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Patrick Moore in one of his books in the index has this list of the smallest crater for a given aperrture and so on.
He ALSO has the smallest or finest rill or rima visible in the same table per aperture. It seems to work out that the finest line or rill in the case of the moon is 1/10th that of the smallest crater.

Ive not yet put that to the test - though Im looking forward to seeing Suess. Any opinions on this?

Pete

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His guide is approximately accurate for craterlets half-filled with shadow (diameter (miles) = 9/D where D is the aperture in inches). I once checked some of the craterlets visible in and around Mare Cognitum (the Riphaeus and Fra Mauro region of the moon) against a copy of the Lunar Astronautical Chart #76, and the smallest craterlets visible in my 8 inch f/7 were pretty much right in-line with Moore's table, although the original information for that table came from work by E.A. Whitaker in 1955. One might do slightly better under some circumstances, but the "9/D" rule of thumb is a somewhat realistic way to estimate minimum crater size for a given aperture. The rille estimate I can't really say much about, as I haven't investigated it all that well. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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... It seems to work out that the finest line or rill in the case of the moon is 1/10th that of the smallest crater.... Pete

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Logic really - when you easily spy a kite thread at 100ft of flea dimensions

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My observatory build-ETX-70 imaging-my videos

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Hi Dave, and thanks Nyte...

Why is it always that half filled with shadow thing? What happens if its all filled with shadow - and wouldnt that a more contrasthy crater. You know, if just the high walls were showing and the interior black.

At any rate, I'm winding up this summer to be an apollo 15 "revisit" along with Apollo 11. I want to see what the smallest feature is I can ail down in similar lighting conditions. Ill probably make an illustration or drawing of it, though I'm tempted to do the video cam thing and stack images.

Thanks again.

Pete

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In some cases, with craterlets that are largely filled with shadow, there isn't enough of a lighted portion to make a huge difference and the craterlet can blend into the nearby topography (especially if that surrounding area is mare). There has to be a rough balance between shadow and the lighted portion to make it stand out when it is near the resolution limits of the instrument. Think of it as two adjacent pixels in an image, one the lighted portion and the the other the interior shadow and you can get the idea. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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