2017 March 20

Using filters for visual planetary observations

Paul G Abel

Introduction

Almost all telescopes today come with a set of optical filters. These are small brightly coloured disks of material which normally have a hard black edge. At first glance, they don’t seem to be particularly useful, but in fact a good set of filters is an essential part of the visual observer’s toolkit.

As is well known, we can think of light as a wave, and white light is in fact composed of all the possible colours in nature. If you take a prism and shine white light through it, you will see that it produces a rainbow – each of these colours corresponds to a certain wavelength of light. The electromagnetic spectrum ranges from very short wavelengths with high energies (like gamma rays) through to the visual part of the spectrum (which our eyes can see) then on through to infrared and longer, low energy wavelengths.

A filter is a specially coated material which is designed to allow some wavelengths to pass through, while stopping others. The types of wavelengths a filter allows through are called its transmission characteristics. When we use them in astronomy, filters allow us to study objects in different wavelengths of light. For planetary observers, this is very useful as the atmospheres of the planets in our solar system can look different in violet light, say, compared to the view in ordinary white light (also known as integrated light).

A number of different filters are available on the market, however not all of them are suitable for visual work. For example, UV filters can be purchased which only transmit UV wavelengths. Imagers use them to capture the cloud markings in the atmosphere of Venus because the Venusian markings are much stronger in UV, however since the human eye cannot see in this part of the spectrum, such filters would be useless for visual work.

In this article we shall confine our discussion to filters which can be used for visual purposes, the Wratten filters, and I will make some suggestions on how they can be used for planetary observation.

Filter basics

Optical filters have been in existence for quite some time – the British inventor Frederick Wratten, who founded the first photographic company, developed them for use in photography. In 1912, Wratten together with his partner, sold their company to Eastman Kodak, and Kodak started manufacturing the Wratten filters, making them available to the general public.

Each filter is identified by a ‘Wratten number’ – if you look at the side of your filter, you will find this number printed there (Figure 1). The number simply identifies the filter – it doesn’t tell us anything about the type of wavelengths it transmits. There are many different Wratten filters available, and I have provided a list of the most commonly used ones in Table 1.

Using a filter is straightforward. First choose the eyepiece you wish to use, then turn it upside down. You will see that the barrel has a screw thread – simply screw the filter into the base of the eyepiece (Figure 2). A word of caution is advisable here – make sure the filter is screwed in tightly, otherwise you risk it coming loose. This is particularly dangerous if you are using a Newtonian reflector as you risk the filter falling on to the secondary mirror, which will do it no good at all!

Most visual planetary observers make drawings, and it is important to record the details of any filters you have used when making your observations (even if you are just making a written account). This is particularly important if your drawings are in black and white, as it will not be possible to tell whether they were made in integrated or filtered light simply by looking at them.
Another thing to note is that not all filters are suitable for smaller telescopes. For example, the W47 is a violet filter, and it only transmits a small amount of light. This means it can only realistically be used with 8-inch [200mm] telescopes or larger – if you use it on a smaller ’scope, the image will be much too dim to be of any value.

Many astronomy outlets now stock filters so it’s fairly easy to build up a large collection of them – personally, I think they are a worthwhile investment as they will allow you to carry out a more extensive observing programme. When purchasing a filter, always check that it is suitable for visual work and that it will be appropriate to use with your telescope.

Using filters for planetary observation

In this section we shall look at how filters can be used to investigate the atmospheres of the planets. I have omitted Mercury from these discussions because it is normally very low down and not a target for beginners. Similarly, I have also left out Uranus and Neptune due to their great distance. I have used filters with some success on Uranus, but that was using the University of Leicester’s 20-inch [508mm] Planewave Dall-Kirkham telescope, and it is safe to say that filter work for the ice giants requires an aperture well beyond the range of most amateurs. We shall confine our attention to the main bright planets, and we start with Venus.

Venus

The second planet from the Sun is well suited to filter work. Venus has an extensive atmosphere, and although the visible markings are subtle, they can be seen by those sensitive to the bluer ends of the spectrum. The thick Cytherean atmosphere scatters the incoming sunlight, and as a result, the observed phase of Venus is always less than the predicted phase – this is called the phase anomaly (or the Schröter effect). The wavelengths of light towards the blue end of the spectrum are particularly affected.

In the 1950s, several BAA observers (particularly Hedley Robinson, Richard Baum and Alan Heath) showed that the phase anomaly was stronger in blue light. They also demonstrated that the cloud markings can vary depending on which wavelengths the planet is viewed in – this is because different filters allow us to see down into different depths of the atmosphere. A W21 (Orange) filter allows us to look deeper into the atmosphere, and it is not uncommon for the cloud markings to look somewhat different in a W21 compared to the view in integrated light.

Measuring the phase anomaly of Venus is one of the main tasks for the Mercury & Venus Section. Observers are asked to use a W15 (dark yellow) filter when making phase estimates as this filter reduces scatter in visual estimation of the Schröter effect. If you use a W47 (violet) filter, you will find the effect is much more pronounced. The cloud markings often show up better in a W47 filter, but if you find the image is too faint, try a W38A (blue) filter instead. A W58 (green) filter can help bring out the cusp caps and the cusp collars when present.

