Take a walk under the stars on a moonless night and take a look up at the sky: the sky can be very bright with little stars, very dark with plenty of stars, and the Milky Way stretching from horizon to horizon, or something in between.
Now, look around and perhaps you can see your surroundings fairly well and even read without a flashlight. Or you can find yourself in pitch-black darkness and a somehow scary environment.
What changed between those scenarios is the sky brightness, a crucial aspect in astronomical observation and photography.
The brighter the sky, the more difficult it is to observe and photograph anything beyond the brightest planets and the Moon, large star clusters, and multiple star systems.
So, are you stuck under a bright sky? Don’t despair just yet, as light pollution filters are your best way to deal with excessive sky brightness from man-made light pollution. Let’s learn more about them.
What Is Light Pollution And Why Is It Bad?
Light pollution is a plague affecting all densely populated areas. It is estimated man-made light pollution renders invisible the Milky Way for about one-third of humanity, including 60% of Europeans and 80% of people living in North America.
Most of the light pollution is concentrated in and around metropolitan areas or, as for Benelux (Belgium-Luxembourg-Netherland), over entire countries, thanks to the main national road network being illuminated at night.
What Can You Do To Fight Light Pollution?
If you want to help solve the light pollution problem, you can support local and worldwide organizations like the Dark-Sky Association, speak to your city council to improve local light pollution, or even start a campaign.
But for now, when it comes to your passion for the night sky, your best option, and the most obvious one, would be to travel to darker locations. Unfortunately, depending on where you live and if you own a car or not, this can be a challenging, costly, and time-consuming solution.
I live in Brussels (Belgium). A trip to a real dark sky would see me driving all the way to France, some 2-300 km away. One way. The best my country has to offer is a Bortle 3 sky in the middle of the woods. Most of the time, a dark Belgian sky would score an unexciting Bortle 5-ish class.
What’s Bortle? We have a full article dedicated to the Bortle Scale. In short, it is a sky brightness classification (typically from class 1 to class 9) based on the faintest celestial object or phenomenon one can see from its location. The less you can see in the sky, the higher the Bortle class.
If you don’t have easy access to a dark place, your next best bet is using the so-called light pollution reduction filters, LP/LPR filters for short.
How Do Light Pollution Filters Work?
LP filters are mostly used in astrophotography with the purpose of reducing the sky’s brightness. But this is not the whole story.
Say you live in the city and you want to reduce the sky’s brightness. You may be tempted to grab that wonderful 10-stops neutral density filter you normally use to create long exposures in the daytime.
And sure enough your ND filter will reduce the sky brightness, but it will also reduce the already faint light coming from that galaxy or nebula you are trying to photograph, with disastrous results.
So what do LP filters do differently? They selectively cut a greater amount of particular types of light and let pass the other types almost unscaled.
Light Pollution Filters For Astrophotography
A camera sensor sees the whole visible spectrum (the type of lights our eyes can see). Plus a fair amount of extra light past the lower end (Ultraviolet light, or UV light) and higher end (Infrared light, or IR light) of the visible spectrum.
Did You Know? We could practically consider ourselves almost blind, as we can only see about 0.0035% of the entire electromagnetic spectrum.
Photographic filters can alter both the amount and relative proportions of the light the sensor can see, and they can be divided into two categories: broadband and narrowband filters.
Broadband filters affect the whole visible spectrum or large parts of it. An example of such filters are neutral density filters, where the intensity of all visible lights, regardless its frequency (or color), is reduced.
If you are a landscape photographer using ND filters, you may have noted a residual color cast, often magenta, in your images: this is because the filter affects some colors slightly more than others.
While this is unwanted for landscapes photography, broadband LP filters for astrophotography do just that, by greatly reducing mostly the light (color) associated with common light pollution.
Didymium filters, for example, aim to reduce the color of the light emitted by Mercury-vapor and Sodium-vapor street lamps more than others.
NarrowBand (NB) filters allow only light within a very narrow region of the visible spectrum to reach the sensor, hence their name.
