USING LOW MAGNIFICATION
Increasing the amount of sky visible in the eyepiece can have some surprising benefits
LOWER MAGNIFICATION INCREASES the area of sky visible in your eyepiece. This allows some of the larger deepsky objects, such as the Andromeda Galaxy or the Double Cluster in Perseus, to look better by giving them a surrounding frame of darker background sky.
Dropping the eyepiece power also has the effect of increasing an object’s surface brightness by concentrating its light over a smaller angular area. As the power is progressively dropped the diameter of the bundle of light leaving the eyepiece – the socalled ‘exit pupil’ – also increases. Eventually a limit is reached where the exit pupil is larger than your own pupil and not all the light can enter your eye.
The magnification that matches the diameter of the eyepiece exit pupil with the diameter of your fully dilated pupil is your telescope’s lowest useful magnification – to find out the size of your exit pupil, see ‘Measuring Your Pupil Size’ on page 37. At this magnification the surface brightness of a deep-sky object reaches a maximum. Some objects, such as the Orion Nebula, may be bright enough to excite some of the cone cells in your eye, which may allow you to see some faint colour. Lower powers, combined with the use of movement, can also help with viewing the extremeties of very softedged extended objects by increasing the contrast gradient at those edges.