(Rim-to-rim diameters 1 km or larger, based on Lunar Orbiter IV images. (Plato Rim diameter from point on the cusp of the inward bowing feature north) of faulted rock (West Rim Bow-in) to east rim edge = 62 miles (99.8 km). NOTE: Letters used below are *not* standard IAU secondary crater designators.
APPROXIMATE CRATER DIAMETERS
(+/- 0.2 miles uncertainty)
The "BIG FOUR" (+1)
A = 1.7 miles (2.7 km) B = 1.5 miles (2.4 km) C = 1.5 miles (2.4 km) D = 1.3 miles (2.1 km) W (on west-northwest wall) = 2.0 miles (3.2 km)
NOTES: Although many amateurs rarely seem to see very much on the apparently smooth dark floor of Plato, the above craterlets are the ones most often reported by those lucky enough to get really good seeing. "A" is the easiest of this group due to its fairly prominent ramparts, and has been reported in a 4 to 5 inch aperture, although the unresolved "bump" of craterlet-A's ramparts is visible in only a 3.1 inch. 3 or 4 of these craterlets can sometimes be observed under low sun angle and in good to excellent seeing in apertures 6 inches and *larger*. These four can sometimes be "detected" as very tiny white spots in 3 to 5 inch scopes during the full moon, although to show them all as true pits often requires a 7 or 8 inch aperture. The "East Wall Pit" is a much larger irregular feature (4 miles across) which often hides in the shadow of the eastern wall during the lunar mornings. It may or may not be an impact crater. There is also a small nearly rimless craterlet "W" low on the west-northwest wall north of the west-rim bow-in which is about 2 miles across. It is considerably more difficult to observe than its size would indicate, as the sun has to be at just the right angle to allow any shadow to fill even part of it to make it visible.
The "Little Four"
e = 1.2 miles (1.9 km) f = 1.0 miles (1.6 km)
g = 0.94 miles (1.5 km) h = 1.4 x 0.8 miles (2.2 x 1.3 km)
NOTES: Craterlet-e has been sighted in a good 8 inch, but craterlet-f may take a bit larger scope to see with any regularity. "e" tends to hide in the long early morning shadows, as "f" does also in the low lunar evening. The Lunar Orbiter shots of Plato show that "h" is a tiny double craterlet with 0.9 and 0.8 mile diameter components, forming an elongated 1.4 x 0.8 mile feature visible in larger apertures, but not fully resolved. Again, very high lunar sun may allow some of these cratelets to be "detected" as tiny white spots near full moon. The "Big Four", and the "Little Four" probably represent most of the craterlets on the floor of Plato which might be visible to amateurs using moderate to large apertures under excellent seeing.
The "Tiny Nine"
i = 0.7 miles (1.2 km) j = 0.6 miles (1.0 km) k = 0.7 miles (1.3 km) l = 0.6 miles (1.0 km) m = 0.7 miles (1.2 km) n = 0.7 miles (1.1 km) o = 0.7 miles (1.1 km) (double craterlet) p = 0.7 miles (triple craterlet) q = 0.6 miles (1.0 km) (double overlapping crater)
NOTES: These are *extremely* difficult to observe even in large apertures as anything other than tiny white spots or rimless pits, although i, j, m, and o2 have been imaged by Maurizio Di Scuillo using a CCD camera on a ten inch Newtonian optimized for high resolution planetary work. Craterlet k has a very small pit to its west and will be tough to resolve easily. Craterlet m is a fairly shallow bowl with little in the way of a rim, so it is more difficult than its size would indicate. "n" is a very small rimless pit just to the east of a tiny white spot which is often mistaken for a crater (may be a small mound or ejecta blanket). Craterlet o is a double craterlet, which forms a 1.3 mile x 0.7 mile elongated feature. p is a triple, consisting of 0.7 mile, 0.6 mile, and 0.3 mile craterlets in close promimity, which might be detectable in very large apertures as a single almost rimless 1.5 mile x 0.7 mile feature (not resolved). Craterlet q is two very small overlapping craterlets which form a single 1.3 mile x 0.6 mile feature. Lunar Orbiter images show a large number of smaller pits down to 0.25 miles across on the floor of Plato, but the above three "families" are probably the only ones which have much of a chance of being seen visually from Earth.
I guess it's all in the illumination and with the moon at 12 days, the brighter rim makes them stand out. I am using a WO 80mm Fluorite APO, so catching them is really at the outside limit here for this size scope (I've not heard yet of anyone having success in anything much smaller and the exquisite optics of this scope make a big difference). Craterlet A was actually an easy catch and spotted immediately at 144x with a Burgess binoviewer (12mm Burgess EP's with 1.8 OCA and 2x Televue Barlow). At that point I went to my 8mm Radians for 216x which, while it does not show much more detail, is still very usable and has the advantage of more eye relief (so I'm not bumping the scope). B, C and D were then observable most of the time and during a few clear moments, I was able to distinguish the CD pair from B, although this was definitely right at the limit and more like noticing a double is elongated even though it cannot be split.
).
