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Why do telescopes use mirrors that simply reflect photons, when they instead could be covered with large sensors to register them? Reflection is all good and well, all thanks to silver and beryllium for that. But wouldn't it be better to electronically register the photons directly instead of after having them bounce around between stupid mirrors? Would any data be lost in a pure CCD-telescope without any mirrors or lenses?

Couldn't a large wired CCD light sensor send the detected signals further in a smarter way than a stupid physically reflecting surface can do? It's the same photons and the telescope itself doesn't generate any new information about the distant galaxies it reflects. Why physically bend mirrors for adaptive optics, instead of bending the raw binary data with an algorithm for the same effect?

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    $\begingroup$ "the telescope itself doesn't generate any new information about the distant galaxies it reflects" - bear in mind a CCD on its own is just a solar panel - it measures light intensity, but not where the light is coming from. It needs focused light, this is really basic optics. Read the answer from DJohnM again as I think he hit the nail on the head. $\endgroup$
    – Andy
    Mar 30, 2016 at 10:01
  • $\begingroup$ By the way there is a CCD that detects direction - a plenoptic camera (plenoptic.info/index.html) but note it uses microlenses on the CCD itself so lenses are still involved. (Plus a whole load of other stuff.) $\endgroup$
    – Andy
    Mar 30, 2016 at 10:06
  • $\begingroup$ To sum it up very quickly: You need to focus the light in order to make an image that the CCD can see. CCDs are like our retinas; they only capture light. But (animal/human) eyes also have a lens, which focuses the light onto the retina. That’s the role of the mirrors in telescopes. $\endgroup$
    – Pierre Paquette
    Mar 9, 2021 at 1:08

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The CCD has no way of recording the direction, the point in the sky, from which a photon is coming.

Say you point your mirror-less telescope at the Moon. Every point on the moon's surface would be reflecting photons onto every part of the CCD at the same time.

You've just created an expensive, sensitive, ambient light meter. There would be no image information whatsoever.

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    $\begingroup$ I still don't get it why stupid silicon in glass as a middleman is better for the photons than the smart silicon in electronics in which it finally ends up in before we can interpret it. Let the CCD have the same shape as a primary mirror would've had, if that helps, and register the photons electronically instead of simply bouncing them. $\endgroup$
    – LocalFluff
    Mar 30, 2016 at 5:50
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    $\begingroup$ It doesn't help.... And the "stupid silicon" is smart enough to receive myriad photons from all over the object and send each one to exactly the right spot on the film or CCD chip to form an image. $\endgroup$
    – DJohnM
    Mar 30, 2016 at 6:38
  • $\begingroup$ The photons are sent from the celestial object. Why re-send them? Is the data loss from reflection maybe too small to be a priority problem? $\endgroup$
    – LocalFluff
    Mar 30, 2016 at 14:29
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    $\begingroup$ The photons from various objects across the field of view are effectively random until the wavefronts are collected and sorted by a "computer" which realigns all the waves and puts them to a flat plane - optics. (and reflection losses are quite small these days.) $\endgroup$
    – Andy
    Mar 30, 2016 at 15:30
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To answer your question, we need to first show the job each mirror is doing.

First up, the Newtonian (lovingly called the "Newt", and invented by Sir Ike Newton):

https://en.m.wikipedia.org/wiki/Reflecting_telescope#/media/File%3ANewtonian_telescope2.svg

Two mirrors in this design, not surprisingly labeled as primary and secondary.

The job of the primary mirror is NOT merely to reflect light, but to concentrate the diffuse photons onto a much smaller point. This makes really dim objects brighter, and is the first step in magnification. (Further magnification is done by the eyepiece, which is similar to a small refracting telescope.)

In the case of the Newt, the secondary mirror reflects the now concentrated photons to a more convenient point for viewing. Without the secondary mirror your head would get in the way of the view. A secondary mirror is not necessary, and in fact many telescopes will place instruments, such as CCD's, at this "prime focus" point.

In the case of the Hubble Space Telescope, the secondary mirror reflects the concentrated photons to the scope's instruments, where they can work their magic.

In all reflecting telescope designs the primary mirror uses the laws of physics to give the end user, whether it's the human eye, or research gear, as many concentrated photons as possible, maximizing what we can see/detect. The bigger the primary mirror, the more concentrated the photons, and the more we have to work with.

When it comes to seeing what we call "the dim fuzzies", bigger IS better!

