Steve wrote:
I hear from Rich that full moon is the best time to view craterlets inside Plato.
Rich wrote:
That suggestion was on saa about a year or two ago. I haven't tried it yet. Some people claim to have seen a number of craterlets in Plato, more than shown in Rukl.
I saw this note earlier in the week and decided to try it out myself. Last night I set up my Starmaster 7 inch Oak Classic near home in Palo Alto. Skies were variable during the night, with lots of high thin clouds floating through. Although transparency was poor, the seeing was pretty steady after 11 pm or so. Double stars were showing very clean splits at high magnification, with nice Airy discs and fairly stable faint first diffraction ring. I have been working on double star projects for many recent near home sessions. Last night, I was able to resolve the BC component of OttoStruve 2764 in Cepheus (0.75 arcsecond separation), something that had not been possible the night before. So seeing was at least subarcsecond, and Jupiter and Saturn looked great as well.
For the Plato test, I began with a Vixen 24 to 8 mm zoom eyepiece ccombined with a Celestron Ultima 2x barlow. That gives a maximum magnification of about 240 in the 7 inch Starmaster. Views were very steady, so I put the same zoom eyepiece into a Televue 3x Barlow instead. At full zoom, that combination gives a maximum magnification of 360x in the 7 inch Dob (just over 50x per inch). Views were still nice and steady, so that's what I used for the rest of the night. When I first looked at the moon, I could make out 4 small craterlets in the otherwise smooth floor of Plato, (a tight double pair, and two single craterlets in a curving arc). I didn't have an atlas with me, so decided to sketch positions for later comparison to Rukl and photographs. As I have often found before, the act of sketching was a big help in teasing out additional detail. As I carefully studied Plato to make my sketch, additional craterlets begin to pop into view. One was near the western edge, as prominent as the first 4 that I had seen. This one had been missed before only because of its proximity to the rim. Then smaller ones began appearing near the original single craters near the center. These were harder to hold, like faint companions of a difficult double star. But I was patient, the sky was steady, and as I remained seated and still at the eyepiece, several small craters became obvious. I had sketched positions of 12 craterlets when the skies finally clouded over for good at about 1:15 am.
When I later checked my sketch against my Rukl moon atlas, I found only 5 craterlets in the Rukl charts. These correspond to the five most obvious ones that I has seen as well. A photograph of Plato near the back of the atlas (50 views of the moon section) shows one more, for a total of 6.
I also have a copy of the classic NASA Lunar Orbiter Atlas of the Moon. Photo 127-3 on page 222 is a nice high resolution shot of Plato, with just over a 100 craterlets visible in the floor of Plato, some of them quite small. One of the small bright spots I had recorded as a craterlet was actually a small hill near the eastern rim of Plato. Around full moon, it can be hard to tell the bright spots of craters from the bright spots of elevations. However, the other 11 spots I had recorded all corresponded to positions of the largest craterlets in the Lunar Orbiter photo. There was an excellent correspondence between their position in my sketch, how easy they had been to see, and the position and size of the craters in the high resolution photo. Based on the photo, I probably missed 4 or 5 additional craterlets whose sizes were not much smaller than the smallest ones I had recorded. I was still finding new ones when the clouds arrived, and suspect I could have uncovered a few more if the skies had remained clear.
It would be fun to try this again under different lighting conditions, and perhaps with an excellent short focal length eyepiece instead of the barlowed zoom arrangement. I suspect the results will vary enormously under different lighting conditions. Many moon craters show bright rims under the direct illumination of near full moon, just as the ejecta rays of large craters are most prominent under similar conditions. This presumably makes the craterlets easier to see.
If anyone else wants to go craterlet hunting, be sure to check out the Nasa Lunar Orbiter photos of Plato for comparison with your counts. The historic Lunar Orbiter atlas is now available on the web, in convenient searchable form.
See http://www.lpi.usra.edu/research/lunar orbiter/img/4-127H3.jpg for a good example of the resolution available for Plato.
