Just when you thought you had the right telescope and a carefully selected bunch of eyepieces you start to hear great stories about telescope filters…

What Are filters?

Filters are designed to block or reject some parts of visible light and transmit only a selective part of the light spectrum. This can be useful when we know that certain light in the sky interferes with the objects we view, and some objects we view transmit light only in certain frequencies. By blocking the frequencies of light we don’t want, the remaining light – if well transmitted – seems enhanced.

What do various filters achieve?

A Neutral Density Filter merely reduces all light and can therefore be useful for reducing Moon or planet glare. A UHC or Ultra High Contrast Filter blocks many elements of light but allows key bands, such as O-III and Hydrogen Beta through. This is excellent for enhancing bright nebula, such as The Swan, The Lagoon. The Trifid etc.

The Light Pollution Filter. Also known as the LPR filter: blocks mercury vapour light transmission and increases contrast in light-polluted skies.

The O-III Filter allows the doubly ionised Oxygen Band of light through and is great for planetary nebula such as The Veil, The Dumbbell and The Ring.

The Hydrogen Beta Filter. A must for the Horsehead Nebula and any faint dark nebula needing contrast.

The Hydrogen Alpha Filter. This is the most expensive and most specialised filter as it is the only filter  which allows solar prominences to be viewed. It allows red light through but can also leave the unweary buyer in the red.

Orange, Red, Yellow and Blue Filters. These are really coloured glass but by enhancing coloration on the planets they make features on Mars, Jupiter and Saturn stand out. An orange filter is useful for planetary viewing in daylight as it darkens the sky.

The Disadvantage of Filters:

Some are very expensive and many, unlike eyepieces, are used infrequently. The specialised ones, such as OIII and H Beta, which severely block out most of the visible spectrum, wipe out too much light in small telescopes. Sometimes filters, although adding contrast in the eyepiece, fail to produce acceptable views of objects, which are pleasantly viewed without them.

What to buy?

Much depends on how avid a viewer you are and whether the price is worth the investment. A regular Moon observer would benefit from a Lunar Filter, which is relatively cheap and makes viewing of the Moon more comfortable when it gets too bright. Many nebulae are enhanced by the UHC or LPR filter. Only purchase the OIII if you have a large telescope and are keen on planetary nebula. The coloured filters are fun when Mars comes around and they can be used on the other large planets. Their price is attractive. Finally, no filter shows the delicate detail as seen by the human eye.