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Section Report for August and September 2017

 This will not be my longest report because the late summer period produced very few observation; all the major planets were relatively poorly placed except the outer ‘Ice-Giants’ of Uranus and Neptune and ‘seeing’ conditions in the period did not allow detailed observation of those distant worlds. For what it is worth I tried to image them on several occasions in the period however turbulent motion in the atmosphere of our summer skies denied me any useful results; I know other observers suffered under similar conditions.

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This does not mean that no useful work was done; both Mercury and Venus received some attention and what was achieved proved to be very valuable.  Visually Venus can appear as a very bland world with its thick cloud layer reflecting sunlight towards us in a smooth and even fashion. Some individuals do have a slightly greater sensitivity than others at the blue end of the spectrum and are able to pick out subtle variations in the shading where slightly darker patches appear in the Venusian atmosphere. I, myself, am completely unable to see these patches so I rely on photographic methods to pick them out.  
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Early in the 20th century it was found that something was absorbing solar ultraviolet light within the clouds of Venus such that reflected light from areas containing this ‘something’ lacked light at near UV wavelengths; making them appear darker. Currently we have no exact data on what this absorber is however it gives amateurs a useful opportunity to study cloud patterns and movements from Earth. Modern telescopes fitted with high frame-rate video cameras and using filters that pass near-UV light can capture these clouds and follow their rotation over hours or even days; the rotation period for any given UV absorbing cloud seems to be around 4 days.
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On the 12th of September Dave Finnigan managed to capture these features using a commercially available UV-filter, the Baader U-Venus filter, on an ASI-120 monochrome camera and using his 305mm Schmidt-Cassegrain telescope.  His image was taken in full day light and this seems to be the best approach as it allows the planet to be viewed at higher elevations and thus through a thinner layer of our own atmosphere; imaging Venus in morning or evening twilight may give higher contrast against the background sky but means there are atmospheric dispersion effects to contend with and, often, turbulence when viewing through a thick layer of air. Dave’s daylight image shows a pale wedge of cloud running around the equatorial to temperate regions on Venus with darker patches at higher latitudes both north and south.
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The ultraviolet is not the only region in which cloud frequencies have been observed, indeed a recent paper published in the scientific journal Icarus (Icarus 288 (2017) p235-239) lists the scientifically useful wavelengths for observing various features within the atmosphere of Venus. Three of these wavelength bands are readily accessible by the earth based amateur. The first, as already mentioned, is the near ultraviolet and deep blue wavelengths running from 350 to 460 nanometres (nm); most commercial Venus UV filters centre on a narrow band around 360 to 370 nm as that offers best contrast of the UV absorbing material.  This band allowed the excellent image produced by Dave.
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The next useful wavelength range cover the deep-red end of the spectrum and into the near infrared (IR) from around 570 to 680 nm and the third is the band from 900 to 1000 nm, which lies at the top end of the sensitivity range for most amateur planetary cameras. These red and near IR wavelengths show contrast features within the atmosphere of Venus where the clouds are absorbing reflected sunlight. I must admit that I have not, so far, been able to image contrast features on Venus in the near IR however Simon Kidd has been much more successful. He used an Astronomik 742 nm IR filter on his monochrome camera to produce the best images of infrared cloud features on Venus I have ever seen. This filter passes more than 90% of light from 742 nm to well beyond 1000 nm so allows the camera to see the higher end of the three observing windows as mentioned above.
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Simon’s observations were taken on the 31st of July (technically in the last period but I have lumped them with current observations for continuity), the 6th and the 31st of August. In each case Simon observed over an extended period and he was able to compile multiple images into animated ‘gif’ files that show the movement of cloud details over time.  Since I cannot animate a magazine article the gifs are shown here as separate images but the subtle cloud movement can be followed. I include his images from the 6th of August so you can judge for yourself.
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Interestingly the UV image and the IR images show different layers in the atmosphere. The Venus Express probe extensively mapped the movement of cloud features in a region from the equator to around the 50 degree latitude lines and the data suggests wind speeds as follows:-
66 km above the surface 370 km/hr or 102 m/s
61 km above the surface 220 km/hr or 61 m/s
45 km above the surface 210 km/hr or 58 m/s
While the Icarus paper suggests the UV absorber layer lies between 63 and 71 Km up and the IR absorber layer is deeper at 55 to 65 km from the surface, Simon’s own calculations, taken from his IR images suggested a wind speed of approximately 120 m/s. His estimate is possibly excessive but not massively so and, in any case, involves a level of spherical geometry more than somewhat beyond my abilities! Future amateur observations can be used to refine these estimates and complement the professional work being done.
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Venus has dominated this report so far and as a final thought on our sister-planet, the Icarus paper suggest that wavelengths of 850 to 1000 nm can be used to pick up thermal emissions from the night-time surface of Venus, radiating away through the clouds and into space. Perhaps a section member would like to join me in trying to image the IR Ashen Light sometime towards the middle of the next apparition?
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Which brings me to Mercury; I was about to put this report ‘to bed’ when Martin Lewis sent me an extraordinary image of the closest planet to the Sun taken on the 22nd of September. Martin took his image around 1015 UT, obviously in full daylight and when the planet was just 13.5 degrees from the Sun. At that time Mercury had a little more than 40 degrees of elevation from his location and the same 742 nm infrared filter, already referred to as useful on Venus, was used to help cut through daytime atmospheric turbulence. Using the standard planetary video-capture techniques he used the best ½ a percent of some 160,000 frames of video to produce his final image! There is no doubt it shows surface albedo features on a target that is almost never well placed for observation in clear and steady air. It has to be said that the contributors for this period have gone ‘above and beyond’ and I roundly thank them for their efforts.
Alan Clitherow.
 

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