Introduction:

One of the finest aspects of visual observing is freely roaming around a dark night sky picking out galactic and solar system bodies, noting patterns of stellar hues and brightness. The binocular is an ideal tool for this pursuit. Use of two eyes, the natural state, is relaxing, easy and relatively quick, although for optimum night-time observing the eyes have to adapt to the conditions which does take time. Whether observing as a beginner or a more experienced astronomer, binoculars are useful in many areas; eclipses, lunar occultations, comets, meteors, asteroids, deep sky, artificial satellites, novae hunting and variable stars.

The BAA Variable Star Section, formed in 1890, has in its database around 1.6 million observations available to amateur and professionals. Programmes of observation include long-standing telescopic and binocular variables, also a list of recurrent variables, a set of eclipsing binary stars and an important area called the 'new variables' based on those stars which Mike Collins has found and which require follow-up work.

Types of programme stars:

Apart from some variables which defy being 'boxed' there are four major classification groups, described as eruptive variable stars, pulsating variable stars, cataclysmic variable stars and eclipsing variable stars. The subtypes are many and differing forms of variation may be occurring in a single star or a stellar system. The main classes and subtypes (abbreviated) of variable suggested for binocular use are: Eruptive (GCAS, RCB), Pulsating (M, SR, RV), Cataclysmic (N, NR, ZAND), Eclipsing (subtypes EA, EB, EW). The General Catalogue of Variable Stars(GCVS) and Information Bulletin on Variable Stars (IBVS) are the source for descriptions of the categories. Dependent on an individual's interests the longer period (interval between successive maxima) objects may be more appropriate for observers with limited time to spend. Those with lots of spare time available may choose eclipsing binaries that show an eclipse in one night; these objects require an estimation of the stellar magnitude every 10 to 15 minutes around the time when an eclipse is predicted. In general the frequency of clear skies will also prove important in selecting the type and number of variables to follow. The basic visual observation is an estimate of the star's magnitude as compared with 'standard stars' at that time.

Time and magnitudes:

Time to the nearest minute, from an accurate source, is normally required and it is useful to have a standard clock, for example, the Rugby MSF (60 kHz) receivers may be acquired as wall clocks or wrist watches. The signal from Mainflingen DCF77 on 77.5 kHz is also being used in similar timepieces. For most purposes time is best recorded in Universal Time, that is British Summer Time minus one hour. Universal Time abbreviated UT (or Greenwich Mean Time, GMT) beginning at midnight is used in nearly all astronomical records. Some observers specialising in eclipsing binaries use the Greenwich Mean Astronomical Time (GMAT) system that can be confusing in relation to the date, as it commences a new 'day' at noon. In the 1850s Norman Pogson (1829 1891) instigated the modern definition of a stellar magnitude scale and a fixed ratio of 2.512 corresponding to one magnitude difference. Five magnitudes therefore relate to a 100 change of brightness. Visual magnitude differences of the comparison 'standard stars' may be appreciated by habitual and systematic observing using a particular specification of binocular. In general, the naked eye under a very clear sky could detect stars to a magnitude of 6.2, a typical 50mm binocular with similar conditions may allow magnitude 10.5 objects to be seen but these values are unlikely in practice

Binocular basics and the eye:

The basic binocular specification such as 830 or 1050 has the power first, the number of times an object is magnified. The second number is the diameter in millimetres of the front lenses, the aperture. Large objectives are for fainter stars and more detailed views. Comparing a 1040 with a 1070 the larger collects 7070/ 4040 or 3.0 times more light, so objects appear 1.2 magnitudes brighter. Atable of fairly common specifications is given as a guide only. The choice of binoculars will involve the observer's interest(s) and age, conditions of use (e.g. if from a dark or polluted site), aperture, magnification, body design (handling appeal), weight and the cost. Size of the binocular exit pupil, that is, aperture in millimetres divided by the magnification, is likely to be the critical factor since it is important to try to match this with the observer's eye pupil under working conditions. The eyes will give the clearest detail operating in the range 2mm to 5mm, their normal size during daylight. Andrew Langley's comments (BAA Journal, 2004 April) on visual acuity in relation to the exit pupil are well worth noting. Maximum pupil size may be 7mm to 8mm but the aging process will reduce this to about 4mm or 5mm. There is an approximate relation of 0.35mm reduction per 10 years as suggested in Sky & Telescope, 1992 May. Sky Publications market a small device that allows the pupil size to be measured. Ten or more minutes after going outside the eyes will have adapted (but not fully) to darker conditions and faint objects and stars will be apparent.

