Observers’ Forum Lunar occultations of double stars – a plea for observations Figure 1. A video frame taken before the occultation of ZC3524. Figure 3. Lightcurve of the disappearance generated by Tangra. The International Occultation Timing Associa- 20cm aperture f/20 tion (IOTA) undertakes a worldwide programme Maksutov−Casse- of video-recording lunar occultations of known grain telescope. Em- and suspected double stars, to ascertain their bedded times − accu- duplicity, position angle, separation and differ- rate to 1ms − were ence in magnitude. generated by a GPS During the 2011 European Symposium on video time inserter Occultation Projects (ESOP) hosted by the Arch- (Figure 1). enhold Observatory in Berlin and attended by Observing condi- members of IOTA’s European Section (IOTA- tions were not good. ES), Tim Haymes and I discussed observing oc- The gibbous Moon cultations of double stars in addition to our regu- was 86% illuminated lar asteroidal occultation programmes. in a hazy sky, with Using predictions from Occult Watcher1 gen- the disappearance oc- erated by Occult2 via a feed from an IOTA-ES curring at a small cusp add-in,3 we have successfully monitored a few angle of 19° (Figure 2) Figure 4. Jan Manek’s reduction of the disappearance, processed with LiMovie. double star events, which did not indicate any which gave a poor sig- duplicity within the temporal resolution of our nal to noise ratio. recording systems. The video recording was processed in published in the Journal of Double Star Obser- However, on 2011 November 6 I recorded the VirtualDub4 and Tangra,5 resulting in the light- vations.9 We will also communicate his results disappearance of ZC3524, a mag 6.9 star in Pis- curve in Figure 3. to the Double Star Advisor of the BAA’s Deep ces. The Washington Double Star Catalogue lists There is an indication of a probable step in Sky Section. the star as having 2 components, with magni- the lightcurve, about 0.4s in duration, just be- Brian has asked that more UK observers un- tudes of 7.4 and 7.9 at a separation of 0.36 arc- fore the total disappearance. This is also seen in dertake this work, to achieve a greater spread of seconds (epoch 2011.8). The recording was made a 2-values moving average. observations at different position angles around with a Mintron video camera operating at 25 A total occultation report was submitted to the Moon’s limb, preferably from observing lo- frames per second, at the prime focus of my Jan Manek (IOTA occultations regional coordi- cations at least 300km apart. nator for Europe) and a double star report to If you are interested in taking part in this pro- Brian Loader (IOTA double star coordinator). gramme, please contact the BAA Lunar Section Jan analysed and reduced the recording Occultations Coordinator [occultation@ using LiMovie6 (Figure 4). baalunarsection.org.uk] who can advise you Jan’s reduction7 confirmed the double oc- on the equipment and techniques required. You cultation, showing that the fainter component don’t have to be a member of IOTA-ES. disappeared first, with a step height of 66% and a time interval of 0.343s. I recorded a Alex R. Pratt magnitude difference (unfiltered) of about References 0.75m, larger than the published value of 0.5m. Brian commented8 that serendipitously, 1 http://www.hristopavlov.net/Occult Derek Breit (USA) had recorded a disap- Watcher/OccultWatcher.html pearance of ZC3524 on 2011 December 4. 2 http://www.lunar-occultations.com/iota/ Our lunar limb position angles differed by occult4.htm almost 90°, the optimum for double star 3 http://www.iota-es.de/OWfeed_intro.html 4 http://virtualdub.org/ analysis. By combining both of our observa- 5 http://www.hristopavlov.net/Tangra/ tions, Brian was able to estimate the posi- Tangra.html tion angle, separation and magnitude differ- 6 http://www005.upp.so-net.ne.jp/k_miyash/ ence of the double star’s components.The occ02/limovie_en.html results have been included in his forthcom- 7 Jan Manek, priv. comm., 2012 March 11 Figure 2. Moon map at the time of the occulta- ing paper ‘Lunar occultation observations 8 Brian Loader, priv. comm., 2012 March 13 tion generated by Occult. of double stars – Report #3’ which will be 9 http://www.jdso.org/ J. Br. Astron. Assoc. 