Light Pollution Filters for Astrophotography
One of the most common questions I receive each day is which light pollution filter I recommend for astrophotography in the city. There have never been so many choices available, with each one offering their own advantages and disadvantages. Couple that with the fact that these astrophotography filters can be expensive, and you’ve got some tough decisions to make.
Over the years, I have had the opportunity to use a number of filters designed to improve astrophotography performance. This includes both deep-sky camera and telescope configurations, and with wide-angle camera lenses. In the following post, I will share what I have discovered about each of these filters when in use for astrophotography.
The majority of my imaging happens in my light polluted backyard, but I have used many of these filters under dark rural skies as well. I’ll provide actual image examples of subjects shot using these filters, so you can decide which filter is best for you.
In my mind, the filter that produces consistent usable data from your location is the one you’ll use most.
A Light Pollution Filter for City Skies
Whether I am shooting with a modified or stock DSLR camera, shooting from the city without a filter is not possible. Unless I want a completely white image of the sky in a 30-second exposure, I need to do something about that horrible city glow.
I live in a red-zone on the light-pollution map (Bortle 7), which makes long exposure astrophotography a challenge. The biggest problem when taking images of space from the city is lack of contrast.
Narrowband filters are an excellent option when it comes to capturing DSO’s (deep-sky objects) through bright city skies. Becuase the wavelength of light they record is so narrow, only the good light that contains the gas from your target is recorded on the camera.
A false-color image can be constructed using a set of filters in Ha. OIII and SII and can be incredibly gorgeous.
But what about if you want to capture a full-color portrait of an astronomical target with the real colors?
Thankfully for astrophotographers in my situation, there are a variety of filter options available. Light pollution filters such as the Skytech CLS-CCD are designed to ignore many of the wavelengths of artificial light that surround my backyard sky. The resulting images have a much better overall contrast, and the deep-sky object can often be identified in the image frame after a 1-2 minute exposure.
Deep sky objects captured from my light polluted backyard in the city
Light pollution filters do not come without their shortcomings. The artificial light-blocking power of these UHC (Ultra-high contrast) filters also affects the natural colors found in the night sky. Specifically, the beautiful blues and cool colors of stars. Strong filters can often record images that show a sea of red stars in the image, whether they are a cool red star like Betelgeuse or hot young star like Bellatrix.
Recommended Filters for Astrophotography (Video)
The astrophotography filters featured in this video are the DSLR clip-in models that install into the camera body, and 2″ round mounted filters that thread intro your camera adapter or field flattener. This gives you a chance to see some of the models referenced in this post first-hand.
Coatings, Technology, and Terms
When you read the product descriptions for astronomical filters, you run into a lot of the same benefits and terminology. For example, you may read that a certain filter includes an AR coating. AR stands for “anti-reflective” and you can understand why this aspect is desirable when photographic bright stars.
The IDAS LPS P2 filter lists its coatings as “IGAD” which stands for Ion Gun Assisted Deposition. This technology states that it reduces the spectrum shift created by standard filter coatings.
Not only do I not know how to prove the subtle differences in technology between filters makes, I’m not convinced I would be able to tell the difference in my images either! It can be extremely difficult to test small changes in hardware when the conditions you shoot in are constantly changing.
The bottom line is, each filter will have it’s own coatings and technology, but the effectiveness of that technology is only appreciated when used in your specific situation.
With that being said, there are some key terms you’ll want to know before investing in a new filter for your telescope.
Commonly used astrophotography filter terms
- LPS = Light Pollution Suppression
- UHC = Ultra High Contrast
- AR Coatings = Anti-Reflective Coatings
- IR Blocking = Infrared Blocking
- UV Blocking = Ultraviolet Light Blocking
What’s the difference between UHC and LPS?
A UHC filter (Ultra High Contrast) reduces the effects of artificial lighting and the skyglow in our atmosphere. The images shot through a UHC filter display a noticeably higher amount of contrast and details of many common deep sky objects. In the past, I have appreciated the results obtained using a Baader UHC-S filter on the Veil Nebula.