Shine a bright white light on a chart or open the 'fridge' door and this will immediately decrease the pupils' size faint stars will now be lost to view. A dim red light will have a less dramatic effect since night-time response to this wavelength is not so adverse and dark adaptation can be 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 eyepieces with rubberised side guards are particularly handy in these circumstances and may help larger apertures to be steadied. Shape of the binocular for hand holding is important and before purchasing, different models should be compared and examined visually and mechanically. A one piece (American styled) barrel design is inherently tough, a genuine Zeiss style (H profile) which has a two-piece body is also strong.

Bird-watching and photographic suppliers of small telescopes and binoculars vary in their sympathy to an amateur astronomer keen to critically inspect their wares so it is probably wise to inform them of the quality expected. Binocular parts and choices Most binoculars have the angular field of view or its dimensional cotangent inscribed on the body housing as a number of feet seen at a distance of 1000 yards. (1 degree of arc or two lunar diameters approximately corresponds with 52ft at 3000ft). For a true binocular field of 5 on an 830, the apparent field is 40. A simple check on the true field of view is useful and this may be simply done by finding two stars that fit across the circle diameter and then check the celestial spacing in degrees and minutes of arc from the scale of a star atlas. The hinged bridge that gives the eyes access to the eyepiece centres should be smooth and capable of staying in the required position. The two circular fields of view should merge into one if both optical barrels are aligned correctly and for all positions around the hinge. Poor collimation is a bad sign and if used persistently without being properly recognised it could affect one's eyesight. It should be possible for an optical specialist to amend this fault but the cost may well be a large proportion of its original value. Large and heavier binoculars that have a long rod extending from the hinge base to a support bridge nearer the objectives are guarding against distortion and bad collimation.

Waterproofing of the binocular is an added bonus since repeated temperature and humidity changes cause condensation and mould to affect the internal parts. Over time with use and possibly the odd accident, optical degradation of stellar images will occur probably due to the prisms moving or coating corrosion. Asuggestion for anyone with ideas about multiple use (daytime and night-time) would be to own two binoculars so the astronomical one is kept for that purpose only. In quality binoculars all air-to-glass surfaces are fully multicoated, which means that ghost images and internal reflections are reduced. The technique of depositing optical coats on polished glass began in the
1930s. The best coatings can be judged by looking down on the objective with a hand over the eyepiece and examining reflections. A totally white reflection means an optical surface is not coated. Multi-coatings reveal reflections that are noticeably fainter. Checking for blemishes on the optics by turning the binocular around and inspecting the objectives against various lighting angles is a useful task. Looking at a thin vertical or horizontal element (a radio mast, or an aerial) through one barrel at a time should display minimal curvature and little false colour to its edges. The eyepiece coatings are also worth checking. Quality lenses including the eyepiece design and fully multi-coated optics with BAK-4 prisms (light crown) are superior to BK7 (borosilicate) prisms bearing in mind relative cost of the type and body style. If the binocular is aimed at a bright background and is held about a foot away the bright exit pupils should have a dark or black surround and no internal parts should be visible.

