THE BAA OBSERVERS’ WORKSHOPS Cambridge Winchester York 2003 February 15 2003 April 26 2003 September 6 Milton Keynes Nottingham 2004 February 28 2004 April 24 Observing variable stars Workshop No. 3: with binoculars York 2003 September 6 by Melvyn D. Taylor Introduction (GCAS, RCB), Universal Time, that is British Summer Pulsating (M, SR, Time minus one hour. Universal Time RV), Cataclysmic (N, NR, ZAND), abbreviated UT (or Greenwich Mean One of the finest aspects of visual observ- Eclipsing (subtypes EA, EB, EW). The Time, GMT) beginning at midnight is used ing is freely roaming around a dark night General Catalogue of Variable Stars (GCVS) in nearly all astronomical records. Some sky picking out galactic and solar system and Information Bulletin on Variable Stars observers specialising in eclipsing binaries bodies, noting patterns of stellar hues and (IBVS) are the source for descriptions of use the Greenwich Mean Astronomical brightness. The binocular is an ideal tool for the categories. Dependent on an individual’s Time (GMAT) system that can be this pursuit. Use of two eyes, the natural interests the longer period (interval between confusing in relation to the date, as it state, is relaxing, easy and relatively quick, successive maxima) objects may be more commences a new ‘day’ at noon. although for optimum night-time observing appropriate for observers with limited time In the 1850s Norman Pogson (1829– the eyes have to adapt to the conditions to spend. Those with lots of spare time 1891) instigated the modern definition of which does take time. available may choose eclipsing binaries that a stellar magnitude scale and a fixed ratio Whether observing as a beginner or a show an eclipse in one night; these objects of 2.512 corresponding to one magnitude more experienced astronomer, binoculars require an estimation of the magnitude difference. Five magnitudes therefore are useful in many areas; eclipses, lunar every 10 to 15 minutes around the time relate to a ×100 change of brightness. occultations, comets, meteors, asteroids, when an eclipse is predicted. In general the Visual magnitude differences of the deep sky, artificial satellites, novae frequency of clear skies will also prove comparison ‘standard stars’ may be hunting and variable stars. The BAA important in selecting the type and number appreciated by habitual and systematic Variable Star Section, formed in 1890, has of variables to follow. The basic visual observing using a particular specification in its database around 1.6 million observation is an estimate of the star’s of binocular. In general, the naked eye observations available to amateur and magnitude as compared with ‘standard under a very clear sky could detect stars professionals. Programmes of observation stars’ at that time. to a magnitude of 6.2, a typical 50mm include long-standing telescopic and binocular with similar conditions may binocular variables, also a list of recurrent allow magnitude 10.5 objects to be seen variables, a set of eclipsing binary stars Time and magnitudes but these values are unlikely in practice and an important area called the ‘new (Table 1). variables’ based on those stars which Time to the nearest minute, from an Mike Collins has found and which require accurate source, is normally required and it follow-up work. is useful to have a standard clock, for Table 1. Theoretical limiting magnitude, resolution example, the and magnification for different binocular apertures Rugby MSF (60 Types of programme kHz) receivers Aperture Limiting magnitude Resolution Magnification stars may be acquired (mm) (2+5logA) (90% effic.) (arcsec) for exit pupil of as wall clocks or 5mm 8mm wrist watches. 20 8.5 7.6 5.8 4 2.5 Apart from some variables which defy The signal from 25 8.9 8.1 4.6 5 3.1 being ‘boxed’ there are four major classifica- Mainflingen 30 9.4 8.5 3.9 6 3.7 tion groups, described as eruptive, pulsat- DCF77 on 77.5 40 10.0 9.0 2.9 8 5.0 ing, cataclysmic and eclipsing. The kHz is also being 50 10.5 9.4 2.3 10 6.2 subtypes are many and differing forms of used in similar 60 10.8 9.8 1.9 12 7.5 variation may be occurring in a single star or 70 11.2 10.1 1.7 14 8.7 timepieces. For 80 11.5 10.4 1.5 16 10.0 a stellar system. The main classes and most purposes 100 12.0 10.8 1.2 20 12.5 subtypes (abbreviated) of variable sug- time is best 125 12.4 11.2 0.9 25 15.6 gested for binocular use are: Eruptive recorded in J. Br. Astron. Assoc. 