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Guides: Colour Filters To Improve Lunar and Planetary Visual Observing

Observing planets through the telescope can at times be very challenging. Atmospherics and other phenomena quite often wash out the features being observed whether that be the surface features of Mars at opposition or the finer detail upon the lunar surface. This is particularly problematic if they are low down where their light passes through more atmosphere. When observing solar system objects through a telescope, the use of coloured filters can make quite a difference to what can be viewed, sharpening, and enhancing contrast of the image. Sometimes, just using a dark or coloured filter to cut down the glare or to cut out some of the other wavelengths of light coming through the eyepiece, especially if using an achromatic refractor, can help make the image disturbed by the atmosphere just that little bit more stable so finer detail pops into view. Filters really can improve the crispness of the image produced by the telescope, making detail easier to see.

There are many types of filter on the market. With so many filters available, knowing which is the best filter for use for the object of interest is difficult. This handy guide will help you decide which colour filter is best used on different objects, to reveal features of interest.

How a coloured filter works.

Filters work by only allowing a certain wavelength, or a small range of wavelengths of light through the glass into the eyepiece. When a coloured filter is used it does produce a colour cast to the image being viewed.

When a red filter is used, it allows red light through, but blocks blue and green light. This results in a false looking red coloured image.

The surfaces and clouds features of the planets are composed of different chemical elements. This gives marked differences in colour between different features. Coloured filters work by reducing the brightness of undesired constituents, and increasing contrast, so features of interest stand out more readily. Selecting the appropriate coloured filter for the feature of interest allows us to enhance the appearance and / or subdue other distractions, making certain features easier to see.

In this case when observing Mars, a red filter will make red features appear brighter, but any features coloured blue or green appear darker. This gives increased contrast to Martian features helping them to stand out more.

The Wratten Number.

Most filters used for astronomy observations are numbered according to the Wratten numbering system. This

means that any filter that is manufactured to a particular Wratten number should pass light through in a predictable manner. Each filter will only allow light at particular wavelengths to pass through to reach the eyepiece. In the list below a #24 filter is red coloured red, so only allows light on the red wavelength to pass through. It blocks Blue and Green Light.

Filters for visual observing usually come in inch and a quarter fittings. The filter is screwed into the end of the barrel of an eyepiece before placing it into the telescopes draw tube to observe.

Some filters are available to fit the bigger 2” eyepieces. As they constitute a bigger piece of glass, they are more expensive.

Once the eyepiece with the filter is in place, the restricted wavelengths of light passing through to the observer’s eye will alter the perceived view. This will darken the appearance of some features and enhance others, depending on the colour filter that is used.

First Light Optics have a number of different coloured filters that are suitable for viewing The Moon and planets. These are not supplied in sets but can be ordered individually to suit an observer’s specific requirements.

Which filter is required will depend on the feature of interest the observer is most interested in viewing.

There is a bit of overlap in the usefulness of filters, but this useful guide will help the observer to decide which filter should be purchased depending on the observation of interest.

Filters can also be double stacked to give a combined result, so it is worth experimenting.

Neutral Density Filters.

These filters do not have any particular colour associated with them, so they do not affect the colour seen through the eyepiece. These filters are used to cut down the amount of light coming through to the eyepiece. This reduces glare from the object being observed, usually The Moon, which can be extremely bright in the gibbous and full phases. These filters come in a variety of densities, depending on the amount of light they are designed to cut out. Neutral density filters can be added together and with careful use can get the result required for the best observation.

#ND96-0.3.

This neutral density filter allows 50% of visible light to pass through, so only half the amount of light entering the telescope passes through into the eyepiece, making the image 2x fainter.

#ND96-0.6.

This neutral density filter allows 25% of visible light to pass through. Only one quarter of the amount of light passes through into the eyepiece, making the image 4x fainter.

#ND96-0.9.

This neutral density filter only allows 13% of visible light to pass through into the eyepiece. This means that the image viewed will be almost 8x fainter.

Polarising Filter.

Like the neutral density filters, a polarising filter is also neutral in colour, so this will not affect the colour seen in the eyepiece. These filters are used when viewing bright objects like The Moon or Venus to reduce glare. They are also used in combination with a Herschel wedge when observing the Sun in white light. Once the filter is fitted the brightness of the image is adjusted by rotating the eyepiece.

Moon Filter.

The Moon can be quite bright especially when gibbous or close to full, producing a lot of glare. This can dazzle the observer making it difficult to see fine detail. This filter reduces the brightness by 18% which helps improve contrast to help enhance surface features. Due to the wavelengths of light it passes, it adds a green tint to the viewed image.

#58A Dark Green Filter

This filter rejects red and blue wavelengths and is useful for a variety of planets.
It helps to increase the appearance of Lunar features.
On Venus it improves contrast in the cloud cover.
Used to view Mars it enhances the appearance of the polar caps and yellow dust storms, if present.
Jupiter’s cloud belts show increased contrast.
Any white spots visible in the atmosphere of Saturn are enhanced.
As it is a fairly dark filter, and has reduced light transmission, so it should only be used on larger telescopes.

#56 Green Filter.

As above, this filter has a similar use on The Moon, Mars and Venus as shown above.
Darkens Red and Blue cloud belt regions, plus Jupiter’s great red spot.
Saturn’s cloud belts, usually dimmed by haze in the planet’s atmosphere, are enhanced.
Again, due to its low transmission of light, this filter does need to be used on a larger telescope which collects more light.

