GETTING TO KNOW YOUR TELESCOPE

From our own cosmic backyard to objects far beyond our galaxy, your instrument will reveal countless wonders of the night sky

The larger a telescope’s aperture, the more light is collected and the more detail is revealed. Telescopic magnification depends on the telescope’s focal length and the focal length of the eyepiece. Magnification is calculated by dividing the telescope’s focal length by the focal length of the eyepiece. The range of useful magnification depends on aperture and the focal length of both the telescope and the eyepiece. Too low a magnification will waste light, as the ‘exit pupil’ of the eyepiece will be larger than the diameter of the pupil of your dark-adapted eye. As an average, an adult pupil will dilate to seven millimetres (0.3 inches) in dark conditions. Therefore the exit pupil delivered by an eyepiece ideally needs to be smaller than this, especially when viewing faint objects, so that the telescope can take in all the light. Exit pupil can be calculated by dividing the telescope’s aperture by the magnification of the eyepiece.

Equatorial mounts allow celestial objects to be tracked as they appear to move across the sky, but they require initial alignment with the celestial pole. Once the polar axis is aligned, an astronomical object centred in the field can be kept there by manually moving the telescope from east to west in pace with the apparent movement of the sky. With a standard German equatorial mount, ensure that your telescope is properly balanced, adjusting counterweights to balance the weight of your telescope. Loosen the axes clamps on the telescope and ensure the telescope’s weight is centred on its own axis. Once the right balance is found, it should respond to a light touch in the desired direction without tipping of its own accord.

The polar axis of an equatorial mount needs to be pointed towards the north celestial pole. If you have a portable instrument and set it up from scratch each session, you’ll probably be content with pointing it towards Polaris, which lies conveniently near the celestial pole. Align the polar axis in both compass direction and angle of inclination, both of which can be adjusted on all types of equatorial mounts. This can be achieved at a basic level by simply squinting along the polar axis so that it’s pointing in Polaris’ general direction. This will allow you to lock onto the object’s altitude so that celestial objects can be tracked in the field of view for half an hour or so. If your mount isn’t electronically driven, an object can be followed as it tracks from east to west by simply nudging the telescope in one direction. A motor drive will permit a ‘hands-off’ experience, enabling you to perform astrophotography.

Many instruments are computer driven and can keep objects centred in the field of view by gradually adjusting the telescope on both axes of rotation. They require their own methods of set-up depending on the system used, whether it’s a simple tracking device, a Go-To instrument or GPS-enabled system. If all of this sounds a little daunting, remember that there’s plenty of online advice from astronomical societies, various forums and discussion sites – astronomers are a friendly bunch and only too willing to help beginners.