These filters are the most aggressive ones when it comes to suppressing light pollution, but they are not suitable for every camera/lens combination nor for visual observation, are costly and require the use of a filter wheel, either manual or motorized, or at the very least, a filter drawer.
Finally, NB filters are not only used to suppress light pollution, but they are the standard for people using monochrome cameras, so to create false color images using the SHO palette (or Hubble palette) and HOO palette.
If you have a color camera, also called OSC in astrophotography jargon, you can buy multi-band filters. These are usually dual-band (to capture Ha and Hb+Oiii emissions only), tri-band (for Ha, Hb, and Oiii emissions), and quad-bands filters (for Ha, Hb, Oiii, and Sii emissions).
NB filters can come in different versions, depending on how broad the bands are. Typically bandwidths are 7nm (less expensive), 5 nm, and 3 nm (very expensive and very aggressive and not for beginners).
The bottom line is that whether you are using a didymium filter or a NB filter, their goal is not only to darken the sky but to do so in such a way to improve the contrast between the sky and your target, so that it will be more visible in your images.
Light Pollution Filters: What Are They Made Of?
LP filters come in two main flavors: with optical glass “enriched” with rare earth elements or with a special glass coating.
The more affordable broadband filters are didymium filters. They work by absorbing the part of the visible spectrum usually associated with Sodium-vapor and Mercury-vapor lights. They do that by using optical glass enriched with a mixture of the elements Praseodymium and Neodymium.
The more costly narrowband filters and high-end broadband filters are interference filters and use a multilayer coating to reflect unwanted light “back into space” and away from the sensor.
If you want to know more, here is a nice tutorial on interference filters from Olympus.
Interference filters can be easily recognized by their mirror-like coating.
What Size Do Light Pollution Filters Come In?
Didymium filters often come in the form of classic screw-on filters, like any photographic filter out there. HOYA has its Red Intensifier (or Red Enhancer) / Red Starscape filter available in all most common sizes for classic photography lenses.
Did You Know? Didymium filters are popular among landscape photographers to photograph foliage during fall. By reducing the yellows, the resulting red of the foliage is much more intense and rich.
If you normally work with square filters for your landscape/architecture photography, NiSi offers its Natural Night didymium filter in square formats (180x180mm, 150×150 mm, 100×100 mm, or 75×80 mm) alongside the classic screw-on format.
If you use micro four cameras, you may have lenses with fairly a small diameter. In this case, if your lens diameter is smaller or close to 48mm, you can use astronomy filters in 2” size (equivalent to M48) in combination with a step up/step down filter.
On my Olympus micro four thirds camera I used a 48mm filter on a 200mm lens 55 mm in diameter with no vignetting.
If you can use 48mm filters, you can then easily use NB, UHC (Ultra High Contrast), and other multiband filters intended for astrophotography.
But those filters also come in the smaller (and cheaper) 1.25” format, perfect for using relatively small sensor cameras with your refractor.
Finally, for some camera models (both DSLR and mirrorless), you can find astronomy filters in clip-in format. These filters are simply clipped in front of the sensor: useful if you have telephoto lenses with large diameters.
Are Light Pollution Filters Worth It?
I will let this image speak for me.
Anti-Covid measures forced me to photograph during the 2021 Galaxy season from downtown Brussels, with my scope flooded by street lights. Thanks to LP filters, the season was not completely wasted.
If you live in the city, LP and NB filters can really unlock your sky for urban astrophotography.
Unfortunately, with light pollution from the newer LED lamps the LP filters can’t do much to improve the situation, as white LEDs are broadband light sources and emit light at all frequencies, although IDAS makes the LPS D2 and D3 light pollution suppression filters that are somehow more promising when used against light pollution from LED lights.
Comparing the transmission spectra of the IDAS LPS D2 with that of the Optolong L-PRO, one can see how the IDAS is the most efficient of the two in filtering out light in the 450-475nm and 550-650nm ranges, two regions of the spectrum where white LED lights have a somehow strong emission.
Still, LED light is still polluting the unfiltered regions of the spectrum.
Are Light Pollution Filters Always Needed?