A CLS filter (City Light Suppression) such as the Astronomik CLS has a wider bandpass than a UHC filter. A CLS filter is suitable for stock (unmodified) DSLR cameras, and work well under moderately light polluted skies. For my modified Canon Rebel T3i, a CLS-CCD filter is a better fit with an infrared wavelength (IR) 700-1100nm cut-off.
You can learn more about Light Pollution Filters with helpful spectral curve graphs on the Optolong website.
Block Light Pollution – Retain Natural Colors
The ideal astrophotography filter for your situation will reduce light pollution, and create images with natural colors and impressive contrast. The amount of light you will need to block of course depends on your imaging conditions. I consider myself to be a good example of someone shooting at the extreme end of the light pollution spectrum, so if I can get around it, you can too.
Baader Moon and Skyglow vs. UHC-S Nebula Filter
The Baader Neodymium Moon and Skyglow filter impressed me in terms of color balance. Using the 2″ round mounted version with my Altair Hypercam 183C resulted in images with a commendable amount of contrast, without destroying the true color of the stars in my image or excessive dimming. In my tests, I found this version to produce better results than the more expensive Baader UHC-S Nebula filter.
Baader lists the UHC-S Nebula filter as having several advantages for visual use, including greater contrast when observing emission nebula. In terms of astrophotography, I found my images retained more natural looking colors when using the Moon and Skyglow variation with my modded DSLR.
The Neodymium glass used in the Moon and Skyglow version of the filter is based on research at Carl Zeiss. The unique characteristics of the Neodymium glass blocks specific wavelengths of light such as streetlights and the moon.
The included IR blocking coatings suppress chromatic aberration when in use for astrophotography. This is an important aspect that is not to be overlooked when shooting with a modified DSLR camera that has had the stock IR filter removed.
Without an IR-Cut filter in front of the sensor, you may have trouble producing accurate, focused stars with your refractor telescope (bloated stars). An external UV/IR cut filter is required when imaging with a full spectrum modified DSLR and an ED refractor.
These attractive specifications are only as useful as the images they produce. The photo below was taken using a dedicated astronomy camera with the Baader Moon and Skyglow Neodymium filter from my light polluted backyard:
The Ring Nebula using the Baader Moon and Skyglow Neodymium Filter | Explore Scientific ED102 CF Refractor – Altair Hypercam 183C
The rich field of colorful stars that surround the Ring Nebula is a great example of this filters ability to retain natural colors. Consider this example image to be a testament to the effectiveness of this filter when used from a heavily light-polluted location.
SkyTech Clip-In DSLR Filters
The SkyTech DSLR clip filters are designed for use with Canon DSLR’s with APS-C sized “crop” sensors. They snap into the body quickly and can easily be swapped out when your imaging conditions or location changes. The clip-in DSLR filters from SkyTech come in 3 variations:
- SkyTech CLS Canon EOS Clip Filter
- SkyTech CLS-CCD Canon EOS Clip Filter
- SkyTech L-Pro Max Canon EOS Clip Filter
These filters are available from Ontario Telescope & Accessories, and my experience using them is thanks to my partnership with OTA. If you have ever dealt with the owner before, you can see why I and many reputable manufacturers stand behind this dealer.
Compatible with Canon APS-C Sized Sensors and EF Lenses
These filters are compatible with Canon EF lenses only, as the rear element of EF-S lenses will come in contact with the filter. Over the years I have slowly built an arsenal of (used) Canon L-Series camera lenses, which are all EF models. I am told that this is not an issue with third party EF-S style lenses, but I have not tested this myself.
SkyTech CLS vs. CLS-CCD Filter
The SkyTech CLS-CCD filter is my goto choice when shooting nebulae with my (Astro-modded) DSLR from home. The included UV/IR filter in the CLS-CCD variation of this filter means that my stars do not bloat, and my full spectrum modified 600D is effective when using my ED refractor.
