Quick Recap - Electromagnetic Spectrum

Before we begin, I wanted to make sure we are all on the same page.  The electromagnetic spectrum is a large range of wavelengths, including Ultra-Violet, Infrared, Visible Light, X-Ray, and more!  Our eyes and cameras are only capable of detecting a small portion of the electromagnetic spectrum.  This narrow range is known as 'Visible Light', and includes all the colors of the rainbow, from Violet to Red.  Colors are measured in nanometers, ranging from about 400nm to 700nm.  This information will be important later on, once we begin looking at the actual astrophotography filters!  For more information on the Electromagnetic Spectrum, click here.

As you might already know, most photography relies on three colors – Red, Green, and Blue.  Blue light is generally from 400nm – 500nm, Green light ranges from 500-600nm, and Red light from 600-700nm.  Therefore, our camera sensors are designed to detect light ranging from about 400nm to 700nm.  Our eyes are also capable of seeing light from about 400nm – 700nm. 

The objects we photograph in the night sky tend to emit a few common wavelengths.  These include Hydrogen Alpha (656nm), Oxygen (501nm), and Sulfur (672nm).  These specific wavelengths contain a lot of valuable information and detail.  Click here for more information. 

Many astrophotographers have special filters that allow them to only capture very precise wavelengths, like H-Alpha.  These “narrowband filters” are great for astrophotography, especially in light polluted areas!  You can capture NASA quality photos, even in the middle of a city!  How? 

Sodium-Vapor lamps, the most common source of light pollution, have a wavelength of 589nm.  Now, if we are just taking a normal photo with a DSLR camera, we are capturing light ranging from about ~400nm to ~700nm.  Therefore, the light from Sodium Vapor lamps is visible in our photos.  However, a narrowband filter blocks out all wavelengths, except for a small range.  For example, only light waves near 656nm will be visible with an H-Alpha narrowband filter.  The 589nm sodium-vapor light will be completely blocked.  Click here to see how an H-Alpha filter will limit the light reaching your camera's sensor.  As you can see, only the limited range around H-Alpha will be visible, every other wavelength of light will be blocked.

Therefore, you could buy H-Alpha, Oxygen, and Sulfur narrowband filters and capture stunning astro images from the city!  You will need to do some special post-processing to turn these images into a great color photo.  However, for backyard astrophotographers stuck in the city, this is amazing!  Who would’ve thought that someone living in LA could get Hubble quality images from their backyard!  That’s the power of narrowband filters.  For more information on narrowband filters, check out Chuck's Astrophotography on YouTube.

To be clear, narrowband filters are not necessarily recommended for DSLRs.  You will want a monochrome CCD-style camera and telescope for this method.  I'll explain this more in a future article.

Picture saved with settings embedded. Rho Ophiuchi as seen from Canyonlands National Park.  The dark skies here allow for amazing images, no filters required! 

*Taken with a stock Nikon D750, Tamron 70-200mm G2, and SkyGuider Pro

Light Pollution Filters

Light Pollution filters are very popular in the astro community right now.  They are designed to block out the wavelengths of light emitted by Sodium Vapor Lamps, the main cause of light pollution.  Now you can now take amazing astro images, even in light polluted areas!  Well, not quite.

First, not all of the light pollution is from Sodium Vapor lamps.  White light from LEDs is becoming more and more common.  While these new LED lights may reduce that ugly yellow/orange glow from cities, they are still producing light pollution.  Since it is white or blue light, it’s also much more difficult to block with a filter.  Therefore, you may still see a lot of light pollution in your images.  Ultimately, the goal of a filter is to block out specific light pollution wavelengths, while preserving light emitted by nebula and galaxies.

There are various types of light pollution filters to choose from.  Some can be screwed to the front of your lens, slid into a filter holder, or clipped inside the camera itself.  There are also filters that require a telescope to be installed.  There are a few companies that are specifically focused on astrophotographers with telescopes, including Astronomik (German company), Optolong (Chinese company), and SkyTech.  These filters usually do a good job.  

