Filters

Astrophotography is full of ways to spend lots of money for a slight improvement on your images. Filters, on the other hand, are a way to spend small or relatively modest amounts of money for a LOT of gain. I cannot think of another thing that can make so much difference for so little investment – some are bordering on pure magic.

The world of filters can seem daunting. There are so many names and types and sizes and wheels and draws and many other things besides. And complicated isn’t a very Everyday way to do astrophotography. So lets try and break this complicated world into some slightly smaller chunks.

In this article, I’ll take you through the different types of filters, the common mountings, and crucially, how to read transmission graphs so you understand what the filter will do for you. They look complicated, but believe me, they are easy to understand once you know a few key basics – see the multi band pass section for more details.

Types of Filters

Suffice to say there are many. However, we can whittle them down to a few key ones that you will want to know about.

• Light pollution filters (also known as CLS, or City Light Suppression Filters)
• Multi band-pass filters
• Narrowband filters
• L-RGB filters

Then there are your three most standard ways the filters are mounted

• 2″ filters (with a threaded frame)
• 1.25″ filters (with a threaded frame)
• Clip in filters (for DSLR cameras only)

These are your core set, and over time you will likely come to find a brand that you like and a way of fitting them that you like.

My filters of choice are 2″ for dedicated astronomy cameras and clip in filters for my DSLR. However I would never buy two of the same filter just because of the different mounting methods. That seems like a waste of money to me.

Light Pollution Filters

For those in light polluted areas, a CLS (City Light Suppression) filter can made a huge difference. The higher up the Bortle scale your sky goes, the more difference it will make. Even in my Bortle 5 back yard it makes an amazing difference.

For those in the higher Bortle numbers, this is the single biggest game changer you can purchase. And they can be fairly cheap.

The enemies of Astrophototgraphy are mercury and sodium (at varying wavelengths). By isolating these particular wavelengths of light, you can block their path to the sensor, so your camera never picks them up.

Think of each pixel as a bucket that can take cups of water. You have 10 cups each of Ha, [OIII], [SII] and 50 cups each of Mercury and Sodium. Without a filter, you will have 130 cups of water in your bucket, but 100 of them are ones you don’t want and because they all mix together, they dilute the other 30 making them almost impossible to see.

The light pollution filter stops those 95 out of 100 cups getting in, so you are only left with the 30 good ones and a small, manageable number of bad ones. Sounds amazing right? It is. Just don’t forget, that by eliminating some wavelengths of light, your camera is not getting a full spectrum, so it will result in a colour cast on your images.

Don’t panic about this colour cast. Just to prove the point, I have just stacked a few light frames (no calibration frames) from that nights imaging, and done some 5 minute editing in Photoshop and by balancing the colours, the cast has gone.

For those in low light pollution skies, I wouldn’t recommend any light pollution filters. It’s just easier to deal with the small amounts in post processing. So if you are Bortle 4 or under, save that money and invest it in a multi band-pass filter.

Multi Band-Pass Filters

Now we are into my favourite category of filters – especially for DSLR and one shot colour cameras. Currently my filter of choice in this area is the Optolong L-eNhance filter.

Multi band-pass is just as it sounds. It lets multiple selected wavelengths of light through, and blocks everything else – including most moonlight making them perfect for those full moon nights.

In astrophotography there are a few key wavelengths of light that you will want to remember;

• 486nm – Hb – Hydrogen Beta
• 500nm – [OIII] – Oxygen III
• 656nm – Ha – Hydrogen Alpha
• 672 – [SII] – Sulpher II

Multi band-pass filters aim to allow as much light through from two, three or four different wavelengths of light. They are dual band, tri band and quad band filters. Let’s look at a dual band pass filter.

The Optolong L-eXtreme is a fantastic example of a dual bandpass filter. The chart above from left to right shows the wavelengths of light. Along the way there are vertical coloured lines that are the points of colours we are interested in. So at 500nm, you can see a line green representing [OIII].

The important part is the white line. It starts at zero at 300nm, stays at zero all the way until just before 500nnm, then drops down to zero again, then peaks again at 656nm, then back to zero until the end of the chart.

Everything UNDER that white line will be transmitted. Everything above that line is blocked. So in this case, everything from 300 – 497(ish) is blocked, then everything from 503(ish) to 653(ish) is blocked, then from 659 onwards is also blocked.

So the only light getting through is a tiny width at 500nm and 656nm, which as we saw earlier, is the transmission lines for [OIII] and Ha – or Oxygen III and Hydrogen Alpha.

The Optolong L-eNhance filter is classed as a tri-band filter because it also allows Hydrogen Beta (Hb) through – though this is done in the same band as the [OIII].

