Popular Astronomy

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August and September 2015

 This reporting period was rather less fruitful than many because of the lack of bright and convenient targets for the section to observe. It’s true that Venus was bright but it went into Inferior Conjunction (between the Earth and the Sun) on August 15th and only really became observable as an early morning object in the second half of September, unless you were prepared to follow it in daylight; so it could hardly be described as convenient. On the other hand Uranus and Neptune were convenient, in that they were observable in the late evening sky and after midnight, but they certainly were not bright and their tiny apparent size makes them somewhat hard to track down and, perhaps, a little disappointing visually due to a lack of obvious detail. However, with Mars, Jupiter and Saturn effectively unobservable in darkness the section did its absolute best to report on what was on view.

 

Starting with Venus; both Martin Lewis and Carl Bowron followed the planet as it sank into the early evening sky with Martin submitting an image taken in Infra-red wavelengths right at the beginning of the period when Venus was just 18 degrees above the horizon. Cloud detail in the infra-red is very hard to capture on Venus and is an ongoing subject of study at the British Astronomical Association. Martin is unsure that he has managed to capture clear IR cloud detail to date as it is easy to over-process images and produce processing-artefacts that can be mistaken for real detail.  I have not yet succeeded myself but attempted to capture IR detail right at the end of the reporting period, more of which later, and I will continue to do so in the next reporting period. At that time Venus will be a striking and obvious morning ‘star’ and well placed for imaging; I would welcome infra-red, ultra-violet and visible-light images and reports from any ‘early-birds’ who are willing to observe.
 
 
Carl Bowron also followed Venus in broad daylight both before and after conjunction and produced striking coloured images showing a beautiful and delicate looking shallow crescent phase with long narrow extensions of the bright crescent right up towards the poles of the planet. Initially he used a One Shot Colour camera but has recently switched to a new monochrome device, a Celestron Skyris  132M high frame-rate camera, with which he hopes to capture ultra-violet cloud detail. Such detail is less subtle and more obvious than the IR clouds already mentioned, but this is still quite demanding work in that the short wavelength UV end of the spectrum is easily dispersed and scattered by movement in our own atmosphere, making focussing difficult and sharp results equally hard to obtain. One has to wait for very steady ‘seeing’ to get good results and, even then, the planet doesn’t always reveal clear UV detail. This is also an ongoing area of research with no-one yet absolutely certain what chemical is absorbing UV light within the Venusian atmosphere and producing the transient dark cloud patterns seen in some UV images.
 
 
 To make things more difficult the narrow crescent phases of Venus, available during this reporting period, only show the cloud tops of the planet illuminated by the Sun from its position behind, as well as offset, from Venus. This makes the angle between the incident rays and the reflected rays quite large; the solar light is glancing efficiently off the high cloud layers and making observation of the deeper UV-absorbing material much more difficult. A late ‘Indian Summer’  across the UK, that is a period of settled high-pressure weather at the end of September, produced some unexpectedly good seeing conditions over the UK and I was able to obtain some morning image-sets of Venus over the space of a few days that did show a little, if faint, UV detail. Sadly my IR images were smooth and featureless, perhaps for the same reason the dark UV detail is faint as discussed above, and both wavelengths will show more detail as the reflection angle reduces and the illuminated phase increases throughout the next reporting period.
 
 
In late September I also had an interesting email discussion with Graham Taylor about the reason why the visual shape of the crescent of Venus, seen directly by observers, doesn’t always match the shapes seen by other observers or the smooth arc-shapes produced photographically. He saw variations in the shape of the terminator in that it didn’t always follow a smooth arc and also that the narrow sections of illuminated phase near the poles can appear elongated and slightly bulbous instead of smoothly fading to a point. We concluded that these may perhaps be explained by certain optical effects, in that humans do vary slightly in their ability to see parts of the visible spectrum, especially near the UV and IR extremes. These variations in colour sensitivity come into play were light is being scattered, such that if you have better than average deep-blue and near UV colour perception you will see more detail and variation along the terminator than someone like myself, who has poor sensitivity in this area. This theory is, perhaps, supported somewhat by the fact that irregularities in the shape of the terminator do seem to vary according to which filter you are observing through.
 