Light pollution creates two main problems for astrophotography.  First, the extra light effectively washes out the sky, lowering contrast and obscuring the faint details of nebula and galaxies.  It also usually adds a heavy red / yellow / orange color cast to your images.  If you've purchased my Star Tracker Course, Astrophotography Post-Processing Course, or Deep Space Course, then you already know how to remove the color casts caused by light pollution in Photoshop!  Using a series of Curves adjustment layers, we can pretty easily remove any color cast from a photo.  The image below shows a bad color cast caused by a CCD camera sensor, not light pollution.  However, I think this is a great example of how you can salvage an image once you understand photo editing.

Before

After

I removed this color cast using a few Curves adjustments in Photoshop!  Check out my Deep Space Course to learn more. 

Special thanks to Rodney for providing this image of the Carina Nebula!

My Experience with the Optolong L-Pro and UHC Filters

While I was traveling through California, I stopped by Orange County Telescope.  I met with the owner, Mike, and had a great talk about star trackers, filters, telescopes, CCD cameras, and more!  Mike and Kenny really know there stuff when it comes to astrophotography and telescopes.  If you need any new gear, or have questions, definitely give them a call!  While I was there, Mike generously provided me with two of the Optolong filters – the L-Pro and UHC.  These are clip-in style filters, which allow you to use them with all of your lenses.  The clip-in filters sit inside the camera, where the mirror usually is.  It can certainly be scary to install them your first time, but it's really easy after that.  Watch how to install a clip-in filter here.

The UHC filter is designed to block a lot of light, mainly greens/yellows/oranges, while allowing the main nebulae wavelengths through.  As you can see from this graph, only blue and red light is allowed through.  (Here’s a graph that shows how the wavelengths and colors relate, if you forgot.)  With these graphs in mind, the UHC filter is clearly designed for photographing nebula.  If you are photographing from your backyard, in the city or suburbs, this filter should cut out any light pollution.  However, due to the heavy light reduction, I would only recommend this filter for deep space use with your telephoto lens or telescope.  Click here to see how weird the colors will look with the UHC filter.  As you can see, the colors get pretty wacky.  The comparison image below shows a comparison with and without the UHC filter.

No Filter

UHC Filter

UHC Filter

f/2.8, ISO 100, 15 Seconds - Nikon D750 with Nikon 14-24mm  @14mm  (Daylight WB, RAWs)

The L-Pro filter is much more precise in its light pollution removal.  As you can see from this graph, it directly targets the main light pollution wavelengths.  The L-Pro filter has been designed to remove airglow and mercury / sodium vapor lamps, while retaining the normal colors.  The comparison images below clearly show the difference.  

No Filter

L-Pro Filter

L-Pro Filter

f/2.8, ISO 100, 15 Seconds - Nikon D750 with Nikon 14-24mm  @14mm (Daylight WB, RAWs)

While the wide-angle shots look pretty good, I did notice some problems upon closer inspection.  Namely, the stars are very deformed in the corners.  In fact, as soon as you leave the center of the frame, there are serious problems with the light focusing.  This appears to be a consistent problem with both the UHC and L-Pro filters.  You can download the RAW photos here for a closer look.

However, there's an important consideration here.  These filters are mainly designed for photographing deep space objects, not wide angle nightscapes.  All of the sample images so far were taken with a Nikon 14-24mm lens on a Nikon D750.  Therefore, it's possible this focusing problem will only be visible with extreme wide angle lenses.  My next tests were using a Tamron 70-200mmG2 and SkyGuider Pro for some deep space astrophotography.