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    $\begingroup$ Bigger is better, but couldn't semi-intelligent electronics de-diffuse the photons better than a stupid geometrically reflective surface can? How is it better that a photon from a distant galaxy hits a silicon atom in a mirror before it hits, and is electronically recorded, on a silicon atom on a CCD or similar light sensitive device? Just like all the points on a mirror can produce a focus, I suppose that with some math (not my job), a CCD array could do that too. $\endgroup$
    – LocalFluff
    Mar 29, 2016 at 20:09
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    $\begingroup$ There's a lot more to astronomy observations than just recording photons. For example, look at prisms, grisms, gratings, and filters. A CCD just records photons, without noting the wavelength. $\endgroup$
    – Donald.McLean
    Mar 29, 2016 at 20:13
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    $\begingroup$ @Donald.McLean Why not capture all the photons directly, why play ping pong with them? $\endgroup$
    – LocalFluff
    Mar 29, 2016 at 20:29
  • $\begingroup$ For much the same reason airplanes take off with a head wind; to give the technology a free boost from Mother Nature. Further, our level of CCD tech is more like a Piper Cub, which can use all the help it can get, than like a jet fighter. Often what we are working on is already on the edge of what we can detect; CCD's by their lonesome are incapable of doing what you propose without the optical assist. $\endgroup$
    – SkyGuide
    Mar 29, 2016 at 21:52
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    $\begingroup$ The math and physics of how and why mirrors and lenses work is actually pretty advanced stuff - MUCH harder to explain than to use. On the flip side, in the radio bands, astronomers have to do all the math that the lenses and mirrors do pretty much for free. The universe is a weird place. $\endgroup$
    – Donald.McLean
    Mar 30, 2016 at 12:24
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Since neither the word "phase" nor "interference" is mentioned in any other answer here, I'll approach it from that direction.

In this answer I said

In an imaging optical telescope (or any imaging system including eyes) every pixel is illuminated simultaneously and directly by all areas of the aperture. From a given point in the distance a telescope will (try to) preserve the phase of all paths reaching the pixel so that the resulting intensity corresponds to the incoming power. This allows the system to obtain the best resolution.

What that means is that the curved mirrors of a reflecting telescope are designed so that all the paths from a distant object in a given direction reach a pixel in phase. The paths from any other point in the sky reach the pixel completely out of phase and cancel to zero. That's why each pixel corresponds to a given direction.

Without those curved mirrors you can't make an image because the CCD's pixels convert the wave's information to intensity only, and loose all phase information. Without any information on phase, there is no way to combine the signals in each pixel to reconstruct the incident wave.

Radio telescope arrays can be though of like your pixels, but those signals are digitized to a bit stream that maintains phase information. The correlator computer takes all those phases and reconstructs the image. If each dish in the array was equipped with a bolometer instead of an RF amplifier and baseband converter, phase information would be lost and no matter how large your baseline you wouldn't have interference.

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  • $\begingroup$ Even though the phase and interference do matter for many things I think this answer is incorrect when it comes to to need of a mirror or lens in a telescope. The mirror or lens is used to guide all photons that originate from single distant source to single pixel on the imaging sensor. The phase of the photon is not needed for imaging but the direction it's going. Even if you had a sensor that could measure phase and frequency of any given photon hitting on it you would still have no idea from which direction that single photon originated from. $\endgroup$
    – Mikko Rantalainen
    Aug 25, 2022 at 15:32
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    $\begingroup$ If you had a sensor that logically did double slit experiment for every incoming photon and could measure the interference pattern caused by that single photon, it might be possible to measure the direction that photon is coming from. I don't think there's a practical way to measure the interference pattern from a single photon, though, let alone to do that for many enough pixels to actually build a telescope out of such sensors. $\endgroup$
    – Mikko Rantalainen
    Aug 25, 2022 at 15:35
  • $\begingroup$ @MikkoRantalainen Interesting! That sounds a bit like a Shack–Hartmann wavefront sensor (see also Thorlabs for example) $\endgroup$
    – uhoh
    Aug 25, 2022 at 21:23
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    $\begingroup$ Yeah, Shack–Hartmann wavefront sensor could do that but the current implementation would lose about 99% of all the incoming light to the mask which is not great for astronomy purposes where photon count is slow to start with. And if you don't use mask, you just replaced your big mirror with millions and millions of tiny lenses. $\endgroup$
    – Mikko Rantalainen
    Aug 26, 2022 at 7:17
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I you just set out a CCD in a room, each pixel will record photons from every direction. With this, you will be able to record the amount of ambient light, but you won't get an image of the room.

