Using Binoculars for VSO

Using binoculars to observe variable stars is an excellent decision because binoculars offer several advantages over a telescope when it comes to observing variable stars. First, they are easy to use and require a minimum amount of effort to set up and take down. Secondly, binoculars offer a wider field of view so that finding comparison stars is relatively easy.

If you believe that binoculars will not allow you to make detailed observations or will in some way inhibit your ability to detect subtle changes in the brightness of stars, take a look at some light curves produced by Kari Tikkanen, an observer from Finland who uses binoculars to observe variable stars. Kari uses 10 x 50 and 12 x 50 binoculars and reports his observations to the Swiss Astronomical Society (BBSAG) as well as the American Association of Variable Star Observers (AAVSO). His timings are published in the BBSAG bulletin.

The first light curve is of the star HU Tau, a semidetached, eclipsing binary of the Algol type (EA/

SD) with a period of 2^056. At maximum light its brightness is about 5"'85 and at minimum about 6™68 (Figure 9.1). As you can see, Kari has been able to plot the eclipse very well with 78 observations. As a testament to his accuracy and ability, Kari records his observation to an accuracy of two decimal points (a hundredth of a magnitude).

The second light curve is of the star U Sge, another eclipsing binary. This star has a period of 3"?38, a maximum brightness of about 6T45 and a minimum of 9T28 (Figure 9.2).

Using 206 observations, Kari was able to observe the full eclipse (down to 9™3) with exceptional accuracy. It should be obvious that binoculars will not limit your ability to observe variable stars! Nor will stars with small amplitudes pose any serious hurdle to serious observers. There are many hundreds, perhaps thousands, of stars suitable for observing with suitable binoculars. Let's look at what makes binoculars suitable for variable-star observing.

The sharpness and brilliance of any image that you see when looking through a particular binocular are determined by a number of different factors, including the interaction of lens diameter, magnification, optical coatings and design. Overall, most binocular observers will agree that the most important consideration regarding binocular performance is the quality of the optics.

Magnification is the degree to which the object being viewed is enlarged. For example, with a 10 x 50 binocular, the number 10 indicates the magnification power. The level of power affects the brightness of an image, so the lower the power of a binocular, the brighter the image. In general, increasing power will reduce both FOV and eye relief (the distance from your f

Figure 9.1. Lighten, of HU Tou. Data providjd by the Kari Tikkanen, USSa VSS. Used wilti permiiiiffi

Figure 9.2. Light cuive of U Sge. Data provided by the Kari Tikkanen. URSA VSS. Used with permission

Figure 9.2. Light cuive of U Sge. Data provided by the Kari Tikkanen. URSA VSS. Used with permission eye to the lens). Remember, if you wear glasses, you can observe without them by using the binoculars as your corrective lens. I wear glasses and contact lenses and I find my vision at the telescope to be noticeably sharper without corrective lenses so I use a strap to keep my glasses close by, for when I need to look at a chart or atlas ... or find my food. When visually observing, 1 do not wear my contact lenses either.

The objective lenses of binoculars are the front lenses. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular binocular, A 10 x 50 binocular has an objective lens of 50 mm. The diameter of the lens determines the light-gathering ability of the instrument with the greater light-gathering ability of a larger lens usually translating into greater detail and image clarity. Doubling the size of the objective lenses quadruples the light-gathering ability of the binocular.

The size of the area that can be seen while looking through a pair of binoculars is described as the field-of-view. Field-of-view is related to magnification, with greater magnification creating a smaller field of view. Low-power, therefore a large field-of-view, is especially desirable when observing variable stars because you will see more comparison stars. It's also easier to figure out were you are in the sky with a larger FOV because more stars are visible.

Eye relief refers to the distance, in millimeters, that a binocular can be held from the eye and the full field of view can still be comfortably observed. Eyeglass wearers in particular benefit from longer eye relief.

The optical elements of the binocular are coated to reduce internal light loss and glare, that in turn insures even light transmission, resulting in greater image sharpness and contrast. Choosing a binocular with good lens coatings will translate into greater satisfaction. Lens coatings range in quality as follows: coated, fully coated, multicoated, fully multicoated. Coated lenses are the lowest quality. Fully coated lenses are economical and may work well for you. Multicoated or fully multicoated lenses are both very good choices but the price is higher. Fully multicoated lenses give the best light transmission and brightest images but the price is higher.

A critical factor in the performance of any binocular is its construction. The security of the barrel alignment and proper internal mounting and alignment of the optics are crucial to producing a binocular that's mechanically reliable, smooth functioning and long-lasting. Check this carefully.

The alignment of the optical elements of the binocular to the mechanical axis is called collimation. Good collimation prevents eyestrain, headaches, inferior and double images while improving resolution. Unfortunately, proper collimation is almost impossible to achieve in very low-priced binoculars that lack quality components and design.

As you see, there are a number of different factors to consider in choosing a binocular. Perhaps the first thing to understand is that binoculars are really just two small telescopes mechanically linked together. All of the facts and formulas that help you to understand telescopes also pertain to binoculars. Each side of a pair of binoculars has a prime focal length, an objective lens, an ocular (eyepiece), an exit pupil, and so forth. Even if you plan to observe using binoculars only, the following section pertaining to telescopes is worth your time.

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