Test 1 - North American Nebula

Before we begin, you can download the RAW files for this test here.  As expected, the filters perform much better with a telephoto lens!  No more odd light distortion or focusing problems!  I took these comparison images from the library parking lot in Stevenson, Washington.  The North American Nebula was my target, and at this time of year it was directly overhead, which should be the darkest part of the sky.  The comparisons below are RAW photos, with no editing applied.  Both images had the exact same camera settings.  The L-Pro filter clearly darkens the image and also adds a slight blue color cast.

No Filter

L-Pro Filter

L-Pro Filter

f/2.8, ISO 400, 5 Minute Exposure - Daylight WB (RAW photos)

Next up is the UHC Filter.  It is drastically darkening the photo, as well as adding a purple color cast.  This is to be expected though.  Remember, it's only allowing red and blue light through.  Therefore all of the images taken with the UHC will be noticeably darker, with a significant color cast.  This might seem pretty bad, especially with these RAW photos.  However, I think you'll be surprised once we edit the images!

No Filter

UHC Filter

UHC Filter

f/2.8, ISO 400, 5 Minute Exposure - Daylight WB (RAW photos)

I'll admit, I was very surprised just how good the Light Pollution filter images looked after a bit of editing!  I used Adobe Camera RAW to increase the Exposure, increase the saturation a bit, add some contrast, remove the vignette, and fix the color balance.  This is still minimal editing though, normally I spend at least 20 minutes messing around with Curves in Photoshop.  Therefore, these photo comparisons show how the images look with just a few minutes of easy editing.  The difference is remarkable!  

No Filter

L-Pro Filter

L-Pro Filter

f/2.8, ISO 400, 5 Minute Exposure - Basic edits in Camera RAW

Pretty impressive right?  Well, the UHC filter did even better!  It really made the North American Nebula stand out from the sky.  Not only is it removing the light pollution, it also helps to remove any other light / colors that might distract from the nebula.  The UHC filter isn't perfect though.  That heavy blue color cast will take some time to remove in Photoshop.  If you don't know how to use Curves, it will likely be a pain to get a good image.  I'd recommend checking out my Deep Space Course if you want to learn more about editing.

No Filter

UHC Filter

UHC Filter

f/2.8, ISO 400, 5 Minutes - Basic Camera RAW Edits

Test 2 - Dark Sky Location

Now that you've seen what's possible in moderate light pollution, let's see how the filters perform under a dark sky.  For this next series of tests I journeyed out to the Alabama Hills, a dark sky region in the Eastern Sierras.  Click here to download the RAW files for a closer look.

First up is the L-Pro filter.  I was a bit surprised by the heavy color cast.  I would've thought the colors would look a bit better.  I suppose the filter is designed to filter out light pollution, so there's not much reason to use it at a dark sky location.

No Filter

L-Pro Filter

L-Pro Filter

f/2.8, ISO 6400, 15 Seconds - RAW Photos, all settings (including White Balance) were identical

Next we have the UHC filter.  As expected, it drastically darkened the scene, and added a unique blue/red color cast.  Now we are starting to see the downsides of using the UHC filter.  Since it blocks so much light, you will have grainier photos.  If you have a star tracker this isn't a problem, just take more photos!  However, if you plan to head out and shoot 15 -30 second exposures without a tracker, you are in for a lot of problems.

No Filter

UHC Filter

UHC Filter

f/2.8, ISO 6400, 15 Seconds - RAW Photos, all settings (including White Balance) were identical

If you'd like to see the edited versions of these photos, click here for L-Pro and click here for UHC.  This should give you a good idea of how the images will look after a bit of editing.  Keep in mind, both filters are blocking light.  Therefore, the filtered photos will have more grain and color mottling.  The best way around this is to capture more light, ideally with a star tracker.  Again, I generally recommend using these filters for deep space astrophotography, not wide-angle nightscapes.  

Test 3 - Lagoon Nebula

This last round was taken in a moderately light-polluted area - Port Angeles, Washington.  This should give everyone a realistic idea of how the filters will work from most shooting locations around the world, where there is some light pollution.  Click here to download the RAW files.

