Why do images of the Moon at night sometimes contain an extra ‘ghost’ Moon off to one side? <\/p>

Ever notice that your photographs of the Moon show a \u2018ghost\u2019 image of the Moon off to one side?<\/span><\/p>\n

Don\u2019t worry, you haven\u2019t accidentally uncovered Earth\u2019s secret second satellite! It\u2019s all a trick of the light.<\/p>\n

Bright astronomical subjects such as the Moon have an overabundance of light.<\/p>\n

A bright Moon photographed against a dark sky is an extremely good source for generating spurious \u2018ghost\u2019 images offset to one side of the main subject.<\/p>\n

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So how to you fix the problem?<\/p>\n
It all depends on what arrangement you are using to capture your lunar images, but let\u2019s say it\u2019s a camera with a standard or telephoto lens attached.<\/p>\n
These lenses typically contain many separate lens elements. Each glass surface on these elements has the capability to cause unwanted internal reflection.<\/p>\n
If the subject is bright on a dark background, like the Moon against a starry sky, and you are using such a camera arrangement, internal reflections can build up and create many offset images.<\/p>\n
Some will be the right way round and others will mirror the main subject.<\/p>\n
If this is the case, try stopping the camera lens down (lengthening the exposure to compensate) or you could try using a different lens or even a telescope.<\/p>\n
A higher magnification (longer focal-length<\/a> lens) can also help reduce the effect, as can lenses with good anti-reflection coatings.<\/p>\n
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Wide-field lenses are particularly prone to causing ghost images.<\/p>\n
If you\u2019re using a telescope as a lens, it\u2019s a good idea to make sure that all of the telescope\u2019s internal surfaces are dark and there are no shiny features that can cause internal reflections.<\/p>\n
If this is the source of your problem, read Steve Richards\u2019 quick guide to flocking a telescope<\/a> for more info.<\/p>\n
Are you an astrophotographer? Whether experienced or complete beginner, don\u2019t forget to send us your images<\/a> and they could end up in a future issue of BBC Sky at Night Magazine.<\/strong><\/em><\/p> <\/body><\/html>\n
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