 
Bright sunlight scattering in the thick atmosphere can also make the tips of the crescent extend dramatically, up to and even past the poles, and when our own atmosphere is slightly unsteady, as it almost always is, these real visual contrast effects can be spread-out by this atmospheric motion making them blurred and appear larger than they really are. This is particularly true when Venus is very close to our line-of-sight with the Sun; at such times the crescents can extend almost all the way around the disc giving some idea of the true thickness of Venus’s atmosphere. Graham has just started to use a planetary imaging camera alongside his usual visual observing routine and his early results on Venus are very encouraging.
 
 
Moving on to the outer ‘ice giant’ planets of Uranus and Neptune, these two planets were well placed for observation throughout the period but there was a bit of a problem; apart from anything else they can be quite hard to find! Uranus is just on the edge of what can be seen with the naked eye from a good dark site but even binoculars struggle to differentiate it from the background stars. Any decent small telescope with a moderate or high-powered eyepiece will show it as a small disc rather than a more point-like star image but it takes a larger telescope to see this disc clearly and, only then, in excellent seeing conditions. Neptune, fainter and more distant, is even more challenging. Nonetheless Simon Kidd embarked on a series of observations of both planets early in the period with some outstanding results. He used his Celestron C14 telescope with an Atmospheric Dispersion Corrector and an ASI224MC one-shot-colour camera which has proven very sensitive in both visible light and IR frequencies.
 
 
 In the last 15 months or so amateurs have started to be able to pick out some detail in the atmospheres of these planets photographically and they have become the ‘distant frontiers’ of what the well-equipped amateur can observe and achieve. Simon’s images of Uranus from early August show very little with visible light but when he moves into the near infra-red faint banding and alterations in shading becomes visible showing that this is a dynamic planet with working weather systems rather than a frozen and inactive world.  The North-Pole of Uranus is noticeably brighter at the moment and large bright storms have been seen rotating at mid-northern latitudes. Tracking these helps with the study of the atmosphere of Uranus and Neptune has similar features; I would welcome any observations seen by section members. My own attempts have been considerably less successful than Simon’s; partly because my own telescope mount is more of a go-near device than a go-to one, especially  when burdened with my 10 inch long-focal length Newtonian, and partly because seeing conditions have not been great when I observed. Nonetheless I managed to track down both planets on several occasions but only on one night was the seeing close to good enough for imaging and, even then, I failed to capture detail. C’est la vie!
 
 
Neptune is particularly interesting because, as previously reported, a very large storm, visible as a white patch in IR images has only recently been seen by amateurs. In still images these patches can be a little unconvincing but animations from images taken some 40 minutes apart do show real rotation of these patches in the atmosphere. Simon’s clearest image of this cloud on Neptune was taken right at the start of the reporting period and he must be congratulated for his excellent work.
 
 
Both Uranus and Neptune remain visible in the next reporting period and, in fact, Uranus reaches opposition on the 12th of October and will be observable all night. If section members have a clear and steady night in the period I would encourage them to try for both targets. Visually Uranus is usually described as blue-green in colour and Neptune as pale-blue but photographically the differences in colour are not very striking. One-shot-Colour planetary cameras show these colours well and most monochrome cameras equipped with a suitable IR-pass filter and on a larger telescope under steady seeing conditions have a chance of capturing these vastly distant cloud features. As Simon has proven recent OSC cameras can also do this work.
 
 
Finally I will move on to Jupiter. No observations of it were reported during this period but I would like to remind section members about sending their Jupiter images to The Juno Project when they do start photographing the planet. NASA’s Juno probe will be inserted into a polar orbit around Jupiter on the 4th of July 2016 and one instrument on board will start taking high resolution images of the cloud tops and features. Until then the Juno team are anxious to see good quality amateur images of Jupiter so they can map the rate of change of various cloud features and plan to make the best use of the probe’s camera. If you visit the website www.missionjuno.swri.edu/junocam you can download a PDF document file explaining how you can contribute directly, with your own images, to this important scientific mission. The preferred image formats and file-types are all explained and I would urge all SPA members to consider helping the Juno project.
 
 
Alan Clitherow.