# Is there a difference between the image captured by a big telescope and a small telescope at several points in spacetime?

Suppose we build a big reflector telescope, with its mirror being the size of a city. That telescope gathers a bunch of photons at a given instant and creates an image of Jupiter. Now, the big mirror can be thought of as being made up of several smaller mirrors, the size of city blocks, for an example. If instead of creating an image which consists of light from several points in space (the big mirror consisting of hundreds of "city block" mirrors) at the same point in time we were to create an image consisting of light from several points in space at different points in time (moving one single cheaper "city block" mirror across the entire city area and capturing images at different times), would those images be of comparable quality? I understand that the second type of image would be asynchronous.

As long as you keep track of the phase information of the light, you can computationaly reconstruct the image from the separate ones. Indeed this is how some radio-telescope systems work. The signals from the dishes on different continents are recorded on tape, and, because the huge amount of data involved, until recently some tapes were shipped by air to a central location for reconstruction.

• Very interesting, thank you. Would this be too hard for an amateur to do with visible light? Commented Dec 3, 2020 at 9:43
• I'm afraid that it is is hard for professionals to do with light. Optical interferometer telescopes have to be coupled in real-time and, althogh they have been attempted, I don't think there are any operating at the moment.. Commented Dec 3, 2020 at 12:43

Here is another question. You have 1 night of observing time with a big telescope. How faint of an object can you image? Bigger mirror = fainter object. This is the primary reason people want big mirrors.

There is a limit to how big a piece of glass you can make and polish into a shape that is accurate to a wavelength of light. The largest so far are 8.2 meters across.

These days much bigger telescopes are being done by making the mirror in pieces. The big challenge is to hold the pieces together precisely enough that they form a single shape with no error bigger than a wavelength of light.

If you have gone to the trouble of making a big mirror out of separate pieces, you really don't point them in different directions to image different objects. It would make more sense to build multiple smaller telescopes for that.

You might look at the same place in the sky for multiple nights to gather more light. This would create a single image.

But you would not combine light from different areas in the sky into one image. It would be like a double exposure photograph.

• Thanks for replying! What I meant is that you would use the individual mirror to image the same object from different points in space at different times, essentially recreating a telescope as large as you'd like with minimal expenses. Commented Dec 3, 2020 at 9:45