Observing the Moon
Section M.11.
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Resolution of the Hubble Space Telescope

I wrote an astronomy article on HST way before it was launched. I did some calculations which seemed to imply that you could count the nuts and bolts on an Apollo Lunar Rover. (At the very least count the wheels) Anyone want to check that calculation?

I am also very interested in seeing the calculations, because I am fairly sure that they are off by a few orders of magnitude.

The formula for the maximum optical resolution of a telescope is:

resolution = distance * wavelength / aperture_diameter

If one plugs in the Earth moon distance (400,000 km), the wavelength of visible light (500 nm) and the diameter of the Hubble (2.5 m) one gets a resolution of 80 m, just like in the recently released Hubble picture of the moon.

If Mars is at its minimum distance to Earth, it is 60 million km away, thus the Hubble's resolution on Mars is 150 times worse, that is 12 km. Mars has a diameter of 6000 km thus a high resolution image of Mars taken by the Hubble is 500 pixels across.

If the Hubble would be able to count the nuts and bolts of a Lunar Rover, say with a resolution of 1 cm, its resolution of Mars would be 1.5 m. That would be better than the resolution of Mars Global Surveyor!

If Hubble would be in a 400 km orbit and would look at the Earth its resolution would be a thousand times better than at the moon, i.e. 8 cm. Of course, the Earth's atmosphere would blur the view somewhat. The US Keyhole reconaissance satellites are very similar to the Hubble, thus their resolution of terrestrial targets is also less than a feet. Good enough to count the wheels on a car, but not the nuts and bolts.

Observing the Moon

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