"Super"moon at perigee

On March 19, the moon was full while at perigee, so its disk was 14% larger than at apogee, and about 30% brighter. Locally, seeing and sky transparency were above average, so it seemed like a good opportunity to try to capture a high-resolution mosaic with the 11 inch SCT, which has a field of view with the camera slightly less than the width of the moon. The above image was compiled from the best portions of 150 images (most images were completely rejected). A printable file, 36x24 inches at 163 dpi, is here.

Because a full moon has direct illumination, the only features visible are those which have a reflectance that contrasts well with the surrounding material. There are some craters in this image that are virtually invisible because they are covered with ejecta, but which are quite prominent under oblique illumination. The area southwest (below and left) of crater Copernicus has some good examples of this. But some very small craters, which are scarcely visible with oblique illumination, have bright white, immature ejecta rims that stand out starkly. There are some of these SW of Copernicus, and inside them one can discern a small dark crater floor. These appear to be the smallest features visible (2 to 3 pixels across in this reduced image). I estimate that the smallest resolvable features in this image are about 3 km across, which corresponds to 1.75 arc seconds. The 11" SCT has a Rayleigh resolution limit of 0.5 arc seconds, and given the limitations of seeing, focus, tube currents, etc. 3 km is probably about the best I could hope for in a compilation like this. At apogee, those crater floors may not have been visible. For comparison, the cratered terrain on the horizon south of Tycho, where the moon's edge is bumpy, has a topography of about 5 km and this was easily resolved. Another interesting feature, close to the limit of resolution in this image, is the twin asymmetric ejecta rays emanating SW of the small crater Messier A.

At this time, the north pole of the moon had swung into view (I have oriented it at the very top). It is just visible on the illuminated back rim of the large crater Peary. There is also rather more of the east (right) side of the moon visible than usual.

Ottawa, Ontario; March 19, 10:15-10:30 PM EDT; -5 C; Canon XSi on 11" Edge HD; unfiltered; seeing very good; selections from 150 images at ISO 800, 1/400 s.



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