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 approximate to5% 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 similar to3 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.