No, they are not: if you are one of the lucky people living under a truly dark sky, say Bortle 3 (or better), you don’t need LP filters, particularly if your target is high in the sky, where man-made light pollution is minimal.
But as with all rules, this also has its own exceptions and you will benefit from LP filters if:
- You have a monochrome camera and you want to create a false color image
- You have a color camera, but you prefer creating HOO and SHO false color images
- There is a big bright moon in the sky
And those exceptions all involve the use of NB and multi-band filters. In dark locations, broadband filters are fairly useless.
What Filter Do You Need For Astrophotography?
To decide which LP filter is the best for you, take a look at your equipment and at the type of light pollution you have in your area. Next, consider what type of targets you want to photograph the most.
But beware: aggressive filters can greatly extend the exposure for your single images (subs) to the point your mount could struggle keeping stars from trailing.
When you start using NB and multiband filters, particularly at dark locations, you will need to guide your mount or tracker.
Also, as we will see in the next two sections, not all filters play well with every type of gear and/or targets.
Which Light Pollution Filter Should I Use For My Target?
Not all the LP filters work well on all targets.
Didymium and broadband filters in general work well with galaxies, star clusters, and reflection nebulae (e.g., the Iris Nebula), as they help retain the natural colors of these targets.
If you need an LP filter for your starry landscapes with a nice shot of the Milky Way or to image Andromeda or the Pleiades, these are the filters to use.
Narrow bands and multi-band filters work better when imaging emission nebulae, often rich in Ha emission. These nebulae are all the most famous ones: The great Orion Nebula, the Horsehead Nebula, the North America and Pelican Nebulae, the California nebula, and many, many others.
Did You Know? NB (Ha) filters can be used to better show emission nebulae in the Milky Way, but also in distant galaxies, like those in Andromeda or in M51. Experienced astrophotographers are known to combine these data with classic RGB images to create astonishing photographs.
Bottom line? Invest in both types of filters to get the most out of your time under the stars.
Light Pollution Filters And Everyday Cameras And Gear
Broadband LP filters can be used with pretty much every camera/lens combination, as they simply block some light to get to the sensor by absorbing it.
Interference filters, on the other hand, do not work well with wide-angle and short focal lenses in general. Due to their wide field of view, short focal length lenses collect lights coming in along very different directions and this creates strong halos in the image.
Tip: Can You Use NarrowBand Filters With Standard DSLR Cameras?
It is commonly said narrow bands and multi-band filters are best used with astro cameras and DSLR/mirrorless cameras that have been modified full spectrum. This modification consists of replacing the stock UV-IR cut filter in front of the camera sensor with a clear glass.
But that does not mean it is a lost cause.
Capturing the OIII band with a stock camera is not a big problem, as the band peaks in the middle of the visible spectrum at 500 nm. Ha and SII bands are, indeed, more problematic, as their peak is toward the red end of the visible spectrum, at about 656 nm and 671 nm, respectively.
The most common NB filters for color cameras are dual narrowband filters, such as the Optolong L-Enhance and they let pass only the OIII+Hb band and the Ha band, which is the strongest of the two signals when photographing emission nebulae.
Collecting this Ha signal with unmodified cameras can prove difficult due to the very long single exposures and total integration time you may need to build up enough Ha signal, but don’t despair just yet.
Here is a simple test you can do to determine how aggressive your camera filter is towards the red end of the visible spectrum (750 nm) and to the near infrared light.
- Set your camera to shoot raw and the ISO to the value you use for your astrophotography
- Then take a lens with f-speed similar to that of the instrument you use at night
- Go to your local photography shop and ask to try a Hoya R72 infrared filter that fits your lens
- Point the camera outside in broad daylight and snap a photo.
The HOYA r72 IR filters let pass only visible light from 720 nm to the near infrared. Hopefully, you will get a reddish image out of your test.
The stronger the contrast and brightness of your image, the more infrared light your sensor can see.
If this is the case, since the near IR begins at higher wavelengths than Ha, your camera could cope fairly well with recording the Ha band and you could try investing in a filter like the Optolong L-Enhance, particularly if you guide your mount and can expose for 3-5 minutes at a time.