#38A Dark Blue Filter

This filter rejects red and orange wavelengths.
This makes it suitable for revealing subtle shading in Venus’ upper clouds.
On Mars, it enhances the polar caps and dust storms, if present.
Used on Jupiter it enhances the appearance of the redder belts and the Great Red Spot. Another bonus is when used on bright comets details in the ion tail are enhanced.
The filter has a low light transmission so should only be used on larger telescopes.

#80A Medium Blue Filter

The Moon’s features really benefit form use of this filter.
Observations of Mercury in the twilight with this filter helps to reveal elusive surface features. Used on Venus it help to reveal subtle shadings in the clouds.
On Mars this filter helps pick out high level clouds and the polar caps.
This filter is the best one to use for both Jupiter & Saturn, enhancing details in the cloud belts, Red Spot and Saturn’s polar regions.
This filter is also useful for enhancing ion tails in bright comets.

#82A Light Blue Filter

This filter has uses on The Moon and most planets, enhancing regions of low contrast but without the cost of reducing the overall brightness of the object being observed.
Used to observe Mercury, Venus and comets, it has a similar performance to filter #80A.
It helps to enhance surface features and polar caps on Mars.
On Jupiter and Saturn it emphasises the contrast between the various dark belts and bright zones.
On deep sky objects it has been used to help enhance detail in spiral arms of galaxies, such as M51 and help to split close double stars.

#15 Dark Yellow Filter

This filter is good for lunar surface details.
It also helps with viewing Mercury and Venus during the day. Also helps to reveal low contrast details in Venus’ clouds at night.
Used to observe Mars it really improves contrast of the surface features and sharpening the boundaries of any yellow dust clouds.
On Jupiter blue tones are darkened, enhancing orange. This improves the appearance of red bands and festoons.
Used to observe Saturn it helps to penetrate the haze to study the polar regions.
If used with a very large telescope it can improve detail of the tiny disks of the remote ice giants Uranus and Neptune. Bright comets benefit by helping reveal detail in their tails.
Needs to be used on larger telescopes.

#12 Yellow Filter

This filter helps to increase the contrast of lunar features when used with larger telescopes.
It also helps to reveal subtle differences in the clouds of Venus.
This is a very popular filter for observing Mars. It helps increase the contrast between light and dark areas, sharpens edges of yellow dust clouds and reveal blue clouds.
Used to observe Jupiter and Saturn this filter enhances red and orange belt and zone features.
Uranus and Neptune details benefit, but only when used with a larger telescope.
Definition of tails in bright comets is also improved.

#8 Light Yellow Filter.

This filter is popular for enhancing the view of lunar details, especially when used with small telescopes.
When used to observe Mars, this filter reduces scattered light from slightly bluer areas. This helps to increase contrast of dark features.
If used with a large telescope to observe Uranus & Neptune it helps improve resolution.
When used to observe bright comets, coma and dust tail details are enhanced.

#11 Yellow / Green Filter.

This filter is used to enhance the dark features on Mars.
Used in Jupiter it helps contrast the dark belts and bright zones.
It enhances Saturn’s details to a lesser extent, but can help accentuate the appearance of The Cassini Division. Again, used with a large telescope, this filter can slightly improve detail on Uranus and Neptune.

#25 Red Filter

A red filter helps to reduce glare.
This really helps observations of Mercury in the twilight when the planet is low down.
If observing the inferior planets during daytime this filter helps to reduce the brightness of the daytime sky, increasing the contrast.
Used on Mars it gives great definition between the dark and bright surface features and polar caps.
This filter gives great contrast between Jupiter’s dark belts and brighter zones.
One disadvantage is that the low light transmission of this filter means that it can only really be used with larger telescopes.

#23A Light Red Filter.

This filter is very similar to filters #21 & #15, but gives a lot more contrast.
However, due to the low light transmission of this filter, it does need to be used with a telescope larger than 6”.
It really helps to improve the contrast between Mercury and a bright daytime sky. This filter is really useful to help reveal the surface features on Mercury during the day.
With Venus it can often reveal variations along the terminator.
Used on Mars, it increases the contrast between the dark and bright surface features.
Jupiter & Saturn benefit by darkening blue features in their atmospheres to help them stand out.
When used on bright comets, this filter helps to improve definition in the dust tail.

#47 Violet Filter.

The most popular use of this filter is to help reveal details in the cloud tops of Venus. It can also help to enhance lunar definition.
Use to observe Mars, it helps to reveal high clouds as well as haze in the polar regions. Used on Saturn it can help to reveal details within the ring structure.
The low light transmission of this filter means that it should only be used on larger telescopes.

#21 Orange Filter

This filter really helps with enhancing lunar features.
The brightness of the daytime sky is reduced significantly, so is useful for observing both Mercury and Venus during the daytime, especially for revealing surface features on Mercury.
Used on Mars, it enhances the boundaries of dark surface features.
Both Jupiter and Saturn benefit by increasing the contrast between their dark belts and bright zones. Jupiter’s great red spot also pops into view using this filter.
Used on bright comets in larger telescopes it enhances the definition of the coma and any dust tail.

#29 Dark Red Filter.

This filter assists observing Mercury and Venus in the daytime, particularly the Venus terminator. Used on Mars it enhances the visibility of dark surface features and polar caps.
Used on both Jupiter and Saturn cloud belt details are enhanced.
It has also been reported to also help view transits of the Jovian moons across the disk.
Being a dark filter with low light transmission means this filter should only be used on larger telescopes.

Coloured Filters - Quick Guide

Reference:
Wratten number - Wikipedia. https://en.wikipedia.org/wiki/Wratten_number