114, 6, 2004 349 Taylor: Observing variable stars with binoculars restored without it being lost completely. A torch’s white light covered with red paint or cellophane is an essential item for visual observers. Those troubled by stray lights often use a black cloth placed over the head and eyepieces to help cut down side reflections. Binocular eye- pieces with rubberised side guards are particularly handy in these circumstances Figure 3. Exit pupils (4mm) of a 20×80 and may help larger apertures to be binocular (courtesy Leeds AS). steadied. Shape of the binocular for hand holding is tion is a bad sign and if used persistently important and before purchasing, different without being properly recognised it could models should be compared and examined affect one’s eyesight. It should be possible visually and mechanically. A one-piece for an optical specialist to amend this fault (American styled) barrel design is inher- but the cost may well be a large proportion Figure 1. Three low power binoculars. Left to ently tough, a genuine Zeiss style (H of its original value. Large and heavier right: 8×40 Swift Trilyte (roof prisms), 3×20 profile) which has a two-piece body is also binoculars that have a long rod extending opera glass, 8×42 Opticron (porro prisms). strong. Bird-watching and photographic from the hinge base to a support bridge suppliers of small telescopes and binoculars nearer the objectives are guarding against vary in their sympathy to an amateur distortion and bad collimation. Binocular basics and astronomer keen to critically inspect their Waterproofing of the binocular is an added the eye wares so it is probably wise to inform them bonus since repeated temperature and of the quality expected. humidity changes cause condensation and mould to affect the internal parts. Over time The basic binocular specification such as with use and possibly the odd accident, 8×30 or 10×50 has the power first, the optical degradation of stellar images will number of times an object is magnified. The Binocular parts and occur probably due to the prisms moving or second number is the diameter in millime- choices coating corrosion. A suggestion for anyone tres of the front lenses, the aperture. Large with ideas about multiple use (daytime and objectives are for fainter stars and more night-time) would be to own two binoculars detailed views. Comparing a 10×40 with a Most binoculars have the angular field of so the astronomical one is kept for that 10×70 the larger collects 70×70/ 40×40 or view or its dimensional cotangent inscribed purpose only. 3.0 times more light, so objects appear 1.2 on the body housing as a number of feet In quality binoculars all air-to-glass magnitudes brighter. A table of fairly seen at a distance of 1000 yards. (1 degree surfaces are fully multicoated, which means common specifications is given as a guide of arc or two lunar diameters approximately that ghost images and internal reflections only. The choice of binoculars will involve corresponds with 52ft at 3000ft). For a true are reduced. The technique of depositing the observer’s interest(s) and age, condi- binocular field of 5° on an 8×30, the optical coats on polished glass began in the tions of use (e.g. if from a dark or polluted apparent field is 40°. A simple check on the 1930s. The best coatings can be judged by site), aperture, magnification, body design true field of view is useful and this may be looking down on the objective with a hand (handling appeal), weight and the cost. simply done by finding two stars that fit over the eyepiece and examining reflections. Size of the binocular exit pupil, that is, across the circle diameter and then check A totally white reflection means an optical aperture in millimetres divided by the the celestial spacing in degrees and minutes surface is not coated. Multi-coatings reveal magnification, is likely to be the critical of arc from the scale of a star atlas. reflections that are noticeably fainter. factor since it is important to try to The hinged bridge that gives the eyes Checking for blemishes on the optics by match this with the observer’s eye pupil access to the eyepiece centres should be turning the binocular around and inspecting under working conditions. The eyes will smooth and capable of staying in the the objectives against various lighting angles give the clearest detail operating in the required position. The two circular fields of is a useful task. Looking at a thin vertical or range 2mm to 5mm, their normal size view should merge into one if both optical horizontal element (a radio mast, or an during daylight. Andrew Langley’s barrels are aligned correctly and for all aerial) through one barrel at a time should comments (BAA Journal, 2004 April) on positions around the hinge. Poor collima- display minimal curvature and little false visual acuity in relation to the exit pupil are well worth noting.