Amateur astronomy is a breath-taking hobby. Whether viewing with the naked eye on a dark, clear night, or scanning the skies with a pair of binoculars in the suburbs; the greatness of the sky is undeniable.
Still, there are ways to take your viewing experience to the next level. The single biggest move you could make is to observe under pristine, dark skies. When this is not possible, the next best bet is to have a complete basic stargazing set-up, and filters are part of this kit.
Often overlooked as an optional add-on to an eyepiece, filters are actually a necessity for getting the best out of the observing the heavens.
How Filters Work and What They Are Used For
Our eyes have different types of photoreceptors which perceive different wavelengths of light. Filters work by controlling the type of light our eyes see (blocking certain wavelengths and allowing specific ones to pass), and by determining the amount of light that reaches our eyes (the percent of transmission).
Whereas one filter would cut glare and make it easier to view a full Moon with greater detail, another would be designed to only transmit specific colours on the spectrum, and improve contrast when observing the planets.
Because the Moon, planets, nebulae etc. all have unique makeups, we need different filters for each. Many filters can be used for viewing the different features of a variety of celestial objects.
Filters are usually threaded to be able to screw onto any eyepiece, so they are easy very easy to use.
Types of Filters
It is possible to safely view the sun. Unlike the other filters discussed here, proper solar filters are not threaded for an eyepiece. The problem with a filter like this is that the telescope will concentrate and focus too much sunlight before reaching the eyepiece filter and possibly damage it. A damaged filter will not protect your eyes.
A good solar filter positions over the front of the telescope, covering the primary optic – the opening of the telescope – completely. Such a filter needs to be fitted to the exact aperture of your telescope to be effective. These filters block out 99.9% of the Sun’s light before it enters the telescope.
Chromatic Aberration Filters
Chromatic aberration is a visual flaw of lenses. Different wavelengths of light are split through a lens at different angles, and do not focus perfectly at a point. This can cause a slight rainbow effect. Chromatic aberration filters help to lessen chromatic aberration.
Light Pollution Reduction Filters
Light pollution has a negative impact on visibility and seeing conditions. The consequences are most noticeable in cities, but also affect suburbs with unwanted light from streetlamps and houses, and the distant glow of the city’s lights.
Broadband Light Pollution Reduction (LPR) filters reduce the effects this excess light has on stargazing. They work by blocking out Mercury vapour, sodium, and other emission lines from manmade objects like streetlamps. This improves contrast and increases the visibility of deep sky objects –like galaxies and star clusters – that otherwise would have been too dim to see.
What LPR filters do not do is make celestial objects brighter, and the certainly do not eliminate the greater impact light pollution has on our skies. However, there is a marked improvement with fainter objects. LPR filters also provide contrast when observing dark skies.
Deep Sky Filters
Narrowband Filters/ Nebula Filters
Narrowband filters work by passing only the pair of emission lines of Oxygen III (OIII), the Hydrogen Beta (H-Beta) emission line, and the wavelengths between the OIII and H-Beta lines.
Narrowband filters are best used for observing varying kinds of emission nebulae. Indeed, they are the best all-round filter for deep sky observations.
These filters also darken the background sky by significantly decreasing sky glow without compromising the views of nebulae.
Using these filters opens up the deep sky: nebulae that are otherwise invisible, faint or unspectacular to view suddenly come to life with brightness, contrast, and detail. Even bright nebulae gain an improvement in contrast using these filters.
Narrowband filters work best under dark skies with good seeing conditions. That being said, they work fairly well under moderately light polluted skies too. They are also better suited to low tthrough medium magnifications.
Some targets that benefit beautifully from the use of narrowband filters include the Lagoon Nebula, Orion Nebula, and the Swan Nebula.
Line filters are very narrowband filters. Where narrowband filters allow the emission lines OIII, H-Beta and all wavelengths between the two to reach the eye, line filters transmit only OIII emission lines, or only the H-Beta emission line.
OIII filters are excellent choices for viewing planetary nebulae and diffuse emission nebulae; and the improvement in contrast with OIII filters is outstanding. Try observing M8, M33, and the Veil and Helix Nebulae with an OIII filter to see the astounding difference it makes.
H-Beta filters can be used for spying structural features of brighter nebulae; however they do not work well on most planetary nebulae – dimming some and completely obliterating others. This type of filter works most effectively with larger aperture telescopes for viewing faint nebulae.
The H-Beta filter enhances the view of M42 and M43, and the California, Cocoon, Horsehead and North America Nebulae.
Planetary Sets/ Colour Filters
As the name suggests, these filters sort the colours of the spectrum; blocking certain colours and allowing others to pass. This is very useful when viewing within our solar system, as planetary surface detail and contrast is increased.