For these tests, I shot from the library parking lot.  Unfortunately, a tree got in the way during the exposure.  However, we can still see how the filter worked, just try to ignore the giant orange blob streaking across the photo.  To be clear, these photos have been edited extensively in Camera RAW and Photoshop in an attempt to bring out the nebula and remove any color cast.  You can see the RAW images here.

No Filter

L-Pro Filter

L-Pro Filter

f/6, ISO 200, 5 Minutes - Extensive Edits in Camera RAW and Photoshop

Next up is the UHC Filter.  I was actually really surprised by how well it performed here!  I could clearly see the Lagoon Nebula much better on the camera's display.  Once I began looking at the photos on the computer, the differences became even more profound.  As you can see, the Lagoon Nebula stands out much better with the UHC filter.  In fact, you can even make out the faint part of the nebula extending out to the left. 

While the nebula stands out much better, it's still not perfect.  As before, there's a noticeable blue color cast to the photo.  After I tried to remove it in Photoshop, using Curves Adjustments, a red color cast became visible in the corners of the photo.  This was a pain to deal with in editing.  It's possible if you use PixInsight, or a different editing workflow, this will not be a problem for you.  Here's a link to my edited version. 

No Filter

UHC Filter

UHC Filter

f/6, ISO 200, 5 minutes - RAW Photos with 'Exposure' increase  (Daylight WB for both)

Final Thoughts

Before we go any further, you can download all of the RAW files here.  This will give you a clear look at how these filters perform.  Alternatively, you can view the L-Pro jpegs here and the UHC jpegs here.

I'll be honest, I thought light pollution filters were a gimmick.  After doing these tests though, I can confidently say that a good light pollution filter can really make a difference!  The nebulae look so much better when using either the L-Pro or UHC filters!  As we saw though, they aren't perfect.  I do not recommend using these light pollution filters for wide angle images, due to the deformed stars.  Also, the filters will introduce a color cast of their own.  The L-Pro is negligible, and can be removed easily.  However, the UHC filter will require extensive knowledge of Curves to remove the blue color cast.  Again, I'd recommend my Deep Space Course which will teach you everything you need to know about that. 

Ultimately, the best thing you can do for your astrophotography is travel to a dark sky location.  Once you are shooting in a dark sky, things will be much easier for you.  Not only will the galaxies and nebulae stand out better, you'll have a much easier time editing your photos.  I usually spend 6 months living on the road each year.  This gives me a unique opportunity to travel to the best dark sky locations in America during each New Moon.  Therefore, I really don't have much need for light pollution filters.  However, I usually spend my winters in Ohio, where we have a fair amount of light pollution.  I'm looking forward to using both the L-Pro and UHC filters when I return home this winter!

I know many people are backyard astrophotographers though, and you will always be dealing with some light pollution.  For you, I'd highly recommend getting a light-pollution filter.  Not only will this make the nebulae stand out better, it will remove some of that annoying yellow/orange color cast from your images. 

Lastly, I do have a special discount available if you plan to purchase these filters!  You'll find the UHC Filter here, and you can purchase the L-Pro Filter here.  Be sure to click the "More Payment Options" button.  From there, you can enter the coupon code: WorkshopStudent to save 5% on your order.  FYI, these are affiliate links.  If you've enjoyed this comparison, you can show your support by ordering and using the coupon code.

Please be aware, these are clip-in filters!  Therefore, your camera may not be supported.  The graphs over on Astronomik should be the same for Optolong, but I always recommend double checking before you buy - Canon Full Frame, Canon Crop Sensor, Nikon Full Frame.  I used a Nikon D750 for these tests and it worked fine with my camera.  Click here to watch how I install my clip-in filters.

That's all I have for you today!  I hope this article gave you a good idea of how the different light pollution filters will work, and if it's worth investing in!  If you have any questions, feel free to leave a comment and I'll try to answer them!