Although the CLS-CCD filter has a wider bandpass than a traditional UHC filter, I have found that color rendition still leans heavily on the red side, meaning that corrections to color balance must be made during the image processing stage. If you are up to the challenge of correcting the color balance of your image afterward, the SkyTech CLS-CCD is a top performer in a city backyard.
When it comes to capturing emission nebulae in broadband RGB, I reach for the SkyTech CLS-CCD clip-in filter more often than not. To enhance my photos of targets like this, I often include details shot in h-alpha and blend the images together for an HaRGB composite. I used the SkyTech CLS-CCD filter for the RGB data I shot on the Heart Nebula back in October:
The Heart Nebula in HaRGB with color data acquired using the SkyTech CLS-CCD clip-in filter
Have a look at the standard RGB portion of the image, to see the difference adding luminance data in Ha made. The complete details of the equipment used for the photo above is outlined here: A Portable Deep-Sky Astrophotography Kit
SkyTech L-Pro Max Filter
I covered these filters in detail a few months ago when I first received a batch from Ontario Telescope & Accessories. The L-Pro Max version let the most light pass through, resulting in less than pleasing results from the city. The L-Pro Max filter preserves plenty of natural light and was designed for Milky Way photography from dark or rural sky locations.
The L-Pro Max is listed as the best option for “nightscape photography” because of its natural light preservation qualities. Unfortunately, I did not get a chance to test this filter under more accommodating conditions. Just look at what my unfiltered backyard sky looks like in a single long-exposure image!
IDAS LPS-P2 Light Pollution Filter (Canon)
The IDAS LPS P2 filter I use in my Canon DSLR was specifically designed for balanced color transmission by Hutech. The IDAS filters use a unique “Multi-Bandpass Technology” to achieve a natural color balance in broadband images. Compared to the Astronomik CLS clip-in filter, I found the IDAS LPS P2 to indeed do a better job at maintaining a natural color balance.
The IDAS filter can be especially effective when shooting from moderately light polluted skies like the ones I experienced this summer. This was the filter I used while capturing the Andromeda Galaxy on a camping trip in August 2017. The image below utilizes data taken years apart, both with the IDAS LPS P2 filter installed in the camera.
2″ Round Mounted vs. Clip-in DSLR filters
When it came to using the Altair Hypercam 183C (or any other non-DSLR camera for that matter), I had no choice but to invest in some 2″ round mounted filters I could thread onto my field flattener. Although they are more costly, I prefer a 2″ filter over the 1.25″ because I like the option of having the widest coverage possible. Anyone shooting with a full-frame DSLR or CCD camera with a big sensor likely will too.
The 2″ round mounted filters are more versatile as they can be used with both DSLR’s and dedicated CMOS astronomy cameras and CCD’s. The clip-in astrophotography filters I have tested were all built for Canon DSLR’s with APS-C sized sensors. The SkyTech and Astronomik filters all actually clip into the camera body, while the IDAS LPS P2 filter needs a specific mounting ring and tiny screw to hold it in place.
The biggest benefit the clip-in filters have is the ability to be used with a camera lens attached. The option of shooting with camera lenses of various focal lengths is an attractive feature. The Astronomik 12nm clip-in Ha filter creates photo opportunities such as ultra wide angle portraits in h-alpha like the photo below.
Wide Angle Cygnus Constellation using a DSLR + Camera Lens with Clip-in Ha Filter
If you primarily shoot wide angle astrophotography shots with a camera lens on a tracking mount, then a clip-in filter is your best option. Deep sky imagers using a DSLR and telescope will find clip-in filters handy when manually swapping out the filters based on the current conditions. For example, during the week surrounding the full moon, I like to capture wide-field images using a 12nm Ha filter in the William Optics Z61 APO.
Lastly, clip-in filters protect your camera sensor and mirror from dust and debris. Much like an external UV lens threaded to an expensive camera lens, it’s nice to have a protective layer over the important bits. Any dust that accumulates on the surface of the filter can easily be blown off by using a simple photography blower.
