  The scientific impacts of telescopes worldwide have been compared on the
basis of their contributions to (a) the 1000 most-cited astronomy papers
published 1991-8 (125 from each year), and (b) the 452 astronomy papers
published in Nature 1989-98. 1-m and 2-m ground-based telescopes account for ~
5% of the citations to the top-cited papers, 4-m telescopes 10%, Keck I/II 4%,
sub-mm and radio telescopes 4%, HST 8%, other space telescopes 23%. The
remaining citations are mainly to theoretical and review papers. The strong
showing by 1-m 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 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 cost > 100 times as much. Citation counts are proportional to
counts of papers published in Nature, but for radio telescopes the ratio is a
factor ~ 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.
