eli5: Why can’t you combine a telescope and microscope?

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This is one of those things that I know doesn’t work, but I don’t know the exact reason WHY it doesn’t work. I remember asking a friend in 7th grade biology why I can’t just put a telescope up to a microscope to see even more detail, but he just laughed and no one I’ve ever asked has given me an answer

In: Physics
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Telescopes are generally limited by the size and perfection of their optics rather than what you can see. You could tack on some optics to look at a narrower portion of the telescope, but the narrow portion wouldn’t contain any more detail. Kinda like how digital zoom doesn’t increase the number of pixels, it just crops away the outside of the image and stretches the center to fill that space.

A microscope relies on photons illuminating a small area to show an image of the really small and minuscule.
A telescope relies on as many photons as possible over a large area, as light tends to scatter over large distances relative to their source.
Also exposure is important as to gather as much light as possible over a given amount of time so the photons can in essence paint a picture on whatever medium your using, be it digital or otherwise.

TLDR: You can. And astronomers do. Sometimes.

[https://en.wikipedia.org/wiki/Airy_disk](https://en.wikipedia.org/wiki/Airy_disk) If you point a (circular) telescope at a star – basically a point source of light. The pattern at the back end of the telescope would be an airy disk. If you look at some part of this pattern with a microscope it wouldn’t really help know anything new. And if you want a smaller airy disk you build a telescope with a bigger aperture (circular opening).

But not everything is a point source. So some people do put a *microscope* at the backend. Maybe you want to use a small telescope to look at Jupiter for instance. The magnification of the telescope is set by its focal length. You can build telescopes with different focal lengths. But if you want a little bit more when you have an existing telescope you could add a Barlow lens to your eyepiece or in front of your camera (2x, 3x, 4x, etc.). [https://lovethenightsky.com/what-is-a-barlow-lens/](https://lovethenightsky.com/what-is-a-barlow-lens/) It’s perhaps not technically a microscope by the standard definition but it is optics and is exactly what you are thinking of.

So yes, theoretically the two are similar, and yes, you might be able to change or add a lens on a microscope to focus on something far away, or change or add a lens on a telescope to focus on something close, but there is a big difference to point out. Most telescopes have pretty big lenses, or the overall diameter. This is because they need to capture a ton of light from really far away.

Now, let’s try to focus that onto something really close. To focus that lens on something just a mm or less away, the light from the far edge of the lens is going to be close to sideways as it trys to focus on the object. While this is theoretically possible, it’s probably going to cause a weird looking image as the image might look like a inverted fish eye lens. Realistically though, the diameter is probably too wide for a lens to focus on something that close with just 1 change or additional lens.

On the flip side, a microscope lens is too small to capture enough light from objects far away. Maybe you could use one to look at a really bright object, like the sun, but in general you need to capture much more light to be visible to your eye, and even most camera sensors. Basically you are trying to look at something super far away through a pin hole. It’s going to be really difficult to focus on that thing really far away when you don’t have enough information (via light) to see it.