making astronomy accessible to all

Binoculars are one of the most useful tools for a new astronomer to own. They can be purchased relatively cheaply for £20-30, but a really good set will cost over £100. For night time use, purchase ones with larger objective lenses but modest magnification (10X50s are used by most people). Take a look at the eyepieces, if they have small apertures (5-10mm or less) don't buy them, as they have small eye relief and will be very difficult to use in practice.

You can buy specialist astronomical binoculars that have larger lenses (75mm and larger) but these tend to be very heavy and consequently only useable when mounted on a tripod. You can mount most binoculars on a tripod, even the very cheap ones, but you will need a binocular mounting bracket to do this with.

Trying binoculars

When buying binoculars, it's essential to try them out first; go to a shop that has a good selection of high class binoculars and select the most expensive large binocular on the shelf and ask to look through them; you will observe a clear bright image, with no tendency for colours to fringe (chromatic aberration) or create multiple images (parallax) and they will be relatively easy to focus. Look at some distant detailed objects in high contrast (signs or notice boards in bright daylight), and if possible one in shadow or poor illumination.

Now select the ones you really want to buy (or the ones you can afford) and try them; look out for the issues above, and decide if the compromise represents good value. Cheaper binoculars tend to suffer from poor alignment (giving rise to multiple images) and dull images or poor contrast (due to missing or cheap anti-reflection coatings). Very cheap binoculars will suffer from difficulty in focussing, and poor focus or chromatic aberration towards the edge of the field of view and the eyepieces may bend in and out of focus as your eyes push against the light shields. Chromatic aberration is less of a problem at night.

Porro prism and roof prism binoculars (both 10X50)

Binocular designs

Opera glasses work in a similar manner to a telescope, using appropriately designed lenses to re-erect the image, but as a result the level of magnification that can be provided is small.

Binoculars like the original Zeiss design use Porro prisms to collapse the focal train and erect the image through multiple reflections. Porro prism binoculars have the eyepieces offset from the objective (front) lenses. Roof prism designs use an advanced form of internal reflections to extend the light path and erect the image. With both designs the alignment and optical quality of the prisms is critical to the overall performance of the binocular. Because roof prism binoculars have the eyepiece in line with the objective lenses, they are more compact and generally lighter, however roof prisms are an order of magnitude more difficult to make and align, hence their cost.

Stabilized Binoculars

Stabs or stabilized binoculars use a gyroscopic stabilisation system to correct the angle of the light path within limits. They are ideal for hand held observations of faint objects, but are prohibitively expensive, costing anything up to 5 times the cost of a good pair of binoculars, and offering little improvement in light grasp (although their optics are usually superb).

Practical Bins

Most astronomers have a couple of pairs of binoculars for field use; it is extremely distressing to lose a very good pair of binoculars, or to sit on them by mistake when returning from an observing session. For that reason most purchase a cheaper large (10X50) pair. Bresser, Celestron, Praktica and Skywatcher are all reasonable brands to choose from.

Using Binoculars

The use of binoculars is straight forwards, but here are a few tips:

• If you wear glasses, it is usual to remove them, but this is a matter of choice and practice. If you keep your glasses on, you will need the eyepiece shields retracted.
• Use the eyepiece shield to keep your eyes at a fixed distance from the eyepiece (these normally screw out from the eyepiece)
• Before using your binoculars in anger, focus them correctly first. Pick an object with fine detail and good contrast, such as a TV aerial, or tree branch against the sky and look through the binoculars at it using your left eye, closing your right eye. Focus the binocular correctly using the main central focuser for your left eye, then open your right eye and adjust the differential focus on the right binocular (this is normally on the right hand binocular, and is a little thumb wheel next to the eyepiece).
• When 'targeting' an object to observe through binoculars, keep looking at the target (don't look through the binoculars first).

Move the binoculars up to your eyes, and position them so that the object you are looking at is central to your field of vision.

Looking through the binoculars first and then searching with them is more difficult, with practice you will be able to 'zero in' on a target more quickly if you keep your gaze stationary.

Article and photo by Noel Clark

Mid-Kent Astronomical Society
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