[GraemeL]

The far side of the moon isn't dark. It has the same day/night cycle that causes the phases of the moon visible from Earth.

On the other hand, it is shielded from radio emissions from Earth and might be a good place for a radio-telescope.

[aflag]

Surely it's darker than the earth facing side. I'm no rocket scientist, but I reckon at least some of the light the sun sends our way gets reflected. From experience, a moonless night sure feels darker than when the full moon is up.

[KineticLensman]

The amount of reflected Earthlight would depend on your location on the Moon's nearside and the phase of the Earth.

The corollary of one side of the Moon always facing the Earth is that the Earth hangs in about the same place in the sky for any given point on the Moon's surface. Over a period of (earth) days the Earth's position would slightly but visibly oscillate due to libration [0]. This might slightly vary the incoming Earthlight (analogous to the Moon's illumination at the horizon vs. at the Zenith).

A much bigger effect, however, would be due to the phase of the Earth. As the Moon orbits it will sometimes be between the Earth and the Sun, sometimes further away. On the sun-side, the Earth would be more 'full' and thus there would be more light to reflect. This would significantly vary the incoming light.

[0] https://en.wikipedia.org/wiki/Libration

[philwelch]

A "full Earth" would coincide with the sun fully illuminating the far side of the Moon, and a "new Earth" would coincide with the sun fully illuminating the near side of the Moon, so wouldn't you only get maximum darkness anywhere on the Moon during a lunar eclipse?

However, counterintuitively, I think you'd be better off with a full Earth and no direct sunlight than a new Earth and full direct sunlight, making the near side a slightly better place for an optical telescope. The near side of the moon even has the opportunity to experience eclipses.

[GraemeL]

You're probably correct, but I also have know idea how much of a difference it would make.

One other advantage would also be the lack of atmospheric scattering, but they have made huge strides with adaptive optics so it might not be worth the extra cost of getting all the components to the Moon and assembling them.

You would also be limited by how close you could point the telescope to the Sun when it's in the sky. I think Hubble isn't allowed to point closer than 50 degrees for fear of damaging its optics/sensors.