SPOT THE DIFFERENCE: ARISTARCHUS (PART 1)

 

                    Here we see the SW rim of Aristarchus crater, a noted TLP site. What differences can you spot here – if any?

 

 

 

SW Rim of Aristarchus

 

 

                    This month we will switch our attention to a prime TLP site, the SW slopes of Aristarchus using images that were taken three years apart (See above – the region we will be examining is in the yellow rectangle) – again we will pick a slope in the hope of detecting fresh boulder trails or landslides, but this time the inclination is only 18°. Although the image pair differ in solar altitude by just 2°, there is a larger difference in the azimuth of illumination amounting to 38°, hence you may see some rotation of shadows, which are clearly not real physical changes. Again we have some strong stereo parallax effects, so when blinking between images, you may see some lateral jumps. Having taken a precursory look at the image pair, I have also noticed some large area differences – I think these are slope angle effects i.e. slopes facing the Sun are brighter than those facing away in the second image, so this 38° difference in illumination azimuth maybe playing tricks on us here.

 

                    I find it useful to look for differences between images by loading them into an image processing program, e.g. Adobe Photoshop, into layers, and then blinking between layers. You may note a slight amount of image shift and this is due to stereo parallax between the two images.

 

                    A couple of suggestions I would like to offer. 1) if the jumping effect from stereo parallax is a problem, then break each image down into smaller tiles, re-register each tile with its temporal pair, then try blinking again. 2) If some areas of the image are low in contract or texture, then do a high pass filter, followed by a contrast enhancement – this will bring out more detail.

 

 

 

                    The LROC images used for Spot the Difference are: (Left) M109548636RC taken on 2009 Oct 27 with: Solar Altitude=64.4°, Solar Azimuth=169.8°, and an image scale originally of 0.5 m/pixel. (Right) M1101516499LC taken on 2012 Sep 05 with: Solar Altitude=62.4°, Solar Azimuth=205.6°, and an image scale originally at 1.3 m/pixel – note this image was taken with a slanted view (not nadir) originally, so the resolution in the horizontal direction differs to the vertical direction.

 

 

                    When you have finished please email me (atc @ aber.ac.uk) back one of the images with the locations of where you have noted changes and I will report on these in the next Newsletter of the Lunar Sections of the British Astronomical Association, and the Association of Lunar and Planetary Observers. The deadline for doing this will be 15th July 2015.