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The primary mirror of the HST has a diameter of 94.5 in (240 cm), and the optics of the telescope are designed so that, theoretically, when making a visible-light observation, the telescope can resolve astronomical objects that are at an angular distance of 0.05 arcsecond apart (see Angle).
After the HST was launched, scientists discovered that its primary mirror had a systematic aberration, the result of a manufacturing error.
Now that the HST has the resolving power it was designed to have, it is capable of performing research such as that necessary to significantly improve the calculation of the rate at which galaxies are receding from the Milky Way as a function of their distances.