Primer - Understanding and Choosing Filters for Visual Use

[Andrew*]

It is very common to see questions posted about filters for visual use on SGL. I have decided to write an article to help people understand the use of filters.

How Filters Work

Filters are coated to alter the light transmission – the wavelengths of light that reach your eye. The eye is sensitive to light from 400nm (violet) to 700nm (red) in wavelength. By blocking out specific wavelengths, or colours, detail in the unblocked colours will stand out more. For example, if you are viewing a planetary detail which is particularly strongly red, blocking out blue and green will improve the contrast on that detail. Thus you need to be aware of what filters work on what targets.

A light transmission curve gives a good idea of what a filter does. This is a plot of percentage transmission over the visible spectrum, with 0-100% transmission on the vertical axis and the spectrum on the horizontal axis. The following is an example showing a UV and IR pass filter. It transmits up to 50% of UV (at ~380nm) and up to 60% of IR (at ~950nm), but 0% throughout the visible spectrum (400-700nm). Of course, this example would be of no use for visual applications as it would appear completely black.

There are two main coating types used on filters: dichroic, which reflects unwanted light, and absorptive, which absorbs the unwanted light. The rest is transmitted. Generally speaking dichroic filters have a more exact and accurate transmission curve, and more efficiently manage the light. They appear very shiny when light is reflected. Where a specific wavelength must be blocked or transmitted, dichroic coatings will be used. Absorption coatings are more general and are used for ND and colour filters, where the filtration need not be so specific. Naturally, dichroic filters are generally more expensive.

Types of Filter

Neutral Density (ND)

This blocks out a portion of light from the entire visual spectrum, thus retaining a neutral colour balance. This cuts down on glare and the dazzling effect from very bright objects – most commonly used on the moon. NB: UNSUITABLE FOR SOLAR USE!! Good manufacturers of ND filters will state the transmission percentage, e.g. 25% means a quarter of the available light reaches your eye (75% blocked).

Polarising

These actually consist of two polarising filters that when rotated against each other will vary the light transmission, so in use acts as a variable ND filter - very useful for tuning the brightness to suit your need.

Colour

These come in all different colours and strengths and are most commonly used on solar system objects to improve contrast. Colours are often stated by their Wratten number. For a guide to using colour filters, see this primer.

Light Pollution

Some streetlights emit light in specific wavelengths. By blocking out the most common wavelengths in light pollution, but transmitting across the rest of the spectrum, a filter can reduce the background brightness in light polluted areas and thereby improve contrast on faint objects. There are many different filters available from different manufacturers, suited to different types of light pollution. Unfortunately experimentation is the only way to find the filter most suited to the type of light pollution you suffer from.

Nebula

Emission nebulae and planetary nebulae emit light in specific wavelengths, so specific in fact, that a filter can block the entire spectrum except a few nanometers centred on the desired wavelength (emission line). Thus everything is dimmed except the light from the nebula. Hydrogen Alpha (Ha), Hydrogen Beta (Hb) and Oxygen III (OIII) are the most common emission lines.

The range of transmitted wavelengths is called the bandwidth. The narrower the bandwidth, the more the emission light is isolated. Emission line filters are commonly called narrowband as they typically transmit a narrow bandwidth (6-13nm). Light pollution filters (above), are called broadband filters because they transmit broad bandwidths. Views through narrowband filters can be very dim and even more so in small telescopes. Some observers prefer to only use them in large telescopes where the view is brighter.

Ha

Unfortunately, despite the fact that Ha is so common in nebulae, it falls in the deep red end of the spectrum where eye sensitivity is very low, and therefore we cannot see Ha in nebulae, and there are no visual Ha filters available.

Hb

Hb is related to Ha, but much fainter. However, it is blue/green, to which the eye is more sensitive. There are only a few objects with a strong source of Hb.

OIII

OIII also falls in the green/blue part of the spectrum. It is bright in many nebulae, on which contrast can be massively increased by use of an OIII filter.

Ultra-High Contrast (UHC)

UHC filters transmit OIII and Hb, and sometimes Ha too. Because OIII and Hb are so close in the spectrum, UHC filters transmit the wavelengths in between too, which makes the band broader. This results in a brighter and more versatile filter, more suitable for smaller telescopes.

Broadband UHC

This is just like a UHC except it is much more broadband, and is targeted for use with smaller telescopes.

Which Filter for Which Target?

Here are some notes on what filters to use on specific targets.

Moon and planets – see this primer.

Nebulae – see this website for use on specific nebulae. The Lumicon Deep Sky filter mentioned is an example of a broadband UHC filter.

Planetary nebulae can be difficult to find as they are sometimes very small and can be confused with a star. A good method of spotting it is to use an OIII filter and place it between your eye and the eyepiece. The starlight will all be almost blocked but the filter will let through the nebula and it will “pop” out.

Galaxies, clusters and stars – stars, and therefore clusters and galaxies, emit light across the entire visual spectrum (continuum). Therefore no filter will help dramatically to improve views of these objects and there is no substitute for good dark skies. However, an LP filter well suited to your kind of LP may make small improvements on contrast.

Did you find this primer useful? Please make any comments here.

Edited December 12, 2009 by Andrew*