So the JW telescope can see billions of lightyears into the distance/past and see countless galaxies in the focal point of a grain of sand, but when it’s aimed at at Uranus or a closer planet, the photos are very low quality.
Why can’t a telescope that powerful capture a good image inside our own solar system?
I understand it sees different wavelengths to typical telescopes but why can’t it take a sharp photo of the light emitting from the planet that’s not blurry?
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The JWST is made to take images of things very far away. So it have gyroscopes that can keep a very accurate attitude when pointing at distant things by finely adjusting the direction. The problem is that these gyroscopes can only do the fine adjustments needed for imagery when moving incredibly slowly. This is fine for things that is very far away, but not for planets in our own solar system. Uranus is basically moving too fast for the JWST. If they take a normal image they get motion blur. So they take short duration photos, essentially “sports mode”, but these settings give more blurry photos.
Planets are much smaller than distant galaxies. Yes, they’re also closer but, if you do the maths, Uranus isn’t close enough for pin-sharp details to be resolved (to your satisfaction) even by JWST. When the numbers are all so literally astronomical, human intuition fails and maths becomes necessary to make comparisons like these.
The answer is simple: the planets are much smaller than distant galaxies, to a degree that their size is a larger factor than distance in determining how large they appear in a telescope. Planetary diameters typically measure in the thousands of kilometers, whereas objects like galaxies or nebulae are measured in light-years—and a single light-year is almost ten trillion kilometers. Nearby planets may be thousands or millions of times closer than distant galaxies, but those galaxies are *trillions* of times larger than planets, meaning the size factor dominates.
Telescopes aren’t designed to magnify so much as capture as much light as possible in order to brighten dim phenomena to a point at which we can usefully observe them. Telescopes are like light buckets: the wider the bucket, the more light it captures in a given time period.
A telescope’s primary optical element (usually a mirror, but can also be a lens) determines the instrument’s magnification. The telescope’s focal length is chosen based on the class of targets it will be used to observe, then it’s mirror/lens is constructed to have the greatest possible areal.
In terms of relative apparent size, planets cover a miniscule portion of the sky compared to more distant (but much larger) deep-space phenomena. So even though planets are physically closer and far brighter than deep space objects, we simply cannot resolve their details past a certain point because the resolution needed to display them is finer than the wavelength of light itself by the time it reaches the telescope.
We don’t use telescopes to make deep space phenomena bigger; we use telescopes to make deep space phenomena *brighter*, or at least less dim.
Generally the most detailed images we have of bodies in the solar system are from spacecraft that travelled very close to them. It’s not really possible to beat them with telescopes on or near earth, similar to how it would be difficult for a spy satellite to get a better photo of your house than you can get with a cheap camera. However, it can still be useful to observe the planets with telescopes, since it’s much easier and cheaper than sending a dedicated probe, and there are always things that haven’t been done with previous missions, e.g. different frequencies and spectroscopy. JWST isn’t capable of observing the inner planets because they’re too close to the Sun from its perspective so it can’t see them from behind its sunshield. But it is being used to observe various things further out in the solar system.
> I understand it sees different wavelengths to typical telescopes
That is part of it. Longer wavelengths of radiation inherently undergo stronger diffraction when they enter and pass through a telescope. So you need a bigger telescope to get the same level of sharpness in an infra red image than you would with an optical image.
Also bear in mind that producing images is only part of the purpose of telescopes like this. They also produce stuff like spectroscopy data that is very useful to astronomers but doesn’t really make for pretty pictures.
Next time you are holding binoculars, test them at a distance of just a few feet (a meter) away from your face. Look sort of blurry? That’s because the lenses were designed to look at things further away, not super close.
If the James Webb was designed to “zoom in” on things that are millions of light years away, then anything in our solar system is considered very close (relatively).
For this same reason, many quality cameras have a “macro” mode so they can take pictures of closer things. But it still has limits.
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