"Gather" may not be used in optics design but it is certainly used in astronomy, to help see dim objects far away.
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From: Caltech Astronomy
http://www.astro.caltech.edu/palomar/faq.html

So, why is a bigger telescope better? Because it gathers more light than a smaller telescope.
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From Astronomy Magazine:
http://www.astronomy.com/asy/default.aspx?c=a&id=787

light-gathering power

the ability of a telescope to collect light; the larger a telescope�s aperture, the greater its light-gathering power
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From Journal of the Royal Astronomical Society of Canada
http://articles.adsabs.harvard.edu//full/1954JRASC..48...96H/0000097.000.html

The primary advantage of a large telescope over a smaller one is in its larger light gathering power.
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From European Space Agency:
http://www.esa.int/esaSC/120370_index_2_m.html

JWST will have a primary mirror with a diameter of 6.5 metres - more than twice that of Hubble�s - giving it much more light-gathering capability.
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