Now if you want to have an image, for each pixel, all the photons have to be coming from the same direction. And for each direction, all the photons coming from that direction have to fall on the same pixel. For that to happen, you can use a camera obscura.

But if you only use the photons coming from one direction for each pixel, you won't be collecting much light, so your image will be rather dark. This is ok if you are taking a picture of a sunny landscape, but if you want to take a picture of stars, you need to collect all the light you can get.

This is where the telescope comes in ! A telescope will collect all of the photons from all of the directions, and reflect them in such a way that all the photons coming from a certain direction will end up on the same pixel. This way, you can have an image that is neither blurry nor dark.

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  • $\begingroup$ +1 This demonstration is 99.44% correct (pardon the oblique reference). There are the whole camera that can see around corners, and coded mask technologies that don't require imaging optics, but it's probably beyond the scope of the current question. $\endgroup$
    – uhoh
    Feb 21, 2020 at 3:00
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I actually found a concept of 2D easy-to-scale telescope some time ago (here's the link). I guess we will slowly abandon refractor telescopes, for, as I understand, we are pushing them to their limits right now and it is getting really hard to make a bigger one (because of how hard it is to make a sufficiently sized mirror of the quality needed). BUT it worth noting, that I am not an expert in any way, so I suggest someone, who knows more about the topic, edit this answer.

EDIT: There's a really good point in the DJohnM's answer, so I thought I'll add that the thing I linked here (SPIDER) isn't just a big 2D array of CCDs; it actually does have a tiny lense over each of the detectors and each of those measures light in a bunch of different wavelength, so it can preserve information about direction and wavelength of light. So the answer to initial question is no, we can't just build a CCD array instead of full-sized telescope, but the idea to make telescopes scalable in two dimensions instead of three seems to be a good one and threre are people working on it.

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Electromagnetic signal from a distant object arriving to your telescope (or your eye) is a Fourier transform of the image of this object. Not the image itself [ref. Diffraction in far field in any book on optics]. The optics in the telescope (or the lens in your eye) is performing the inverse Fourier transform so you can get the image again. After this step CCDs are placed.

Yes. You can put detectors to record the Fourier transform and later invert it on a computer. For this you need to record the amplitude and the phase of the signal. This is how long distance interferometry works aka VLBI, Event Horizon Telescope and others. But they use special detectors not ordinary CCDs.

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What you propose could work. However you would still need lenses. What you could do is cover the CCD matrix with a mask that has tiny little holes in it. Basically this would allow the CCD elements to be affected by the "pinhole effect" thus facilitating a lens.

Problem with that is that you would lose some light as the photons hit the non-transmissive areas of the pinhole mask. And finally, the biggest drawback would be the loss in resolution for the size of the CCD array versus a lens based analog system.

If we could create superconducting CCD arrays then it would be way more efficient than anything we have now available to the public. Especially when it comes to light gathering, however still not 100% perfect when you take resolution limitations into account.

The day we can make CCD elements millions of times smaller than what we have now. Then your idea will probably work to some extent. Although, I am sure the designers of such future technology have already thought of it before you.

I think that by the time that happens, we will have a technology far more superior. Such as creating an energy field using the Bose-Einstein Condensate effect that can suspend photons in a field. This would allow us to analyse incoming photons suspended in a matrix, and we scan the image at the resolution of a photon.

NASA has already proven the Bose-Einstein Condensate theory, so it will not be long now. With that type of resolution, we might be able to observe something as small as a fly cleaning itself on the surface of mars in Hi definition. The Bose-Einstein Condensate effect basically causes quantum matter to act as if it was photons/waves, while the universe behaves relatively as a superconductor.

In other words, as electrons can travel super fast through a superconductor, the matter affected by the Bose-Einstein Effect stands almost still relative to the Universe, while the Universe travels relatively super fast around such quantum matter affected by the Bose-Einstein condensate field.

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    $\begingroup$ Hello and welcome to this site! We tend to prefer sourced claims to opinions. Perhaps you could back some of your statements with a source? $\endgroup$
    – usernumber
    Mar 8, 2021 at 15:55
  • $\begingroup$ Maximum amount of information that you could get from imaginary fly from the surface of the Mars would be limited by the total amount of photons radiated from that fly. As the number of all the photons ever radiated by said fly is finite and the smallish detector on the surface of the Earth is a tiny tiny fraction of all possible directions those photons could go, there will be very limited photon count on the sensor even if it detected every photon ever emitted from that fly. As such, you wouldn't get high definition image due lack of data. $\endgroup$
    – Mikko Rantalainen
    Aug 25, 2022 at 16:01

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