Quad-band filters are also available – but can often be costly. I don’t own or use any quad-band filters, but a good example is the Radian Triad Ultra filter. The transmission chart slows clearly the four peaks that are allowed to pass, everything else is blocked.

These incredible specific quad-band filters are not cheap though. The Radian Triad Ultra comes in at about £1,200!

The Triad Ultra is also closer to dedicated narrowband.

One of the keys to these filters is that, when used with a one shot colour camera, they produce true colour images. Whereas narrow band is generally used to create false colour images – so lets dive into narrowband filters.

Narrowband Filters

Narrowband is where things start to get serious. And expensive. Generally, narrowband filters are used with mono cameras.

Narrowband filters do just what they say, they isolate a single part of the spectrum and block out all other light. For example, if you used a Hydrogen Alpha (Ha) filter, then you would only get a small amount of light passing through at 656nm. So your image would look very red. Overall these produce

There are a number of colour pallets that you can use, but the most famous is SHO – also known as the Hubble Pallet. It was made really famous by Hubbles image of the Pillars of Creation in the Eagle Nebula.

SHO stands for Sulphur II [SII], Hydrogen Alpha (Ha) and Oxygen III [OIII]. They map across to RGB in that order, too. So [SII] maps to the red channel, Ha to the green channel and [OIII] to the blue channel. You can therefore already tell that this is a false colour image. We know from its wavelength that Ha is red, so by mapping it to green, it’s already false.

However, don’t let that detract you, many false colour images are utterly beautiful in their own right. So long as you maintain your integrity and don’t try and claim it’s a true colour image, then what’s the harm? It’s images like the above that inspired me – and probably you, so why be purist about it now?

There isn’t much more to narrowband filters. Expect maybe that you can use a Ha filter, for example, to enhance your RGB images. So if you took a photo of a nebula, you could then take the same picture with a Ha filter and combine the two to really bring out the red. This becomes an HaRGB image.

L-RGB Filters

It stands for luminance, red, green and blue. These are used with mono cameras to create a true colour image by collecting the R, G & B one colour at a time. You then also have a luminance filter which helps to improve the overall contrast.

Why would you want to use a mono camera to create a colour image this way, I hear you ask. Why not just use a one shot colour camera? It’s all to do with how your camera pixels work.

So that’s a lot of lost photons. With a mono camera, they don’t have an RGGB matrix. Every pixel collects all photons that come through. So if you put a blue or red or green filter in front of it, all pixels become that colour. so you get this;

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Whereas before you only had one red pixel, you now have four times that, same for the blue. Even the green has doubled. This means that your images are much sharper as every pixel is collecting every colour.

The luminance filter simply provides additional contrast in your final image. It’s not a necessity, but frankly if you have gone to the trouble of spending hours collecting data in each colour, you may as well do that last bit and get the luminance data!

DSLR Clip-In Filters

My DSLR has a SkyTech CLS clip-in filter. You cannot get more Everyday than this – the filter literally clips into the front of the camera and that’s the job done. I paid under £70 for it and I wouldn’t be without it.

When you first get a clip in filter, there is a chance that it wont seem to fit in your camera. The first time I received a clip in filter, I thought something was wrong. Don’t panic, there’s nothing wrong. You simply need to adjust the two ‘wings’ on the bottom of the filter. I used a small flat headed screwdriver and you just gently prise the wings slightly further out until you have a nice snug fit.

These, for me, are the simplest of filters to use. No adjustment to your imaging train, no additional filter houses. Clip it in your camera and you are good to go.

There are many types of filters available as clip in, including CLS, narrowband and multi band. So you will not be short of options when it comes to these filters.

They do, however, have one big limitation. They only fit into your DLSR. So if you ever upgrade to a dedicated imaging camera and decide to do away with your DSLR, those filters are now useless. So if you are thinking of upgrading, there is every chance you should consider mounted filters.

Mounted Threaded Filters

Nowadays, these are my go-to filter of choice. They come in two sizes 1.25″ and 2″. Everything I use is 2″, so that’s what my filters are.

There are a number of ways that they can fit into your image train. That could be through filter wheels, filter draws, or as simply as they screw onto the end of your field flattener like this;

Mounted filters are by far the most versatile. They can be used in many ways across many cameras and scopes. For example, the William Optics SpaceCat51 (and RedCat51) allow you to attach a filter directly to the telescope and then attach a camera. Nothing else needed.

Summary

Filters come in all shapes and sizes, and whilst you can spend a fortune on them, some of the cheaper ones really can deliver a huge boost to your images. Think carefully about what you want to gain from your filters, how you plan to develop your imaging equipment and your conditions.

There is so simple answer to questions such as ‘which CLS filter will work best for me’. Sadly it can be trial and error. And read what other people say about them in comments and chat sites. People in the same Bortle skies are usually the best to look for, but a lot of the time it’s down to trial and error.