Filters are used for blocking some frequencies of light, and allowing some to pass through. Even just looking at the moon and planets, you will definitely benefit from one. Most fit into the eyepiece barrel, but there are some exceptions. This is where you put the filter in:

What kinds of filters are there?

There’s a huge amount to browse, and many sizes too, so you’ll find the right filter for you. They can be used for any type of telescope, a guide to them can be found here.

Light Pollution Reducing Filters

If you’re looking at the moon or other planets, a good filter to have around is a Light Pollution Reduction filter (sometimes written as LPR). They block the wavelengths of light from street lamps; nowadays our street lights use sodium vapour, so the LPR filters let all wavelengths of light pass except wavelengths emitted by sodium vapour. This produces a clearer image and the night sky will look darker.

Polarising Filters

Reflected light is partially polarised (it’s now oscillating in one plane), which makes it easier to deal with. Polarising filters reduce reflected light by, you guessed it, polarising the light.

(This filter works just like the GCSE and A-Level practical many people have done, involving two filters. When you rotate them, the light passing through varies.)

Since these polarising filters have a plane of polarisation, you can rotate the filter until the plane matches with the polarised reflected light, and block it!

The moon reflects a lot of light from the sun, especially when full, so polarising is quite useful for a balanced image.

Neutral Density Filters

Neutral density filters don’t alter the colour of images or views, but reduce wavelength intensities. This is especially useful for photographers trying to reduce glare from photos.

When I look at a full (or near full) moon, the light reflected from it over-saturates my view so I like to use an ND filter . This proves very beneficial to those with high aperture telescopes. With this, I can see lots more detail in the moon’s surface. This does NOT mean that an ND filter increases the contrast, an easy misunderstanding to make.

An important thing to note when shopping is that a neutral-density filter may also be called a “moon filter”.

Colour Filters

As well as LPR filters, colour filters also are brilliant for planets and the moon. They can add some contrast or bring out details in planets. They’re also called RGB filters. The best way to learn about them, is by using them! Many companies offer planetary bundles for fairly cheap prices, made up of different colours, mostly the primary colours, so it may be good to invest in one and start practising! A good thing to note is that filters can be stacked, however I will warn not to stack lots on top of each other; filters BLOCK light, so stacking too many will block all visible wavelengths of light.

This list may be a good start for some:

• Neutral Density is great for the moon, as is a polarizing filter.
• More details of the moon’s surface can be found using yellow-ish filters.
• Both Mars and Jupiter are really good for practising with all your colour filters. There’s so much to extract from them! It’s worth looking at both with lots of colours.
• Venus and Saturn are similarly “all colour” types though not so much as Jupiter and mars, and many features that would have been “invisible” tend to pop up in dark reds, violets, and blues
• Yellows and light green-y yellows are really useful for adding contrast when viewing Uranus and Neptune. Since they’re both blue, a blue filter isn’t that useful.

Looking at Distant Objects

When a certain element is present in a distant object, it will emit a few wavelengths (or just one) of light. We can put these in an emission spectrum, and here are 3 examples:

If you observe a long distance, your images will have no colour, so adding narrowband filters and colour filters will bring back some of it. It’s easy to think you are “cheating” with imaging, or altering them, but the filters only give back the original colour of the object, so you haven’t actually tampered anything. If however, you take an image and tint it with a photo-shopper, then you will have tampered with the image.

Narrowband Filters

A Narrowband filter only emits a specific range of wavelengths through, so don’t stack these. A quick search on the elements within what you want to observe will help you choose the right filter for viewing.

Or if you want to be more specific, line filters work just like narrowbands, but only emit 1 or 2 wavelengths.

Check out the work of Robert J Hawley. With the before photo there’s only the use of colour filters, but in the after picture he has used a narrowband to bring out an element, here it’s Hydrogen-alpha.

Many objects will have the same elements, for example hydrogen, so don’t feel as though each filter can be used for one object only. Try practising with lots of deep-sky objects and get to know them!

Solar Filters

These are an exception, where the filter usually goes on where the light first enters the telescope (the front). The sun is too bright to observe without any filters, and I don’t recommend it until you have a filter and projector, or if you prefer going permanently blind.

These block out a lot of light to produce a safe view of the sun. Along with a solar filter, I’d definitely recommend a projector as well, especially for solar eclipses. Instead of being viewed directly through the eyepiece, the image is transferred onto a screen.

How much do filters cost?

It all depends on the type of filter. Moon filters are generally very cheap and affordable, and buying colour filters in bundles reduce costs greatly! These are good to start with. Neutral density and polarising filters tend to cost between £10-£30. Some brands like to charge almost 3x that, but there’s no need to spend so much on a regular moon filter.

The most expensive filters are narrowband and line filters. The same narrowband will range from £80-£200! Bear in mind that some are easier to manufacture than others.

Rather than buying new, how about looking at thrifting online or looking at astronomy buy, swap, sell groups on Facebook? I found a small bundle of narrowbands at around £70 each, which is pretty good! Have a good nose around before making a purchase, or why not borrow from a friend?

That’s all for filters today, stay tuned for more!

Happy Stargazing!