Ordinary Meeting, 2005 May 25
CCD Observations from the Bunbury Observatory
Upon his return to the UK in 2004 January, after several years abroad, the time had seemed ripe to realise his long-held dream of setting up a fixed observatory, Dr Shears explained. This talk would recount the experiences of his first year as a newcomer to the art of CCD astronomy. He recalled with amusement the reaction of his estate agent when first told that minimal light pollution was to be a major requirement in his choice of home, and, when a house in the town of Bunbury, 15 miles south of Chester, had been chosen, the surreal scene as the postman had had to weave a path through the morning traffic of the narrow local roads with his seven-foot dome.
For his primary telescope, he had chosen a Takahashi FS102, a 102-mm apochromatic refractor with 820-mm focal length: he had acquired something of an affection for them whilst travelling the Far East, where their portability was highly valued. His CCD observations were made with a Starlight Xpress MX716 CCD array, a Skysensor 2000 goto unit providing good tracking. He anticipated that some might greet his choice of instrument with surprise: he had been in part curious himself to question whether it was possible to achieve scientifically useful results with a mere four-inch aperture. He left that question to the audience, and continued with his account.
Over the past year, he had made observations on 88 nights, around 25%. His imaging methodology, he explained, had rapidly settled upon a standard technique of taking one-minute exposures of all objects, and stacking them where longer effective exposures were required. He controlled the CCD array and performed basic calibration using AstroArt III; post-processing was done in Adobe Photoshop. Proceeding to show a gallery of the fruits of his deep-sky imaging efforts, he commented how well the wide field of the Takahashi framed the Dumbbell Nebula in its star-field. The Veil, the Cocoon and the Crescent: all were imaged with similar fidelity. Stephan's Quintet was reason for a brief pause; the speaker remarked that capturing it had long been a dream: its association with the Redshift Controversy had caught his imagination.
Turning to comets, he remarked upon the fine examples which had graced the sky of late. In late summer, while imaging the fine tail of 2001 Q4, perhaps the most magnificent of all, he had discovered the technique of stacking his one-minute exposures not on stars, but rather on the cometary nucleus: while this did produce star trails, the resolution of the tail was greatly enhanced. Other comets had produced quite different shows – 2003 K4 with its stubby tail in June, and 2004 T4 with its diffuse coma in December. Machholz had provided perhaps some of his most stunning images, in early 2004, though its tail was much too large to fit into a CCD frame.
Schwassmann-Wachmann 1 was a comet which had long intrigued him: quite apart from the impracticalities of its name, the widespread publication of its ephemeris seemed unjustified for so faint an object. Its regular outbursts to mag 11 seemed to provide the explanation, and he proceeded to tell the tale of his own observations. In early September, it had taken a rather uninspiring, faint and diffuse appearance. Eleven days later, the view had been completely changed: it had appeared much brighter, almost stellar in appearance. Over the following week, a diffuse coma had gradually appeared.
Supernovae had also caught Dr Shears' attention: initially he had set out only to image 2004dj in NGC 2403 after hearing of its discovery in 2004 July, but, having obtained images, he had become curious to try photometry. Pleased with his first results, he had continued to monitor it from early 2004 September through until April, when it had faded below mag 15.7, and reliable photometry had proved impossible. He had also been able to monitor 2004et, discovered by Doug Rich in the sweeping photogenic spirals of NGC 6946 on 2004 September 17, for eight months.
His successes with supernova photometry had spurred him to try variable stars, and it was to these that he finally turned. He had observed two types of objects for the Variable Star Section: undertaking time-resolved photometry of those known to be in outburst, and periodically monitoring others for the Recurrent Objects Programme, checking them for outbursts, with a detection limit of mag ~17.5. The latter search had yielded one discovery: an outburst of dwarf nova CG Draconis to mag 15.6 on April 9. He remarked that, in view of the first talk, this search might soon be obsolete, but Dr Norton, in the audience, remarked that SuperWASP's detection limit would be an inferior mag 16.
Dr Shears displayed several light-curves that he had produced, remarking how easy it had proven to be. Having obtained a series of images, he had imported them into the AIP4Win1 software package, which could perform aperture photometry on them all in a single operation. One of his earliest successes had been with dwarf nova V1113 Cyg, whose light-curve he had studied with the Peranso2 Fourier analysis tool, identifying a 111-minute period. This was within 3% of its published superhump period – that of variability attributed to precession of its accretion disc. Whilst not new science, this demonstrated the accuracy of which his set-up was capable.
The speaker closed with the light-curves of several further variable stars, most notably Bernhard 1, a newly discovered dwarf nova in Camelopardalis, identified from plate archives. An outburst had been detected on March 16, and he had had an opportunity to study it on March 19 between 19h30 and 23h29 UT, during which time he had captured three complete superhump cycles, his observations suggesting an unusually short 81-minute period, varying between mag 11.7 and 12.0. On March 25 he had observed quite different behaviour: rapid flickering with its brightness changing by up to 0.5 magnitudes within minutes. After initially doubting his observations, then finding re-reduction to effect no change, he had discovered that others had noted the same phenomenon.
He noted with awe the work of Mr Gary Poyner, who had made visual observations over a 24-minute period, estimating its magnitude every 10 seconds. Known as an observer par excellence, Poyner had reported variations by almost a whole magnitude, though the speaker was unsurprised by his decision not to continue any longer. A very mysterious star, members were urged to watch it as and when it was next active.
The speaker wished to leave with two thoughts. Firstly, to urge all deep sky enthusiasts to consider making variable star observations: he pointed out that many variable stars on the Recurrent Objects patrol list shared fields with deep sky wonders, not to mention active galaxies, which were themselves deep sky objects in the truest sense. Secondly, he left members with his opening question of whether science was possible with a four-inch aperture; he wasn't yet sure himself, but felt he had certainly come close. He finally thanked everyone within the Association who had made his steep learning curve of the past year such an enjoyable experience.
After the applause for Dr Shears' inspiring account, Mr Mark Armstrong asked what colour filters he had used in his photometry. Dr Shears replied none: he had tried using a V-band filter, but it had reduced his limiting magnitude by 1-2, severely restricting the science that could be achieved. With so small an aperture, he needed all the photons he could get.
Expressing his personal congratulations to Dr Shears on his fine work, the President then adjourned the meeting until June 25 at the Cavendish Laboratory in Cambridge.