AAVSO Binocular Program

Welcome to the AAVSO Binocular Program, an initiative of the AAVSO's Long Period Variable Section.  Stars that form this program are selected as being ideal for observers with binoculars -- both for magnitude range and for arrangement and field size for comparison stars. 
You may find a list of stars in the current AAVSO Binocular Program in the files section of this website.

The AAVSO is pleased to note that the AAVSO Chart Plotter ( http://www.aavso.org/vsp ) now includes an option to plot binocular charts for these program stars.  These charts come with preselected comparison sequences and field sizes ideal for most binoculars.  We hope you find these useful, and hope you can take advantage of them in your observing sessions tonight!


by David B. Williams

(Adapted from AAVSO Eyepiece Views, June, 2006)

The meaning of the word binocular says it all: two eyes.  There are two types of binoculars, field glasses and prismatic binoculars.  Field glasses (and the low-power version, opera glasses) are "straight-through" optical systems with only objectives and eyepieces.  Prismatic binoculars include a set of prisms that fold the light path between the objectives and eyepieces, creating a more compact optical system.

Simple Binoculars

My first optical instrument was a pair of 6x30 binoculars.  These revealed many Messier objects and I also used them to make my first variable star estimates.  Many amateurs begin with binoculars, because they are often available in the household when the initial interest in astronomy is born.

The Sky and Telescope web site has a feature titled "Binoculars, Halfway to a Telescope."  This is a calumny, because binoculars are actually twice a telescope!  Binoculars aren't second-choice substitutes for "real" telescopes, they are simply small telescopes.

What's good about binoculars?  They are small, eliminating considerations of storage and transportation.  And they can be put to use quickly - no set-up time!  When the temperature is far below freezing, I can step outdoors with my binoculars and make several estimates of familiar variables and jump back indoors in less than five minutes.

Binoculars are inexpensive.  Reasonably good binoculars can be purchased for just a few dollars on eBay (I bought my secondhand 7x35s for $17).  They are multi-use instruments that can be used for sports and nature viewing as well as astronomy.   And binoculars are wide-field, erect-image instruments, making it easy to find your way around the night sky.

What's bad about binoculars?  Not much.  They have small apertures and low magnifications, but these characteristics are also advantages when you are observing bright variables and need a wide field of view.Best of all, binoculars let you use both eyes!  Since the evolutionary development of the eye 600 million years ago, vision has been based on the combination of two visual signals.  Your brain is wired to process two

visual channels, enhancing resolution and contrast.  When you think about it, looking at anything with just one eye is a very strange thing to do.  Monocular telescope users are sometimes advised to keep both eyes open, covering one with a hand or eye patch, to relieve the deficiencies of viewing with just one eye.

Binocular Specifications

Binoculars are designated by their magnifying power and their aperture in millimeters.  Thus, my first pair of 6x30 binoculars provided a magnifying power of 6 with an aperture of 30 mm.  Most binoculars come in power-aperture ratios of 1:5 (6x30, 7x35, 10x50) or 1:7 (7x50, 9x63, 11x70).

Dividing the aperture of binoculars in millimeters by the magnification gives you the diameter of the optical system's exit pupil.  Thus, 1:5 ratio binoculars produce an exit pupil of about 5 mm, while the 1:7 ratio produces an exit pupil of about 7 mm.  The maximum aperture of the dark-adapted human eye is about 8 mm, which shrinks with advancing age (that's why children can usually see fainter stars with the unaided eye than adults, another example of how youth is wasted on the young).  Their larger exit pupil is why 1:7 ratio binoculars are sometimes called "night glasses," because they provide the largest exit pupil that the human eye can accommodate, and therefore the brightest images.

This means that 1:7 ratio binoculars give the brightest views of nebulae and star fields in a dark sky.  (Binoculars with a ratio greater than 1:7 would produce an exit pupil larger than the human eye can accommodate and some of the collected light could not enter the eye.)  But in a moonlit or light-polluted sky, 1:7 binoculars also provide the brightest sky background, drowning out faint stars and nebulosity.  So for variable star observers, a power-aperture ratio of 1:5 or smaller is preferable.  We want to darken the sky background and see fainter stars.

Nonetheless, all types of binoculars are useful for variable star observing.  Use whatever binoculars you have!  My own working set includes 4x30, 7x35, 10x50, and 20x60, and I occasionally use a pair of 25x100 at a club observatory.  It should be noted that binoculars can magnify too little for optimum astronomical use.  I can't see stars much fainter than about magnitude 6.0 with my 4x30s, because they don't magnify enough to fully exploit the available aperture, but these small binoculars are excellent for estimating variables in the 4th to 5th magnitude range.  These variables are a little too faint to estimate easily with the naked eye in a bright sky and too bright for easy observation in more powerful binoculars.  The 7x35s reach magnitude 8.0, the 10x50s magnitude 9.0, and the 20x60s magnitude 10.0 in a fairly dark sky.

Some observing tips can improve the observation of variable stars with binoculars.  First and foremost, many modern binoculars are now produced with orange or green objectives to penetrate haze - do not use binoculars with colored lenses to estimate variable stars!

Also keep in mind that binoculars are altazimuth instruments (with hand-held binoculars, you are the mount).  This means that field rotation effects cannot be avoided.  To minimize errors caused by the changing angle between the variable and each comparison star, bring each star to the center of the field of view before deciding on your estimate.

Binoculars are intended for hand-held use, which is fine for stars well above your binoculars' limiting magnitude (though one disadvantage of hand-held binoculars is that you have to reacquire the field every time you look away to check a chart).  When observing dimmer stars, you can increase your faint limit by at least 0.5 magnitude by resting an elbow on a fence post or bracing the binoculars against the side of a utility pole or the corner of a building.

Mounting binoculars on a photo tripod produces an even steadier view, but it is difficult to use tripod-mounted binoculars when observing high in the sky.  The best observing investment I ever made was the purchase of one of those "parallelogram" binocular mounts for my 20x60s.  It allows me to effortlessly raise and lower the binoculars, lets me get underneath to observe near the zenith, and stays fixed on the field of view when I look away to consult a chart.

Unless you observe from a desert location, dew is the greatest enemy of binocular observers.  Binoculars should be kept capped whenever they aren't being used.  Styrofoam cups make excellent insulating caps. I pop them over the objectives whenever I am not actually looking through my mounted 20x60 binoculars.  I keep smaller binoculars in a covered storage tub when I'm not actually using them.  Simple dew shields for each objective can be made from rolled cardboard and adhesive tape.  During the warm, humid summer months, I have found that electric anti-dew heaters are essential during long observing sessions (and dew heaters keep frost from forming in cold weather).  Alternatively, a hair dryer is noisy and has to be used again and again, but it will do the job.

The AAVSO program includes hundreds of variables that can be observed through all or part of their range with binoculars.  Some of our top observers have never used any other instrument.

Can variable star estimates made with binoculars contribute to "real science"?  Binocular observations are just as useful as estimates made with large telescopes.  A variable of a particular type has exactly the same scientific potential whether it is 6th magnitude or 16th.  If you observe stars that are appropriate for your aperture, quality of the estimates is the only consideration.