Guide: “Which filter should I use with my OSC camera?”

I’m often contacted with lots questions, and am always happy to help beginners. Perhaps the most common theme that crops up is which filters to use. I’m writing this guide to provide some useful information so that new astroimagers can understand which filters to use, and why.

A seemingly simple question…

I think that lots of beginners are expecting a simple answer to the question of which filter to use for their OSC camera. Normally the question will be something like, “I’ve got terrible light pollution, which filter is best?” As is the case with everything astrophotography-related, it’s a little more complex than you’d first think!

To begin with, let’s be clear that this guide is for OSC cameras. Mono is different as far as specific filter recommendations are concerned, although the background information we’ll cover is still relevant. Also, this guide is about deep sky objects (galaxies, nebulae, etc.) Planetary (like Jupiter, Saturn, the Moon) is quite different.

With that done, we can begin. Not all astronomical objects are the same. We can, in general, split them into two categories, based on the light that they emit. Knowing this is essential when it comes to filter choice. The two categories are broadband and narrowband.

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Broadband

Broadband targets are those that emit lots of their light in the visible part of the electromagnetic spectrum. This is the part that our eyes naturally detect, and so do our camera sensors. The main types of broadband targets are stars (including clusters), galaxies, and reflection nebulae (clouds of gas and dust that we see because they’re reflecting the light from nearby stars). Here are three images I’ve taken of broadband targets:

Because our OSC camera sensors are naturally geared up for broadband targets, it’s not essential to use any filters to take good images of them. It can be beneficial, however, to use a UV/IR cut filter. This can help to produce sharper images, and reduce star bloat. Note that many astro camera, including my beloved ZWO ASI2600MC Pro, have a UV/IR cut filter built in. If this is the case, you don’t need to buy an extra one. If you’re not sure about your camera, scout online and find some people that use the same type. They’ll be able to advise on whether you need a UV/IR cut filter.

You might be wondering about light pollution. By far the most effective way of combatting light pollution for broadband targets isn’t actually anything to do with filters. The trick is to gather lots of data and clock up a high total integration time. Think 20 or more hours from a city. This might sound unachievable, but with the right approach it’s not that hard. To help, I’ve written a whole article about how to get long integration times. Now, look again at the three broadband images above. They were taken from a Bortle 8 city centre, and I didn’t use any filters at all. Long integration times are the key!

There are lots of light pollution filters available, and given this article is about filters, I should mention them! It’s a bit of a minefield, and their effectiveness varies based on where you’re imaging from, and so your local sources of light pollution. I’ve tried lots and have only found one that actually provided me with any benefit: the Optolong L-Quad Enhance (review here). I can’t say for sure whether it would work for you, because your exact sources of light pollution will be different from mine. Consider whether your money would be better spent on an accessory that makes it easier to achieve long integration times. Here are some ideas.

So, let’s sum up advice for broadband targets:

• If your camera doesn’t have an in-built UV/IR cut filter, then consider one. Check in with other users of your camera for recommendations.
• Long integration times are key to beating light pollution. The worse your light pollution, the longer the required integration time.
• A light pollution filter is very much optional, even if you’re in the middle of a city.
• You might see some benefit from a light pollution filter. Try different ones to test if they actually help. I’ve found the Optolong L-Quad Enhance to be the best for me, but what works for me may not work for you. And remember, there’s no substitute for long integration times.

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Narrowband

Now let’s talk about the other type of target: narrowband. These give out most of their light in very specific wavelengths, hence the “narrow” in narrowband. The most common wavelengths are called Hydrogen-alpha (Ha), Oxygen-III (OIII), and Sulpfur-II (SII). Emission nebulae are excellent narrowband targets, and are plentiful in the sky.

If you were to treat such an object as a broadband target then you might be hard pressed to see any detail at all (other than the stars, which are broadband in nature). But capture the narrowband wavelengths and the target comes to life! The slider-view image below demonstrates this. Both pictures show the same target, but broadband is on the left and narrowband is on the right.

Narrowband wavelengths are quite distinct from the wavelengths that light pollution occupy, which is very useful for light-pollution plagued astrophotographers because if you can filter out everything except the key narrowband wavelengths, not only can we do justice to targets like emission nebulae, but we can also partly sidestep light pollution!

So, how do we go about capturing narrowband data using an OSC camera? That’s where filters come in. We OSC users commonly use dualband filters (although some triband ones are available). Dualband filters capture two narrowband wavelengths simultaneously, usually Ha and OIII.

There are lots of these Ha/OIII dualband filters available. The Optolong L-eXtreme was previously considered to be the best of these, but a newer generation has now been released. The Optolong L-Ultimate is the best I’ve used, but also the most expensive. I’ve also tested the Askar Color Magic and found it to be quite good.

