Why is capturing meaningful signal from extremely distant objects difficult despite large mirrors?
They appear as tiny points occupying a minuscule angular area.
When observing objects millions or billions of light-years distant, two combined effects make signal collection difficult. First, due to the immense distance, the object subtends an extremely small angular size in the sky—it appears as a virtually dimensionless point source. Second, the light reaching the telescope is inherently faint because of distance attenuation. Even with very large mirrors designed to collect a greater amount of this faint light over a wide aperture, obtaining enough photons to build a signal strong enough for meaningful spectral or photometric analysis requires extremely long exposure times. Furthermore, the resulting image remains only a point, lacking visible structure.
