how do telescopes work? why does a bigger mirror mean deeper images? how can we focus a telescope on a specific point in the sky?


i understand how light and photography works, but it baffles me how telescopes can be so precise, and how they can ”collect” light from so far away. Idk please help me understand them a bit better

In: 5

A bigger primary mirror means more light is collected. This lets us see dimmer objects, and the further away something is the dimmer it appears.

Additionally a larger mirror allows us to distinguish smaller features; large telescopes have better angular resolution. This is because of diffraction and is beyond my ability to explain. Suffice to say it is a fundamental property of light. The further away something is the less of the sky it takes up, so the better the resolution you need to see useful things about it.

They focus on a particular point on the sky by moving to look at that point. Imagine a very zoomed in camera that you point at the region of interest. They are focused at infinity because that is where the stars, galaxies etc are (sort of).

Just imagine light as billions of photons sent out in all directions at once. When you “see” something, that means some of those photons have reached your detection equipment, i.e. your eyes.

If you see something “very far away”, that just means that the photons you picked up have travelled for a looooooong time to reach you, but they are still the same as the photons from short distance objects. However, since you are much farther away from the source, you probably only picked up very few of these photons compared to if you were very close to it (remember, it sent out photons in all directions, but still just a finite amount). So while the individual photons are the same as others, you have much much fewer reaching your detection equipment.

Now with a large telescope, you simply increase the size of your detection equipment, so that you can now pick up not just the photons which hit your eye, but also those that would have flown by your eyes (i.e. you would have missed them). So all the telescope is doing is increasing the chance of catching photons from that far away object, and thus make it appear brighter in the picture.

A telescope is just like a camera lens with a large diameter that can collect a lot of light and can have quite a high level of magnification.

You can if you like attach a regular DSLR on the end of a telescope and capture images that way, it is not an uncommon thing to do for amateurs even if specialized astronomy cameras are better for many tasks. One common feature they have that a DSLR does not have is a cooling system to reduce the sensor temperature so noise gets reduced. Astronomy cameras are also more like webcams in the usage they have very few if any controls on them and no internal storage so you control them remotely with a computer. You tend to control DSLR remotely too because touching them results in vibrations.


There is one fundamental difference between most telescopes and normal camera lenses. Most telescopes are Reflecting telescopes so the direction of light is changed by curved mirrors. Refracting telescopes that used curved glass lenses like normal camera lenses exist to but are less common.

Refracting telescopes have some drawbacks like that large-diameter lenses are heavy and only supported from the side so they start to deform from their own light. Light also not to pass through the glass that is not perfectly transparent. The curved mirror on the reflecting telescope has the reflecting material on the top so no light passes through glass. You can also support the mirror from the back and not just the sides.

Collecting light from far away is not a problem because optics do not care at all how far light has traveled until reached it. You can with your naked eye see the core of the Andromeda galaxy that is 2.5 million lightyears away. Stuff that is far away is very faint so you need a large diameter aperture to collect a lot of light and long time exposures to collect enough of it. Exposures that are hours in length are not uncommon even if they tend to break down to multiple shorter exposures perhaps 30 seconds each.

Focusing on something far away is not a problem, any camera can do that. Normal camera lenses reach often have a focal length setting on infinite because you quickly reach the point where the depth of field extends to that length. It might be that everything from 5 meters to infinity is in focus on a camera lens. If you can take an image of the moon in focus you will have the setting that is the same or extremely close to something 14 billion lightyears away.

Lots of stuff in the sky is also larger than you expect and the magnification you need is not extreme even compared to good camera lenses. look at [the moon and andromeda galaxy to scale.]( The andromeda galaxy is the second larger object in the sky after our own galactic core, it is just very faint and you can just see the core of it as a white blob with your naked eye. To capture an image of it you need to have long exposure and capture a lot of light.

So telescopes are not that different from camera lenses. The primary problem is to capture enough light so long time exposure and large lens diameter. If you understand photography the problem is the same as taking photographers at night. You like to have a lens with a large aperture and long exposure to get a good images.

Imagine someone is spraying a hose. You’re trying to collect water. You take your cup and hold it near the nozzle and it fills up really quickly. Great!

Now imagine you can’t get too close to them because there’s a wall (or you’re trying to be socially distant or something). You hold your cup out, but it barely gets any water in it. The hose is spraying everywhere, so the same size of cup can’t collect as much water from far away. One solution is to stay there holding the cup for longer. That’ll work, but it means you can do anything else for a long time. Another solution is to get a wider cup. Maybe you get a funnel and put that in your cup. This can collect water from a wider area and focus it down to your cup.

This is the basis for bigger telescopes. The light from distant objects gets weaker as it gets more spread out. To compensate and collect enough light, we either need to hold the telescope in the one spot for longer or use a bigger mirror/lens to funnel in more light.