Scientific Impact of Large Telescopes

The scientific impacts of telescopes worldwide have been compared on the basis of their contributions to (a) the 1000 most-cited astronomy papers published 1991-1998 (125 from each year) and (b) the 452 astronomy papers published in Nature during 1989-1998. Ground-based telescopes of the 1 and 2 m class account for ~5% of the citations to the top-cited papers; 4 m telescopes, 10%; Keck I/II, 4%; submillimeter and radio telescopes, 4%; HST, 8%; and other space telescopes, 23%. The remaining citations are mainly to theoretical and review papers. The strong showing by 1 and 2 m telescopes in the 1990s augurs well for the continued scientific impact of 4 m telescopes in the era of 8 m telescopes. The impact of individual ground-based optical telescopes is proportional to collecting area (and approximately proportional to capital cost). The impacts of the various 4 m telescopes are similar, with the CFHT leading in citation counts and WHT in Nature papers. HST has about 15 times the citation impact of a 4 m ground-based telescope but costs more than 100 times as much. Citation counts are proportional to counts of papers published in Nature, but for radio telescopes the ratio is a factor of ~3 smaller than for optical telescopes, highlighting the danger of using either metric alone to compare the impacts of different types of telescope. Breakdowns of citation counts by subject (52% extragalactic) and journal (ApJ 44%, Nature 11%, MNRAS 9%, A&A 6%) are also presented.