Friday, October 11, 2013

'Unusually dark Plato floor', reported by the late Patrick Moore, is perfectly normal.

On 1984 Feb 14 P. Moore (Selsey, UK) observed that Plato was darker than the nearby mare and no detail could be seen on the floor or the eastern wall - the latter was obscured. At 23:40UT some dimming was still present on the north east wall and still no detail on the floor of Plato. Cook noticed that the eastern floor close to the wall was misty and also noted no detail on the floor. Amery though noted that all parts of the floor were sharp although some darkening was visible in the north west and a hint of obscuration. The east wall though was quite sharp. Moseley could see the central craterlet but from 8-6 o'clock tricky to define (Foley says that this effect has been seen at this colongitude before). Streak ray across the floor of Plato seen (North) - filter measurements made. Cameron 2006 catalog extension ID=241 and weight=5. ALPO/BAA.

 CCD image of Plato taken by Charles Galdies on 2013 Jul 20 UT 22:11 with north towards the top. (Left) locations of three craterlets. (Right) Darker region of the floor marked.

I carried out my verification of this LTP as reported by P. Moore on 20 July 2013 at 22:11UT using a Nexstar 8SE and DMK21AU.

The seeing was IV and the transparency average. The gain setting of the camera was adjusted to emphasize the detail in the crater floor. A total of 3000 images were obtained at a frame rate of 30fps, stacked using Registax 6 and he then used wavelet transform to obtain sharper images. Basic histogram levelling was performed using Adobe PhotoshopTM to emphasize contrast of internal floor relative to adjacent mare.

In my full report, which can be downloaded from here, a streaked ray is described on the northern part of the crater floor shore on the NE, next to the upper “c” in Fig above (left) and the same illustration shows that there are at least three craterlets on the floor. The right figure above illustrates the darker area of the floor.

Given the description of the LTP above, at least some detail should have been visible on the floor back in 1984, though that would have depended upon observing conditions experienced by individual observers. Tony Cook, who reported this observation on ALPO's The Lunar Observer September issue, argues that some detail on the E and NE is visible, but it is not as contrasty as on other parts of the rim.

Tony Cook concludes that the 'unusual dark Plato floor', originally reported by the late Patrick Moore, can now be considered as perfectly normal. In view of these facts Cook reduced the ALPO/BAA weight of this LTP report from a 3 to a 2.

This report has been published in the September issue of the September issue of ALPO's The Lunar Observer.

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