Scientific impact of large telescopes

The scientific impacts of telescopes worldwide have been compared on the ba sis of their contributions to (a) the 1000 most-cited astronomy papers publ ished 1991-1998 (125 from each year) and (b) the 452 astronomy papers publi shed in Nature during 1989-1998. Ground-based telescopes of the 1 and 2 m c lass 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 mai nly to theoretical and review papers. The strong showing by 1 and 2 m teles copes in the 1990s augurs well for the continued scientific impact of 4 m t elescopes in the era of 8 m telescopes. The impact of individual ground-bas ed optical telescopes is proportional to collecting area (and approximately proportional to capital cost). The impacts of the various 4 m telescopes a re similar, with the CFHT leading in citation counts and WHT in Nature pape rs. HST has about 15 times the citation impact of a 4 m ground-based telesc ope but costs more than 100 times as much. Citation counts are proportional to counts of papers published in Nature, but for radio telescopes the rati o is a factor of similar to3 smaller than for optical telescopes, highlight ing the danger of using either metric alone to compare the impacts of diffe rent types of telescope. Breakdowns of citation counts by subject (52% extr agalactic) and journal (ApJ 44%, Nature 11%, MNRAS 9%, A&A 6%) are also pre sented.