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Filters used in astronomy 

Visible light is emitted in a range of about 380 to 780 nanometres: from the short wavelength blue-violet end to the long wavelength red end. When observing a particular astronomical object in a telescope, you can often improve contrast by the use of an appropriate filter. The filter achieves this by blocking certain regions of the visible spectrum and letting others through. There are filters in all conceivable colours that are often used for Lunar and planetary observing. You can make specific details on the surface of planets stand out more via their use.

Moon filter

A neutral, grey or moon filter is used to lessen the intensity of bright moonlight and to slightly increase contrast. Anyone who has ever been to an observatory and looked at the Moon through a large telescope without a filter will vividly remember the experience and know why this filter is so important. Observing the moon without a filter will not cause any damage, but it is so bright that it really dazzles you. If you then turn away from the telescope and look into the darkness you will often still have a ghostly afterimage of the moon in the eye you observed with. Although this afterimage will gradually fade, it is still very irritating.
Of course these filters are available in different light reduction levels. They range from a light transmittance of about 8% up to 50%. The filters with a high transmittance are suitable for the smaller telescopes and those with a low transmittance are suitable for larger telescopes.

Adjustable polarizing filters are the luxury version of Moon filters. This is not just one, but two filter elements, which are connected to each other. Rotating one filter element relative to the other continuously adjusts the amount of darkening. Most polarizing filters allow light transmission levels from 1% to 40%. They can be used to set the optimal balance between light level and contrast for the size of telescope you are using.


Nebula filter 

There are also special filters available for deep sky observing. These are quite complex and costly to produce. They consist of multiple dielectric layers which are vapour-deposited onto high-quality optically flat glass. They have the task of only passing a well-defined range of the light spectrum, depending on the type of observing the filter is to be used for. As a rule, spectral regions which are of no interest are absorbed and the spectral regions in which the objects mainly radiate are allowed through. The image seen in the telescope is darkened slightly.

All nebula filters block the spectral regions where streetlights radiate in. This is, for example, the case above 530nm and extends to about 630nm. When you look at the transmission curve of any of these filters, you will notice that there is a significant drop in transmittance in this area and the curve only rises again above 630nm. These filters are enormously effective as they block the street light while, at the same time, increasing contrast in the object being observed.


Using filters with a telescope

Using a filter with a telescope is quite simple: you have a choice between using 1.25" or 2" filters. These sizes correspond to the eyepiece format on your telescope and all you have to do is to screw your filter onto the thread in the eyepiece barrel provided for it. You then put the eyepiece, including the filter, into the focuser of the telescope and you can start observing immediately.

There is a huge range of astronomical accessories out there, and this book only covers a small selection – those which are of interest to beginners in amateur astronomy. But have a look at our online shop at to discover other accessories, ranging from those for solely for visual observing to special astrophotography equipment. You can also find out about other accessories in our comprehensive glossary. And if you have a question you would like answered in person, then ring our experts or send us an email – we would love to hear from you.

On behalf of, I wish you much enjoyment in your new hobby of astronomy. The universe is out there, just waiting to be discovered by you!


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