I mentioned that there are three main narrowband wavelengths: Ha, OIII, and SII. Most dualband filters cover Ha and OIII, so what about the SII? This wavelength is less common and often weaker in astronomical objects, so some OSC imagers basically forget about it and make bi-colour images using their Ha/OIII dualband filters. It’s also possible to create pseudo-SII from your data during processing, in order to create very colourful images from just Ha/OIII. It’s not as scientifically accurate as incorporating genuine SII data, but it looks striking!

It’s a good time to be an OSC imager though, as now there are some SII/OIII dualband filters available. These can be used if you want to create genuine Ha/OIII/SII images using an OSC camera. You collect data on your target using your Ha/OIII dualband filter, and then using your SII/OIII dualband filter. Combine the results during processing and hey presto, you have an Ha/OIII/SII image! The SII/OIII dualband filter I use is the Askar D2.

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Mix and match

By now hopefully you’ve got a good grasp on the difference between broadband and narrowband targets, and how you should choose filters based on the specific target you’ve got in your sights. However, sometimes things are a little more complicated, and you may want broadband and narrowband data.

Let me give an example. I stated that galaxies are broadband targets, and that’s true. However, some galaxies have Hydrogen-alpha regions (where stars are forming). Hydrogen-alpha is narrowband, so are we missing out on it by treating the galaxy as only broadband? Let’s conduct an experiment to find out.

Below is a slider image of The Pinwheel Galaxy. On the left we see it imaged as a broadband target (with no filter). On the right is the exact same target, but imaged using an Ha/OIII dualband filter. (There’s hardly any OIII so we can ignore that). Notice how the broadband view contains lots of detail. The dualband view is nowhere near as impressive, but does highlight those areas of star formation in the galaxy’s spiral arms:

So what to do? As an astroimager, you have a decision to make. You could just stick with broadband data. Or, you could also collect some narrowband and add that in. The slider below shows these two options in fully processed form. On the left is broadband. These are often called RGB images (Red Green Blue). On the right is the RGB plus Ha; we call it an HaRGB image. Is it worth the extra time and effort of collecting the Ha? That’s for you to decide.

Here’s another example. I was recently imaging The Flaming Star Nebula. Some quick research showed that it’s both a reflection nebula (broadband) and an emission nebula (narrowband). Use the slider image below to compare my broadband image (Optolong L-Quad Enhance) on the left, with Ha/OIII (Optolong L-Ultimate) on the right.

Here’s what the two images look like combined: an RGBHaOIII image:

And finally, just when you thought you had it all figured out, let’s mention star colours. Stars are broadband targets, although they’re so bright that they show up in narrowband too albeit not as well. Their colours, however, are lost when imaging in narrowband, and they all appear as white dots. Broadband captures star colours well. So, if you want to image a narrowband target but have accurate star colours, then you may want to add a little bit of broadband data just for the stars. This is all comes together when you process the image.

Here’s a final example photo. It’s The Cygnus Wall captured using an Optolong L-Ultimate (Ha/OIII), Askar D2 (SII/OIII) and a little bit of broadband (no filter) just for accurate star colours.

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So, which filter should you use with your OSC camera?

Phew! That’s a lot of info to take in. See how the question “which filter should I use?” is actually quite complex? Let me sum up the advice:

• If you have an OSC camera but no spare budget, then you can still image broadband targets with no filter at all (or just a UV/IR-cut filter if necessary).
• A broadband light pollution filter may be beneficial, but you won’t know for sure until you test it under your local sky conditions. I use an Optolong L-Quad Enhance. I consider a broadband light pollution filter to be a low-priority purchase as you’ll probably get more bang for your buck by spending money on accessories that make it easier to get long integration times.
• The worse your light pollution is, the longer your total integration time should be. This is particularly true with broadband imaging.
• If you want to image narrowband targets — which I recommend, because they’re plentiful, interesting, and narrowband works well even under light-polluted skies — then first buy an Ha/OIII dualband filter. The cheapest option worth having is a second-hand Optolong L-eXtreme. The best premium option is, in my opinion, an Optolong L-Ultimate.
• If you’re confident with Ha/OIII and want to develop more advanced skills then the next logical step is to buy an SII/OIII filter. I use an Askar D2.
• Choose your filter based on your target. Do some research to find out what type of object it is. Galaxy? Emission nebula? Globular cluster? Reflection nebula? This will inform your filter choice.
• I use an Optolong L-Ultimate (Ha/OIII), Askar D2 (SII/OIII), and Optolong L-Quad Enhance (broadband RGB).

Useful extra reading

OSC vs Mono from a city
How to get long integration times
Light pollution filter shootout (IDAS LPS D1 vs P3 vs D3)
Dualband filter shootout: L-eXtreme vs L-Ultimate vs Colour Magic 6nm
Review: Askar Colour Magic D1 and D2 duo band filters (Ha/OIII & SII/OIII)
Review: Optolong L-Quad Enhance
Top 10 upgrades

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