Light pollution is encroaching more and more on where we live and where we do astronomy. Cities and towns grow, business parking lots and football stadiums appear, and an ever-increasing amount of light finds its way up into the sky and eventually into your telescope objective. Sometimes we can escape to a dark-sky site for some relief, but what is one to do in the meantime? Unlike the yet-elusive cloud filter, there is a type of filter that can help reduce the fog of city lights: light pollution filters!
Effect on Astrophotography
There are a lot of “dim fuzzies” that gain brightness and detail when observed through the sensor of a camera even when they are difficult to pick out from the background sky in the eyepiece. When there is light pollution in the camera’s view, however, that light also gets collected by the camera’s electronics, and the deep sky object can still be lost within the background light, even after processing.
Narrowband filters allow one to pierce through the heavy cloud of spurious light and detect faint detail of the clouds of space, but they are only useful for specific types of targets, not to mention their much greater cost. Galaxies and reflection nebulae are left out in the cold, unable to be seen through the light pollution, and not on the correct wavelengths for narrowband filters.
How Do Light Pollution Filters Work?
The majority of skyward-deflected light comes from poorly-shielded streetlights and parking lot lights, which are often sodium and mercury bulbs. Sodium and mercury (like all chemical elements) glow on specific wavelengths – for sodium, that yellow-orange color you are familiar with. Fortunately for astronomers, most of the interesting objects in space to image don’t glow in that part of the spectrum, which makes it a prime target for some filtering.
Filter manufacturers take advantage of this fact by creating filters that have a “hole” in the spectrum where city lights typically glow.
Astrodon L2P filter spectrum, annotated by the author
Several filter manufacturers sell light pollution filters, such as Astrodon and Lumicon, and their spectra vary. The Astrodon L2P filter has several holes for specific common streetlight wavelengths, while the Lumicon Deep Sky filter blocks a wider swath.
Lumicon Deep Sky filter spectrum
Here is an example of a light pollution filter at work on the California Nebula, using a ZWO ASI294MC Pro on a Takahashi FSQ-106N f/5 refractor under Bortle 7 skies north of San Francisco, CA.
8x300s exposure, luminance filter
8x300s exposure, light pollution filter
As you can see, with the same camera and exposure time, the light pollution filter aided in bringing out much more of the nebula color and detail. Additional processing on the image would help reduce light pollution even further.
With many cities switching to more energy-efficient LEDs, and with many choosing white or cooler color temperatures for their lights, blocking light pollution with filters is becoming increasingly difficult. Light pollution reduction advocates, like the International Dark-Sky Association, promote the use of warm-spectrum LEDs instead, which are not only easier to block with light pollution filters, but are also better for human and animal health. They also advocate for better shielding of lights and making sure they only point down to reduce the amount of light scattering into the atmosphere in the first place.
What to Look For in a Filter
There are a few things to keep in mind when choosing the proper light pollution filter for your camera. The first is what type of camera you have. If you are using a full-spectrum-modified DSLR, a monochrome astrophotography camera, or a one-shot color astrophotography camera, then you will need a light pollution filter that also blocks UV/IR. Otherwise, you can get halos around your stars from the not-in-focus UV or IR light, particularly if you are using a refractor or a reflector with corrective optics. Unmodified DSLRs already have the UV/IR-blocking filter built in.
You can find out whether a given filter is UV/IR blocking either from the product description or by looking at the spectrum chart provided by the manufacturer, such as the Astrodon one up above, which does block UV/IR.
If you are using an unmodified DSLR, or a modified DSLR that has only had the spectrum filter removed and not the UV/IR filter, then you do not need a light pollution filter that also blocks UV/IR. However, using a UV/IR blocking filter will not hurt anything, and will allow you to use that filter with other cameras in the future. (Unless, of course, you are trying to image in IR light!)
Another thing to consider is how much of the light spectrum is blocked. The more light that is blocked, the less light pollution will appear in your image, but there will also be less light from your target, particularly if that target is a galaxy or globular cluster.
Filters come in a wide variety of sizes with a dizzying array of numbers attached – 2”, SCT rear cell, 31mm, etc. There are a few things to keep in mind when selecting filter size.
Filter Wheel/Tray/Drawer Size
Your filter will need to be attached to your camera, and there are multiple ways of doing this. Users of monochrome cameras will typically have a filter wheel with multiple filters, while users of color cameras may or may not have a filter tray or drawer.
Filter wheel carousels come in a variety of sizes, so you will need to check the specifications for the filter wheel and choose a filter that will fit. Also check the height of the filter to make sure it will fit in your wheel, especially if you have a particularly narrow model. Some 2” filter wheels, which are M48-threaded, will also accept 50mm unmounted filters, which you will need filter masks and screws to attach to the carousel.
Camera Sensor Size
If you are trying to decide which filter size you need for your camera, be sure to check the sensor diagonal size on your camera. You want a filter that covers the whole sensor and does not cut out the corners. For example, APS-C sized sensors (such as DSLRs and some astro cameras) are around 24x16mm in size, which yields a diagonal of 28.8mm, so 31mm (1.25”) filters will work fine. However, large-format sensors, such as those on recent ZWO and QHY releases, will require larger filters. The new ZWO ASI6200MM Pro, for instance, has a sensor diagonal of 43.3mm, so 31mm and 36mm filters will not be large enough. Instead, 2” or 50mm filters are ideal.
If you are using a Canon or Sony DSLR, you can also use what is called a clip-in filter, which sits inside the camera body just above the sensor. This also allows you to use a light pollution filter with a camera lens.
How Do I Process Images Taken With a Light Pollution Filter?
If you look through a light pollution filter with your eye, you will notice a blue-green color cast. It will throw off the white balance of your images if you use a one-shot color camera, but it is not difficult to fix. Tweaking the Levels and Curves in Adobe Photoshop or GIMP can restore a natural color balance, as well as the multitude of color correction tools in PixInsight (DynamicBackgroundExtraction, ColorCalibration, and PhotometricColorCalibration).
Rosette Nebula, before color calibration (using a screen stretch with linked color channels)
After applying PhotometricColorCalibration in PixInsight
Unless you have the great privilege of either living at or having a remote telescope at a dark sky site, light pollution filters are a must for astrophotography. While imaging under light pollution is never as good as being at a dark site, you can at least improve your images and expand your list of accessible targets. Happy imaging!