The Wratten System is used for ordering the different colour filter types.
Sets of planetary nebulae can be added to over time. A basic four colour set is ideal for a beginner, and will usually include blue, green, red and yellow. The extensive list below indicates the function of each filter.
#8 Light Yellow
- Enhances details on the Moon’s surface
- Shows better contrast of Mars’ maria
- Allows red orange features in Jupiter’s belt to stand out
- Improves the resolution of Uranus’ and Neptune’s discs, effective with larger apertures
- Darkens maria on Mars
- Helps surface details of Jupiter to stand out
- Good for viewing clouds on Saturn, the Cassini Division, and the planet’s red/blue contrasts
- Can be a useful lunar filter
- Helps show blue clouds in Mars’ atmosphere, as well as lightening the planet’s red/orange features
- Brings out red/orange features of Jupiter and Saturn
#15A Dark Yellow
- Brings out surface details of Mars, and good for viewing Martian polar ice caps
- Useful for observing polar regions of Jupiter and Saturn
- Useful for improving orange/ red features of Jupiter and Saturn
- Shows low contrast cloud detail on Venus
- Reduces the transmission of blue and green wavelengths
- Increases the contrast between blue/ green and orange/red/ yellow features
- One of the best filters for viewing Mars
- Good filter for viewing Jupiter’s belts and Great Red Spot
#23A Light Red
- A good all-round filter that helps planets stand out against the sky
- Improves the view of Mars’ maria
- Enhances Jupiter’s belts and polar regions
- Enhances Saturn’s belts and polar regions
- Helps some planets stand out from the sky
- View some of Mercury’s features
- Used to observe Venus’ terminator
- Good for Mars’ maria
- Sharpens contrast of Jupiter’s features
#38A Dark Blue
- Observe Martian surface and dust storms
- Good filter for viewing Jupiter; increases contrast of belts and the Great Red Spot
- Used for observing the rings of Saturn
- This filter blocks green, red, and yellow wavelengths and is a good all-round filter
- Can be used for viewing the Moon
- The best filter for observing Venus
- Observe Martian polar ice caps
- Can be used for Jupiter
#56 Light Green
- View Mars’ polar ice caps and dust storms
- Better contrast of Jupiter’s atmosphere and cloud belts
- Blocks blue and red wavelengths
- Improves contrast of Mars’ polar ice caps
- Enhances Saturn’s polar regions and cloud belts
- A solid all-round filter
- Can be used as a lunar filter
- View Jupiter’s cloud belts and Great Red Spot
- Observe Saturn’s belts and polar regions
#82A Light Blue
- A great all-round filter
- Can be used to observe the Moon, Mars, Jupiter and Saturn
- A great filter for observing brighter galaxies
ND96 Neutral Density
- This neutral filter transmits all colour wavelengths uniformly
- Reduces glare
- Great for viewing the full Moon
- Splits double stars
Best Telescope Filters
This quick overview of a few choice filters makes it easy to purchase and build up a selection.
The filter optimizes solar system views by significantly reducing the amount of bright glare. This makes it ideal for observing the full Moon in greater detail. The other handy feature is that you can adjust the amount of light transmission from 1 % to 40% by simply rotating it.
This is a good choice of filter for viewing deep sky objects under moderately light polluted skies. This filter blocks all forms of light pollution emissions such as the Mercury vapour and Sodium emission bands. This is helpful for detecting dimmer emission nebulae.
This neutral filter transmits light across the complete visual spectrum. At 13% transmission, it is perfect for reducing glare and viewing a variety of planetary bodies. This filter also separates nearby double stars (when their levels of brightness differ).
A quality filter that reduces sky glow effectively, and boosts contrast for sharper and clearer views – even from the suburbs.
Astronomania’s H-Beta filter cuts out light pollution while still maintaining over 90% light transmission. This means you can get solid views of fainter targets. This filter works best with larger aperture telescopes, and when the seeing is good.
Gosky brings you a basic colour filter set including blue, green, red and yellow filters, as well as a ND96 filter. The kit is great value for money: you can view an extensive range of planetary features from dust storms to cloud belts, maria and more.
Safely view the sun with this quality Orion solar filter. It fits the front aperture of correctly sized telescopes perfectly, blocking out 99.9% of sunlight before entering the tube. The filter rejects ultraviolet and infrared radiation, and improves contrast without losing the natural colours of the Sun. This filter can also be used for photographing the Sun.
The best starting place is to have a four colour planetary kit, a narrowband nebula filter, and a solar filter. This start-up collection will be all you need for many nights of happy viewing.