D and again for very small angles we can approximate θ=1.22λ/D

A small θ means a high resolution. The 1.22 is when the first rings of “blurriness” start. See the double slit experiment as to why that happens.

This means that for larger λ (longer wavelengths) θ gets larger. For larger D (larger telescopes) θ gets smaller.

We can’t control how far away things are or what kind of light they emit, so we are very limited in the kind of objects we can see.

For example, let’s say Jupiter is orbiting Proxima Centauri, 4.24 ly away. In order to be able to visually tell it is separate from the star, we would need a telescope about 40m across (ignoring the fact that the star would drown out any light coming from Jupiter). In order to actually be able to see the planet in isolation we would need one 430m across. (Keep in mind this is for visible light and I did a lot of rounding)

Another problem is wavelength. The JWST is designed to look into deep space, but since space is constantly expanding, that means the light from distant objects is redshifted. This means it has a longer wavelength than when it was created, so the JWST is designed to pick up those longer wavelengths, which aren’t useful for observing nearby stars which are in visible light ranges.