Mars

The fourth planet is smaller than the Earth, and its somewhat elliptical orbit means that it only comes to opposition about once every two years. When Mars is well placed in the night sky, the dark surface markings (known as albedo features) can be quite prominent – the ‘V’-shaped Syrtis Major is perhaps the most famous marking. However, you might find a W8 (yellow) filter can help boost the contrast of the albedo features, and a W25 (red) can bring out the polar caps.

Although the Martian atmosphere is tenuous by terrestrial standards, it is still sufficient to produce a variety of of phenomena which can all be investigated using different filters.

As Mars orbits the Sun, due to its having a similar axial tilt to the Earth it undergoes well-defined seasons of winter, spring, summer and autumn. Both the north and south poles have polar ice caps, and when spring arrives in a given hemisphere the polar cap begins to heat up and evaporate. This returns large quantities of volatiles such as carbon dioxide into the atmosphere. As a result, we see an increase in white cloud activity.

These white clouds can be rather magnificent (I never tire of looking at them), and they tend to collect in low-lying areas like the Hellas basin or around the vast volcanoes in the Tharsis plateau. You will find that white clouds are best observed in a blue filter like a W38A or a W80 (light blue) – they can be dynamic objects and it is not uncommon for those on the morning terminator to melt away as the rising Sun causes them to disperse.

Another phenomenon for which Mars is famous is dust storm activity. There are three types of dust storm: Local, Regional and Global. The global dust storms are very impressive – the Martian winds kick up vast amounts of dust which sits in the atmosphere obscuring the whole planet and rendering the albedo features invisible, often for many months at a time.

Regardless of the final size of a dust storm, they all begin life as small yellowish coloured clouds, and filters can be used to increase the contrast and make them more prominent. In particular, try using a W21 (orange) or W25 (red) – these are well known to help enhance the view of dust clouds.

Jupiter

As we move out beyond Mars we enter the realm of the gas giants – Jupiter and Saturn. These worlds are very different and somewhat alien compared to the rocky inner planets. When we look at Jupiter, all we are seeing are the top layers of a very extensive atmosphere. Here wind speeds in the zonal jets can blow at 100 m/s. Some storms, like the Great Red Spot have lasted for many hundreds of years, while others are more short-lived.

A cursory view through a small telescope will show Jupiter as a flattened disk crossed by dark bands and lighter zones. A 4-inch [100mm] telescope will easily show the two main equatorial belts, and larger telescopes will reveal many more. The dark belts are a brownish colour and are the deeper, warmer layers of the Jovian atmosphere. In contrast, the bright zones are higher clouds formed from frozen ammonia crystals.

On a good night, a 6-inch telescope or larger will reveal a lot of detail in the belts and zones – it is not uncommon to see white ovals, and dark barges, and various irregular bright sections. These can all be enhanced with different filters.

A W8 (yellow) is a good all-round filter for Jupiter, and helps to enhance the belts and the zones. The Great Red Spot is well seen in a blue filter like a W80A (medium blue) – for best results, make sure the GRS is well onto the disk (ideally near the central meridian). The belts of Jupiter tend to be reddish-brown in colour, so a blue filter can help to enhance them further by making them appear darker. This can also increase the contrast of any localised storms within the belts.

It is not uncommon for the Equatorial Zone to contain a number of bluish storms called festoons. These tend to start in the southern edge of the Northern Equatorial Belt and extend out into the surrounding Equatorial Zone, and a W25 (red) filter can help increase their contrast.

Saturn

Initial views of Saturn seem to imply that its atmosphere looks like a somewhat quieter version of Jupiter’s. However, Saturn’s atmosphere is every bit as dynamic, but the features appear more muted due to the existence of a petrochemical smog which sits above the main atmosphere, making the belts and zones in Saturn’s atmosphere appear much less well defined.

When Saturn is well placed, and its axial tilt allows, it is normally relatively easy to pick out the two main equatorial belts, but other belts and zones may be harder for the eye to define. Nonetheless, filters can help with a visual study of Saturn. Like Jupiter, blue filters help to bring out reddish-brown features and similarly, red ones help to bring out bluer features. In general I find a W58 (green) helps to enhance the belts and zones.

An interesting phenomenon associated with Saturn is called the ‘bi-coloured aspect of the rings’. This occurs when the rings on one side of the planet appear brighter than those on the other side when viewed in a red or blue filter. Although the underlying reason for this phenomenon is still obscure, a number of reliable observers have reported it over the years and it is a good idea to see if the effect is present when making a visual study of Saturn and its rings.

If you intend to specialise in visual planetary observing, you will find that a good set of decent quality filters can greatly extend your observing programme. Even if you specialise in imaging or are just a casual observer, you will find that a selection of coloured filters will enhance the views of the planets through your telescope, allowing you to see them, quite literally, in a new light!

Paul G. Abel