Shooting Narrowband with a Color Camera
If you read my post about shooting narrowband images with a color camera, you’ll remember that the Bayer matrix design of a color CMOS camera sensor is not as effective at collecting narrowband light as a Mono sensor is.
A color camera like the Hypercam 183C trims the light gathering ability at these wavelengths down to 1/4, meaning that less detail is recorded in each frame. It’s not an ideal situation, but these filters can still benefit the backyard astrophotographer on a budget, who wants to try narrowband astrophotography with an existing color camera.
The Veil Nebula – Captured using 2″ Narrowband Filters
One of the most interesting and powerful filters I’ve ever used for astrophotography is the STC Astro Duo-Narrowband Filter. This filter isolates two wavelengths of light, Ha, and OIII. When used with a color camera like the ZWO ASI294MC-Pro, it does an incredible job at capturing the two prominent gases at the same time. The image below of the Crescent Nebula was shot using this filter.
The Crescent Nebula – Captured using the STC Astro Duo-Narrowband Filter
Finding the Right Color Balance
I am a big fan of images with the real, natural colors of a nebula or galaxy. Emission nebulae with deep reds can often be a little easier to process from light polluted skies. The rich vibrant color of the hydrogen gas shows up with impressive contrast against a city sky on targets like the Omega Nebula. Because of this, it can be easy to ignore what has happened to the cool blues of the stars and background sky of the image.
Bright Emission Nebulae show up well, but even blue stars can appear red
A modified DSLR camera will record more red details in the hydrogen gas in emission nebulae. The camera used for the photo above was a full-spectrum modified Canon EOS Rebel T3i (600D) with a SkyTech CLS-CCD clip-in filter attached. A stock DSLR camera with the original IR Cut filter intact would be better suited for the SkyTech CLS clip-filter.
Other examples of emission nebulae that benefit from the strong light pollution filter like the CLS-CCD include the Eagle Nebula, California Nebula, and the North America Nebula. These deep sky targets are all primarily red hydrogen gas that shows up in long exposure images with a welcome amount of detail and contrast.
Reflection nebulae, especially dim ones like the Witch Head Nebula, are much harder to capture from a light polluted area. A modified camera has no advantage when it comes to capturing these dusty DSO’s with colors that reside in the blue area of the spectrum. I would suggest using a less-harsh light pollution filter on these types of objects, as retaining the natural cool colors of the target is advised.
Fixing Star Color in Post Processing (Photoshop)
With the RAW broadband color data captured, your images may display the signs of an astrophoto with a strong broadband light pollution filter. There are many ways to tackle this issue during the image-processing stage of your photo, but my preferred method is to make all adjustments to the final stacked image.
There are many chances to adjust color balance in your image along the image processing stages. The point you choose to make adjustments is up to you, and often experimenting with your captured data can result in a more effective workflow.
Ways to adjust color balance in your image:
- Adjust the RGB sliders in DeepSkyStacker in stacked image
- Adjust your RAW images using Adobe Camera Raw before stacking
- Adjust your RAW images using RGB levels in Photoshop before stacking
- Adjust your final stacked .TIF using RGB levels in Photoshop
- Adjust your final stacked .TIF using ACR filter in Photoshop
- Adjust your final stacked .TIF using Select / Color Range in Photoshop
As you can see, there are countless ways to handle this task, each with slightly different results. The methods above only reference stacking and processing images using Adobe Photoshop, as that is still the way I prefer to process my astrophotos.
List of astrophotography filters I have used:
Broadband LP 2″ Round Mounted
Baader Moon & Skyglow
Explore Scientific UHC
Broadband LP DSLR Clip-in
IDAS LPS P2
SkyTech L-Pro Max
Narrowband 2″ Round Mounted
Astronomik 12nm Ha
Astronimik 12nm OIII
Astronomik 12nm SII
Narrowband DSLR Clip-in
Astronomik 12nm Ha