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These tiny accessories can make a huge difference to what you can see
If there is one seriously neglected and underused tool in your astronomical toolbox, then it surely has to be the filter. These often-colorful discs are available in 1.25-inch or 2-inch sizes and screw into the barrel of an eyepiece, the end that attaches to a telescope. The point of filters is that they alter how astronomical targets appear. They work by stopping — filtering out — some wavelengths of light from passing through your telescope's tube, changing what you see through the eyepiece.
This flies in the face of conventional astronomical wisdom that every photon counts, that more light means better views. But this is precisely the point of filters: they remove the light you don't need and only deliver the wavelengths you want for a given situation. It's for this reason that there are so many filters out there. Some are colored, some are clear, but each has a slightly different effect and is designed with a different purpose in mind.
Sometimes the effects can be remarkable: there are, for instance, filters that can enhance the polar caps of Mars and reveal the subtle shadings of clouds on the otherwise bland disc of Venus. It has to be said, though, that filters can't perform feats of magic. There is still no filter in existence that acts as a cure-all for poor seeing or poor sky transparency.
Where to Begin
Most astronomers, whether budding or experienced, are aware of color filters, which are used for planetary work to tease out hidden details like the ones already described. But there are many more types, and they can cut down on lunar glare, help to reduce the insipid orange glow of streetlighting and even block out all light bar one specific wavelength, which can work wonders if you are keen on examining the denizens of the deep sky. We cover all of these in a bit more detail below. Some filters can even be used together for an enhanced effect, but bear in mind that stacking them in this manner will further dim the view.
If, once you have read up on all the different types, you decide filters might be of use to you, where should you begin? Many suggest a neutral density filter — otherwise known as the Moon filter. This one simply dims the view, and as such it is great for observing our close companion when it is in its dazzling fuller phases.
As always, if you can try before you buy, then do so — astronomical society events and the larger star parties around the country are both good opportunities to get some hands-on insight into the kinds of changes filters can deliver.
Once you're committed to making a purchase, be aware that you don't always have to buy filters individually: colored filters in particular are often sold in sets. If you get to the point that you are using a lot of filters, you may wish to consider another accessory, the filter wheel. Some are motorized, some are manually operated, but the basic premise is the same — they allow you to swap filters without having to remove your eyepiece each time.
Do be aware that all of the filters discussed here are intended for night-time use only. Under no circumstances should they be used to view the Sun, as they will do nothing to mitigate the dangerous intensity of its light. But, just as with any other object in the sky, the Sun's appearance changes once viewed through different filters. Never view the Sun without a solar filter in place.
Color filters are designed for planetary work. They are often referred to by their Wratten number, written in the style '#1'. This is based on the original Eastman Kodak filter range, which had 100 shades, although for most astronomical purposes only a few are required. Among the most useful are:
- #8 (Yellow) — For cloud bands on Jupiter and Saturn
- #25 (Red) — For picking out surface detail on Mars and cloud details on Venus
- #47 Violet — Useful for Venus and enhances the Schröter effect
- #58 (Green) — For improving red features such as Jupiter's Great Red Spot
- #80A (Blue) — For Martian dust storms and clouds, and the belts of Jupiter
Like narrowband filters, ultra-high contrast filters improve contrast, making the background sky darker and helping deep-sky targets to stand out better. They pass both hydrogen-beta and oxygen III light, so will improve your view of a wider range of nebulae than any single narrowband filter.
Light Pollution Reduction
These are designed to suppress the specific wavelengths giving the orange glow from sodium streetlights, with the result that they darken the background sky. This can help you see deep-sky objects better, particularly nebulae and galaxies, as they are more easily washed out than the planets are.
As the name suggests, narrowband filters cut all but a few select wavelengths of light — typically the ones emitted by bright emission and planetary nebulae. By blocking the rest, they help to improve contrast and so bring out subtle detail. Typical narrowband filters include hydrogen-beta and oxygen III.
Both of these filter types reduce the glare of bright targets — the Moon, Venus, Mars, Saturn and Jupiter. Neutral density filters reduce the intensity of all wavelengths, but are particularly favoured for lunar use. Polarizing filters can often be tuned to control the amount of dimming they deliver.
These filters can help you to combat the chromatic effects often seen through achromatic refractors, which most commonly take the form of noticeable blue or violet halos around bright stars. Hence these filters are also sometimes referred to as 'minus violet' filters. They can be used on any target.
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