Of course, the faster your imaging instrument is, the better.
Oh, and since you are in the shop, you may want to grab that IR filter. IR photography is the best way to turn boring and common landscapes into magic places. But this is another story 🙂
Tips: How To Use Light Pollution Filters
LP filters are fairly simple to use and depending on the type of filters, they can be screwed in front of the lens or in the camera adapter for your telescope, clipped in front of your camera sensor, or simply inserted in your filter holder.
The most important thing is keeping them free from fingerprints, as manipulating them at night can prove challenging.
And, as with anything for optical use, avoid scratching them and excessive cleaning.
Tips: How To Clean Your Filters
Filters will get dirty, no matter how careful you are in handling them. Dust, specks and fingerprints are the common problems.
Small dust flakes and specks are easily removed by blowing some compressed air on the filter or by using a lens blower.
With fingerprints, swabbing the filter with some isopropyl alcohol should do the trick. Just don’t get obsessed with cleaning them to avoid removing or scratching the filter coating.
To know more about cleaning your filters and optics have a look at these tips on cleaning filters from the Optolong company.
What Are The Best Light Pollution Filters?
Here are some of the best LP filters you should consider buying.
Didymium and broadband LP filters
They are best used on broadband targets such as galaxies, stars, star clusters, reflection nebulae, and starry landscapes.
Pros:
- They are suitable for almost any camera/lens combination;
- Fairly useful for starry landscapes and light pollution from old street lights;
- Affordable;
- Colors stay fairly natural;
Cons:
- They do also absorb some wanted light from our targets;
- Can’t remove Moonlight;
- May create halos around the brightest stars;
Some reputable filters are:
NarrowBand Filters
They are best used on emission nebulae, such as M42, the Horsehead nebula, etc. True narrowband filters are for monochrome cameras and are intended to capture different emission bands, such as Ha, Hb, OIII, and SII.
Pros:
- Very performant in suppressing light pollution
- Allow for some good astrophotography images from the city
- Allows to create false color images when using a monochrome camera
- Can suppress Moonlight
Cons:
- Expensive
- You need multiple filters and a filter wheel to rotate them during your session
- You can capture one emission band at a time
- Time-consuming and demanding editing
- Not suitable short focal lenses
- Natural colors are not reproduced
- Not suitable for galaxies, star clusters, and reflection nebulae
Some of the most reputable manufacturers are:
- Astronomik
- Optolong
- ZWO
- Celestron
- Baader Planetarium
Multiband filters
These are LP filters that can be used with color cameras and capture more emission bands at the same time. Again, they are most suitable for emission nebulae, having strong Ha emission.
Pros:
- Very performant in suppressing light pollution
- Allows for very good astrophotography from the city
- Allows to create false color images as well as “true” color images
- Can capture more emission bands in one time
- Less expensive than a set of NB filters
- Can suppress Moonlight
- Clip-in version available for some DSLR/mirrorless cameras
Cons:
- More expensive than broadband filters
- Natural colors are not reproduced
- Not suitable for galaxies, star clusters, and reflection nebulae
- Not suitable for short focal lenses
- Increased exposure time
- Work best with astro cameras and DSLR/mirrorless cameras that are astro modified
Some reputable multiband filters are:
One Filter, Four Different Looks
With narrowband filters, you can separate different signals and recompose them in different ways to create images having very different looks.
Take the Optolong L-Enhance filter, for example. You can use the images as they come from the camera, and you can create a black and white version of it. But you can also extract and save the Ha and Oiii bands as single images, to create images with an HOO or HHO palette.
Here is a video I made to illustrate the process of creating such images.
Conclusions
Light pollution filters are a must-have for (almost) every astrophotographer on this planet. They help deal with the evil orange sky glow from light pollution that is robbing the night sky of its treasures.
You should probably start with a good quality broadband filter and then move to a dual narrowband filter or better when you decide it is time to up your game.
For me, living in bright Belgium, where the darkest spot barely scores a Bortle 3 class, LP filters were the best investment I could have ever made for my astrophotography.