This question is inspired by the NRO's Orion Satellites. These are surveillance satellites placed near geostationary altitude that measure radio emissions.

Importantly, they have been described as the "largest satellite in the world" (quotation cited from previously included link) thanks to their ~100 meter dish.

If this dish were instead configured to focus optical wavelengths, what spatial resolution could be achieved in images of the Earth's surface? How much of the dish would need to be reflective to collect useful images? (e.g. one solid 100m mirror or a matrix of smaller mirrors)

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    $\begingroup$ The resolution of the largest existing spy satellites is not diffraction-limited but limited by atmospheric distortion. The resolution limit is determined not be optical size but by the efficacy of your adaptive optics or deconvolution algorithms. $\endgroup$
    – antlersoft
    Aug 23 at 19:34
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    $\begingroup$ Thanks for your comment, @antlersoft. This makes sense for spy satellites in LEO or certain Molnyia orbits. Does it hold true for the sat I describe? i.e. a 100m dish at GSO (36,000 km from earth's surface)? This is a MUCH greater distance than most earth imaging satellites operate at. $\endgroup$
    – A McKelvy
    Aug 23 at 20:34
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    $\begingroup$ Worth noting that the largest optical telescope in the world is nearly an order of magnitude smaller than this. $\endgroup$
    – ikrase
    Aug 24 at 2:55
  • $\begingroup$ I did some googling on satellite resolution a while ago. Existing geostationary satellites have a resolution of 1-2 km. Low earth orbit ones can manage all the way down to 10 cm resolution. So this thing could probably do better than existing geostationary sattelites but I don't think it would come anywhere close to the resolution of low earth orbit satellites which are around 100 times closer to the earth. $\endgroup$
    – quarague
    Aug 24 at 11:14
  • $\begingroup$ @ikrase as usual you've piqued my interest How are the Extremely Large and Thirty Meter Telescopes coming along ("planned 2027") $\endgroup$
    – uhoh
    Aug 26 at 0:39

1 Answer 1


The resolution of an optical telescope can be calculated using the Rayleigh criterion: this is the theoretical limit: an optical system cannot be better than this.


θ is the angular resolution (the smallest angle between two objects at which they can be resolved as separate objects), λ is the wavelength of the observed light, D is the aperture of the telescope (basically, the diameter of the mirror).

if we pick λ is 500 nm (green, ~ in the middle of the visible spectrum), D is 100 m, we get $6.1E^{-9}$ radians.

Now we convert that angular size to a linear dimension on Earth's surface:


L is the distance, 36,000 km. This gets us a linear size of 0.21 meters. That's pretty close to the resolution of low-altitude spy satellites (KH-11: theoretical resolution of 0.06 m, in practice it's worse due to atmospheric distortion).

Then we get to the practical problems:

  1. The required accuracy of the mirror depends on the wavelengths you want to observe at. So the radio antenna on the Orion sat can have variations in shape on the order of centimeters, while an optical mirror has to be accurate to within tens of nm. For radio, you can get away with an umbrella-type unfolding mechanism with a mesh in between the ribs. An optical mirror has to be thick, accurately ground, and placed on a frame with actuators so you can adjust its curvature. The largest mirror in space is on JWST, at 6 m diameter.

  2. You could segment the mirror to make it easier to launch, but that adds lots of optical noise (diffraction spikes, ie light that reflects off the edges of each segment, this shows up as spikes if you use e.g. hexagonal mirror segments, or an Airy disk if you use circular segments). This reduces the resolution.

  3. To create useful images, most of the surface area has to be reflective. A radio antenna can use a mesh, if the holes in the mesh are smaller than the wavelength of the radio signal. For optical mirrors, it's not practical to make a mesh mirror. You can use a segmented mirror, see point 2.

  • $\begingroup$ Precisely what I was looking for. Thank you, Hobbes. $\endgroup$
    – A McKelvy
    Aug 24 at 13:09
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    $\begingroup$ Probably it was implied in your answer but I think it is still worth stating, the JWST mirror is segmented, it is made up of eighteen 1.3 meter mirror segments which were joined together after launch to create a 6 meter total mirror diameter. This results in the diffraction spikes that you mentioned. $\endgroup$ Aug 25 at 23:35

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