The diameter of the smallest lunar crater that can be distinguished as a crater is considered the resolution of a lunar image or a visual observation. In 1955 Ewen Whitaker distilled his experience into a table of what resolution a visual observer could normally achieve with a decent telescope and good seeing.
I have converted resolutions to metric units and rounded to meaningful significant figures.
Whitaker pointed out that the eye is much more sensitive to high contrast lines than round craters, accounting for strikingly better resolution for rilles.
These numbers for crater resolution may be about right for visual observations, but modern imaging does much better. I am guessing that the best imagers get about 1 km resolution with 8" scopes, and 16"-18"telescopes sometimes achieve 500 -600 m resolution for craters. This suggest that video imaging effectively doubles a scope's resolving power compared to visual observing.
I do not think that the table is correct for rilles, perhaps because the widths of rilles were not well known in the 1950s. Based on images submitted to LPOD I think that 8"-10" scopes resolve rilles about 1 km wide, and 18" scopes may reach 500-600 m. What do others think? You can comment (after simple registration) by clicking the 'discussion' tab above.
- tychocrater Jan 13, 2008
Based on a S&T summary from May, 1971 (p. 323).
Calibrated Crater Sequence Images by Mardi Clark.