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Filters - the very basics

18 September 2023

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Graph showing OIII Filter Bandwidth Comparison

OIII Filter Bandwidth Comparison

Someone asked on the WhatsApp group about using filters. It's a huge topic, but here is some very simple information on them.

Firstly - this doesn't cover solar filters. Please don't use anything here to look at the sun, ever.

Secondly - this is about visual filters, not astrophotography filters. Astrophotography filters are a whole other game.

Filters are simply (!) thin circular discs of glass with a metal rim that are screwed to the end of your eyepiece (the end that goes into the scope). For 1.25" eyepieces you'll need 1.25" filters; for 2" eyepieces, well... you'll need 2" filters. Good filters are expensive; good 2" filters are even more so.

Filters are generally used to block out certain wavelengths - colours - of light to make other wavelengths more obvious. That means that almost all filters reduce the brightness of the whole of the view, and that can be a little disconcerting at first - everything gets dimmer. You almost certainly can't use a filter to find an object; you have to be sure you are pointing in the right place, then add a filter and hopefully be able to tease out the object you are looking for.

There are some exceptions - a neutral density (ND) filter blocks out all wavelengths to the same amount for example, and is sometimes used for the moon or planets to reduce their brightness as they can just be too piercingly bright. If you are half-way through an observing session and you put Jupiter in your field of view it is bright enough to ruin your dark adaptation, so perhaps add an ND filter to make sure you can find that dim planetary nebula immediately afterwards. They're available in different strengths.

Most other filters are either bandpass or narrowband filters. Bandpass filters tend to try and improve the general contrast you see in order to block something out - like light pollution glow. They let most light through, but prevent just some distracting wavelengths from reaching your eyes - so you see the faint stuff more easily. You'll see names like 'UHC' (Ultra-High Contrast) or CLS (no idea what it stands for, but they're quite good) or LPR (Light Pollution Reduction). These latter LPR filters work really well for old-fashioned sodium or mercury vapour streetlights that have a single colour that can be blocked out, perhaps less so for more modern LED lights that put out a more white light.

Narrowband filters are quite object specific: they let through only the light from a specific type of object. For instance, the Veil nebula is an area of glowing gas (a supernova remnant) where the light emission is caused by Oxygen molecules being heated - that gives off light in a certain wavelength called the OIII (or 'trebly-ionised oxygen') line. So, if you view the Veil through an OIII filter you see only that light - even the stars are dimmed. It can give very good views of the Veil, or planetary nebula like the M27 Dumbbell. Others filters can be very specific, like the filter needed to see the famed Horsehead Nebula (B33) - it glows on a 'line' (a wavelength) that almost no other objects glow in and is very faint: it really needs a Hydrogen-Beta filter to be seen with the naked eye, and that's a very expensive filter for this one object!

From the above I'm sure you can see it's a large and complex topic. Personally I've got an ND filter, an CLS filter, a UHC filter and an OIII filter. That covers pretty much everything I need. If you're starting and you're battling light pollution then look at one of the bandpass filters for that purpose, but check your streetlight type and make sure it is covered. For general use a contrast-enhancing bandpass filter is good to get at faint objects (in my experience Astronomik UHC/CLS filters are both pretty good; Baader are also good); if you have reasonably dark skies then an OIII is a good-value narrowband visual filter for many objects.

I hope this helps a little. Please do ask if you want to know any more.