Whitepeak Observatory, Tacoma, WA

Resolution Standards in Lunar Imaging

As there is a great deal of interest in lunar imaging and the resolution thereof both in the amateur and to a lesser extent in the professional communities, I examined the state of the art at various levels of imaging to see what the current limits actually are. This information can be useful for the purposes of determining what level of resolution one can realistically expect from a given aperture and as a benchmark for evaluating the quality of lunar images generally--if a given image seems poor there is good reason to search for a better one; conversely, if a given image shows details in line with the averages detailed below, then one need not waste time looking for something superior.

I thought i'd share this image as it seems a lonely example of the (current?) state of the art per the 'really big guys':

(press release image)

This image represents "0.07 arcsec resolution". This is a portion of the floor of Taruntius obtained through ESO's 8.2meter YEPUN telescope using AO (That's right--no star-- they used a bright lunar albedo point to calibrate on instead!) The ground resolution is 130meters. The largest crater near the top is Cameron, 10km diameter. Shame (but not too suprising) is this is the *only* Moon image they have bothered to take (or at least publish) in the last three years--and this dispite the fact they promised more when this one was released. So, not 'never', just 'almost never' do the pros image the Moon. More's the pity as there are definate 'holes' in the LO image sets that could do with some organized 'filling in' at such resolutions. (See the morphological differences between secondary and primary impacts here?)

For comparison purposes, Lunar Orbiter IV delivered ~65m ground resolution, a little better than twice as good as the ESO obtained. (Other orbiter missions have gotten down to *1 meter* ground resolution.) Hubble runs around 0.10"arc resolution. The Consolodated Lunar Atlas images, the film era photographic standard, run to ~3km resolution, at least on the full size tiff's published by the LPI--perhaps the originals are better? A survey of the very best amateur images (indexed to measurements taken off the cooresponding LOPAM images) shows an overall limit of about ~700m/0.4"arc resolution in the >350m aperture class. One other example of ultra high res lunar imaging, but not strictly 'amateur', is the work of Bruno Daversin using a publicly owned professional grade 24" cassagrain at the Ludiver Observatory in France. Clavius Floor This instrument delivers ~0.2"arc resolution (claimed) on the lunar surface. (I feel this is based on pixel resolution not feature resolution however; I measured the Petavius image and find resolution to be ~1.2km ground or about 0.6" arc.) Typically amateur images that exceed 0,5" arc resolution are exceptional...the overall amateur hi-res median seems to be in the 1"arc/1.8km neighborhood, but resolution seems to decrease faster than imaging aperture decreases. Get down to 4~8" aperture range and I found no examples with better than about 3km/1.7"arc resolution--most come in around 4-5km.

If the above overview indeed reflects reality, I have to wonder why, exactly, the smaller instruments seem alone in being unable to image closer to their diffraction-limited resolutions. Image resolution to theoretical resolution ratios seem to widen disproportionately with decreasing aperture, dispite possessing the same advantages of webcams and image stacking. The reason must lie elsewhere...I wonder why this is? Perhaps the answer lies in the availability of shorter exposures/frame rates thus enabling better 'seeing penetration' in the larger apertures due to their ability to deliver higher levels of field illumination? Or the larger scopes seem to be used more often in concert with better imaging equipment and/or simply possess better optical quality overall? Or perhaps the possibility that larger, more expensive instruments are simply more likely to be sited at locations that offer typically better seeing conditions than smaller instruments? Whatever the reason(s), it would be interesting to know what the factors really are and how they influence these uneven ratios.

back to main page