122, 3, 2012 177 Observers’ Forum Deep Sky Section NGC 6826 − the Blinking Planetary Light June nights are not ideal for deep sky as- When you look straight at tronomy and many observers restrict their tar- the planetary you will be gets to the brighter showpiece objects, leaving using the central cone cells more challenging quarry until darker skies re- in your eye which, although turn. However, whatever you like to observe, sensitive to colour, are not challenging or not so challenging, the summer particularly sensitive to faint constellation of Cygnus will almost certainly light. The result is that the feature in your plans, and even without fully outer nebula shell disappears dark skies there are many wonderful objects while the central star remains within its boundaries to enjoy. Planetary nebu- bright. To see the shell you lae are some of the most beautiful objects in the need to use averted vision, sky and many being small and bright are ideal that is to look to one side of targets for lighter summer skies. Cygnus con- the object rather than straight tains many superb examples with NGC 6826, at it, thereby bringing your Andrea Tasselli commonly known as the Blinking Planetary, being light-sensitive rod cells into play. The nebula This is caused by the so-called snow plough one of the best. will then reappear. Switching between direct and effect, when fast material ejected from the dying When William Herschel discovered it in 1793, averted vision can make the nebula appear to star collides with slower material ejected earlier. he catalogued it as number 73 in his class IV blink on and off, hence its name. The Blinking Planetary is not an easy object objects: planetary nebulae. Herschel’s classifi- Observations of NGC 6826 have been received in which to see detail visually, and to see any of cation system was morphological, and planetary in the Section from Andrew Wilson, Cliff this structure, or even hints of it, will require a nebulae to him were anything that appeared Meredith, David Wagstaff, Ian Haygreen, Andrea telescope in the half-metre class, very steady small, round and sometimes greenish in colour, Tasselli, Peter Grego, Dale Holt, Paul Downing seeing, high power and almost certainly an OIII i.e. anything that looked planet-like and particu- and Peter Taylor. Andrew Wilson’s image was filter. Peter Grego’s drawing shows how the larly like the planet Uranus that he discovered in taken with a Meade LX200 coupled to a MX916 nebula appears through a smaller instrument. 1781. The term coined by Herschel has stuck, CCD camera and is composed of 10×60s frames. Sketched through his 20cm SCT at ×75 it ap- even though planetary nebulae, confusingly, have In addition to the bright central star and the outer pears like a very small unresolved globular clus- nothing whatsoever to do with planets. halo, two small spots of nebulosity can be seen ter: or perhaps, with a bit of imagination, like The Blinking Planetary lies at RA 19h 44.8m on either side of the central star. These can be Herschel’s planet Uranus that led him to coin and Dec +50° 31' (2000.0) which puts it just over seen in more detail in Andrea Tasselli’s fine image the term ‘planetary nebula’ so many years ago. 5° ESE of kappa Cygni, the magnitude 3.8 star taken through his 20cm Intes Micro M809 f/10 forming the western wing tip of the Swan, i.e. the Mak−Cass fitted with a SX SXV-H9 CCD cam- Stewart L. Moore, Director opposite wing to that containing the Veil Nebula. era. Exposures in this image were 205×20s L plus Or, if you are observing from a dark site and can 24×25s each RGB. see fainter stars, it lies just over 1° east of mag 4.5 These spots are known as Fast Low Ioniza- theta Cygni. At magnitude 8.8 with a diameter of tion Emission Regions or FLIERs for short, and around 25 arcsec it will be immediately obvious, they are seen in many planetary nebulae. Their even in quite a small telescope, as a greenish-blue exact nature and formation is still not properly slightly out of focus star, and if the atmosphere is understood, although it is likely that they result steady allowing high power to be used, it will from material ejected from the poles of the dy- appear slightly elongated. ing star at some point in its evolution. The Increasing telescope aperture to something like Hubble Space Telescope image of this planetary 20cm will make the outer shell very obvious, (http://hubblesite. org/gallery/album/pr199 but only if you try not to look directly at the 7038d/) shows them particularly well. Also magnitude 10.6 central star. It is this that gives shown in Andrea’s image is an elongated ring of the nebula its name − the Blinking Planetary. brighter (denser) material around the central star. Andrew Wilson & Peter Hewitt These contributions are copyright © the Journal of the British Astronomical Association, www.britastro.org/journal. If you wish to reproduce them, or place them on your own Web page, please contact the Editor: Mrs Hazel McGee, hazelmcgee "at" btinternet.com Peter Grego 178 J. Br. Astron.