What limits the ability to directly image distant objects with telescopes?

[Holocene]

Why can't a telescope be built that has the capability of directly imaging extremely far away objects...say a planet in another galaxy?

Is light collection the issue?

 

[mgb_phys]

There are two limits.
1, Resolution - this depends only the diameter of the mirror ( assuming you are in space - on Earth you are limited by the atmosphere). The largest current telescopes would resolve the largest nearby stars.
To resolve stars in nearby galaxies would require telescope appertures the size of the Earth.

2, Signal limit - this depends on the area of the telescope and the noise in the detector * the length of the observation. We can detect quasars (very bright galaxies) out to pretty much the edge of the observable universe. The only limit for individual stars would be the size of the telescope and how long you are prepared to observe for.

 

[Parlyne]

For ground-based telescopes, there is an additional issue in seeing faint objects. The sky itself is not totally dark, due to the diffusion of local and astronomical light sources. So, if you're integrating for a long time to try to see an extremely faint object, tou'll evetually end up with an image that's totally washed out.

 

[mgb_phys]

Even for space based telescopes the ultimate limit is probably zodiacal light. This is light reflecting off dust left arounf in the solar system.