Eye relief relates to how far the eye may be from the eyepiece and still see the full field of view. Observers who have astigmatism and use spectacles will need a long eye relief; those who have near or far sight normally take spectacles off and focus without any thought. The modern focusing mechanism is to have the left eye adjusted first by the wheel, with this one then shut (or objective capped) the right eyepiece is turned on its housing and the focus refined. This technique also corrects for different eye strengths. However, a binocular that has individual eyepiece focus could be considered slighter superior mechanically and optically.A wheel centre focus that turns both eyepieces at once is fast, individual ones a bit slower. Long dewshields extending from the objectives and large enough not to lessen the field of view will reduce reflections into the objective lenses, assist seeing faint objects and delay dew forming on the glass.

Free or fixed binoculars:

A magnification of up to about 10 if hand held and steadied may give suitable views, but over 12 a stellar image will be 'jittery' and it is advisable to have the binocular secured to a mounting for critical observations. There are two commercially made clamps that connect a binocular to a photographic tripod, the view will then be superior to a hand held one, fainter objects will be seen better and you are hands free. A screw connection in the base of the hinged bridge is a better position than one to either of the barrels. Several large binoculars use a centre bar with sliding clamp that is screwed into the tripod mount. There have been a number of mountings devised on an individual basis by binocular specialists with balanced/stabilised ones available from instrument suppliers. A sturdy tripod with fittings to mount the instrument in alt-azimuth mode is one method, but for real comfort a chair or recliner with a binocular attachment is potentially ideal. Urban observers hounded by permanently bright night skies may find that a higher magnification will show fainter stars, and in this respect large, custom-mounted instruments with interchangeable eyepieces would be an option. Charts, atlas and a torch or logbook can be used more easily if the binoculars are mounted. Some observers dispense with a record book and use a voice record, thereafter writing the observations directly into a log book or on a computer file for personal use and/or sending to a variable star organisation. The ultimate test for the casual observer is on focused stars. The changing aspect of a focused star image as the observer takes it across the field of view will make for interesting comparisons higher quality instruments should show these tiny circlets of light to near the field edge. The technique of bringing variable star and comparisons to the optical centre is a standard method that tends to offset the mercenary comment that follows. Image stabilised binoculars have a reputation for producing virtual pinpoint images from edge to edge, however their higher cost has to be considered in relation to a conventional instrument of larger aperture mounted firmly. Resolving close stellar pairs is an interesting pastime and another check on the binocular performance.

Variable star names:

The system started by F. W. Argelander (17991875) has, for example, the first-named variable star in Scutum as R Scuti, then proceeding with additional ones to Z Scuti. There were hundreds or thousands more found in particular constellations so after Z, RR, RS to RZ came about. Thereafter SS to SZ, and so on. From ZZ the formation went to double form AA to AZ and so on but J was not used because of its similar character shape to I. The double lettering to QZ provided for 334 designated variables. Identifying and naming additional objects then started at V335, in every case followed by the genitive form of the constellation or its accepted abbreviation. The 88 constellations have standard short forms using three letters, for example, V465 Cas (or in full Cassiopeiae).

Charts and sky scale:

The Variable Star Section observing charts use an analogy of stellar magnitude differences by the commonly used procedure of showing brighter stars with a larger diameter in comparison to fainter ones. Night-time stellar observers are looking at the smallest of tiny circles of light asdefined by the diffraction of light across the objective's clear diameter. By placing the programme stars UU Aur, U Boo, RX Boo, ST Cam, W Cyg, VY UMa have appeared in the BAA Journal. It is very useful to have access to a star chart or software to print large areas of sky with bright stars that are then used in conjunction with a variable star chart. The charts include the right ascension and declination of the variable, and a guide to the scale of the chart, usually either 18, 9, 3 or 1 size plots. In order to assist in finding objects the chart has an arrowed marker to the north-south axis. One of the first steps is to hold the chart so that its north-south marker is approximately aligned with the north pole star, to appreciate the chart's orientation. How large or small things appear on the sky, the scale, is not easy to grasp at first. Adegree of arc, about twice the lunar size, is shown as a bar on the chart so it may be compared with the observed field size. Atypical 1050 binocular field of view is 5 across the diameter, so ten http://www.britastro.org