The Steavenson Award

 

Observing Comets

Mr Shanklin set the scene with an introductory history of the Comet Section. Founded in 1891, only one year after the BAA itself, it had always been a staple part of the Association. Since the Second World War, around 300,000 observations had been amassed of over 400 separate comets. The first Director of the Section had been W.F. Deeming, whose work had earned him the honour of having a comet named after him, though it had since been lost. It seemed likely that a survey such as LINEAR would rediscover it whenever it made its next return. The speaker was the 12th Director that the Section had seen.

This long heritage was profoundly beneficial, because it allowed the Section to directly compare observations made using the same equipment at different epochs. It was found that the results obtained from different instruments often showed more discrepancy than might be imagined, and the Section could draw much better comparisons by taking advantage of this continuity. The speaker explained in particular that modern CCD cameras did not always give results that were directly comparable with visual observation. Thus there was still a significant opening for visual comet observers to contribute.

The Section communicated the latest news to its members via its website, which could be found at http://www.ast.cam.ac.uk/~jds. A paper newsletter, The Comet's Tale, was published twice a year, and sent to members at a cost of £5 for two years' subscription. Results were communicated to the rest of the Association primarily via papers in the Journal and talks at meetings. Notably, Nick James and Gerald North had recently produced a handbook, Observing Comets, published by Springer, which the speaker warmly recommended.

Mr Shanklin then proceeded to discuss four of the most important stages of cometary astronomy, and first of all, the discovery of new comets. This was a field where amateur comet-hunters now faced significant competition from robotic surveys such as LINEAR and NEAT, which covered vast areas of the sky. However, the speaker urged amateurs not to be too disheartened, for these surveys did not quite cover the entire sky, with the northern polar twilight zone being omitted. Furthermore, there were presently no robotised surveys sweeping the southern sky at all. Among the Association's most famous comet discoverers were George Alcock, and Albert Jones. Jones' recent co-discovery of 2000 W1 had put him into the record books on two counts: both as the oldest ever comet discoverer, and as having the longest span between his discoveries – his first comet discovery had been in 1946.

The SOHO satellite, launched in 1995, had introduced exciting new possibilities for the discovery of comets which were too close in the sky to the Sun for ground-based observation. Whilst SOHO was primarily intended to probe solar phenomena, two of its on-board instruments had comet-discovering potential: the SWAN (Solar Wind ANisotropies) detector, and the two Lasco solar coronographs. The former provided an Hα map of the entire sky, which was particularly well suited to detecting the hydrogen coma of comets. It had discovered a modest number of previously unknown objects. The Lasco coronographs had had greater discovery potential, covering a 10° field of view around the Sun, and had found around 640 new comets. However, they were now significantly past the end of their design lifetime and were suffering technical difficulties. Presently, there were 20 days in every three months when they were unable to return data.

The second stage of cometary astronomy was the follow-up observations which were required after each discovery. These were needed in the first instance to verify the initial discovery, and later on to refine our knowledge of the object's orbital parameters. Other questions which needed addressing were how fast the newly discovered object was brightening, and whether it was an asteroid or a comet. This latter distinction was determined primarily upon the location of the predicted perihelion of the orbit. The process of gathering follow-up observations was initiated as soon as the discoverer informed the Central Bureau for Astronomical Telegrams (CBAT) of their suspected new object. An anonymous observation report would then be posted by CBAT, with a request for other astronomers to report both positive and negative observation attempts.

Once follow-up observations had been gathered, the provisional orbit inferred from them could be used to predict how the comet's brightness would vary in the future. However, comets which were approaching their first perihelion were frequently observed to show anomalous lightcurves as their surfaces warmed for the first time. For example, they might be unusually bright on approach, and then much fainter after perihelion. This was a common reason for comets proving to be disappointing. An accurate knowledge of the orbital parameters was particularly important for those objects to which there was potential interest in sending spacecraft to rendez-vous.

The third aspect of the Comet Section's work was determining the morphology of comets: looking at the structure of their tails and comas. The human eye was notorious for its subjective judgement in this regard, and consequently the focus tended to be on CCD work. Streamers were frequently identifiable within ion tails, as were jets and halos.

The final category of work within cometary observation was the estimation of visual magnitudes, which remained an important exercise. The basic principle behind such estimations was to find reference stars of known brightness close to the comet, and to use these as comparisons to the comet itself. The brightnesses of reference stars could usually be looked up in either the Guidestar or the Tycho catalogues. A potential hitch arose if one of the reference stars turned out to be a double star, but the mathematics of adding the two component magnitudes to find the total brightness of the binary system was straightforward. Usually the aim was to find one reference a little fainter than the comet, and a second a little brighter than it, and to scale subjectively between these two reference points. However, matters were usually complicated because the comet's light was spread over a finite area of the field, with a hazy coma and tail. Hence some caution was needed when comparing its total brightness to those of pointlike stars, since a comet would thus subjectively appear fainter than a star of the same magnitude, because of its light being more widely spread. Mr Shanklin suggested that more accurate photometry could therefore be achieved if the eyepiece was placed a little out of focus when observing the reference stars, so they appeared blurred by the same amount as the comet.

For some comets, the visual magnitudes were prone to undergo dramatic changes as inhomogeneities within the nucleus were revealed, and as different layers boiled off in turn. In other cases, such as 2P/Encke, the absolute magnitude appeared completely unchanged over 50 years of observation. The speaker demonstrated this point with George Alcock's artwork of the 1954 return, with appearance entirely consistent with the present day. He returned to his initial point that the Comet Section benefited greatly from having such a rich historical archive, allowing this kind of comparison to be made readily, often even using observations from the same instruments.

Following the applause for Mr Shanklin's comprehensive discussion, the final talk of the afternoon was presented by Dr Chris Baddiley on the Association's crusade against the menace of light-pollution.

Fairfield

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