USING HIGH MAGNIFICATION
Knowing when to boost the magnification of your setup will help to reveal hidden detail
ONE MIGHT IMAGINE that using lower eyepiece magnifications would make detail in deep-sky objects easier to see by increasing the object’s overall surface brightness. However, this is generally not the case: higher magnifications usually allow you to see more detail.
As you increase your magnification two changes occur that affect the visibility of low-brightness and low-contrast detail in deep-sky objects. The most obvious change is that the detail gets larger, clearly making it more visible. The second change is that both the background brightness and the object brightness fall. Even though the relative contrast stays the same, this decrease in brightness makes things harder to see. Up to an optimum magnification, the benefit of increased size more than outweighs the fall in brightness – you’ll find that increasing magnification actually allows you to see more detail. Beyond the optimum, the benefits of further boosting the size tails off and things then become harder to see overall because the falling brightness wins out.
The brain and eye work best when the detail you want to see appears 1-2° across. So experiment at the eyepiece, increasing the magnification until image details are about this apparent size. In a large complex object like the Orion Nebula or the Whirlpool Galaxy, where there are a range of different detail sizes, brightnesses and contrasts, you will probably find that you need to use a range of different magnifications to pull out all the different detail present – lower magnifications for the really large features and higher magnification for the finer detail.
Targets with high surface brightness and fine detail, such as the Eskimo Nebula or the Cat’s Eye Nebula, can benefit from really bumping up the magnification to see all the detail present, but do pick a night where the atmosphere is steady. High powers can also be used to advantage to enable you to see the faintest stars in globular clusters or to detect small faint galaxies. Such objects at the threshold of vision are regarded as point sources by the eye and brain. This means that increasing the magnification leaves them unaffected but darkens the sky, so increase the effective contrast and allow them to pop into view.