Optolong L-eNhance and L-eXtreme Filters: Should I Buy Them? Review and Analysis of Results
Let’s take a look at the details of two filters from Optolong – the L-eNhance and L-eXtreme – and compare them through tests in my heavily light polluted yard. Being that these filters are geared toward amateur astrophotographers, all tests have been performed on relatively basic (affordable) setups, and are aimed at a goal of producing aesthetically pleasing images. NOTE: Optolong has also recently released another update to this filter series, the L-Ultimate, but at the time of this post inventory was not available, so tests and review of that filter will follow when they become available.
THE PROBLEM
Astrophotography from within a city / heavily light-polluted area is incredibly difficult at best, if not impossible. Then there's the Moon - as beautiful as it may be (and responsible for tides, influencing life on Earth, etc, etc) – is often a rather unwelcome guest 3 out of 4 weeks each month.
THE SOLUTION
Fortunately, technology has allowed us to nearly eliminate the negative influences of these factors through the use of narrowband filters, specifically, the Optolong L-Ehnance and L-eXtreme tri-band and dual-band filters. Both of these filters isolate highly targeted wavelengths of visible light, effectively blocking unwanted artificial light and moonlight, allowing only the light from emission nebulae to pass through (specifically, Hydrogen-alpha, Hydrogen-beta, and Oxygen-III in the case of the L-eNhance, and Ha + OIII with the L-eXtreme). Both the L-eNhance and L-eXtreme filters are excellent choices for color CMOS or CCDs, and DSLR / mirrorless cameras. While these aren't exactly cheap filters, their relatively low cost for the quality (compared to many other filters out there) make them an easy choice for hobbyists to be able to create beautiful images of deep space objects (DSOs) even under a full moon and in heavily light polluted areas. At the time of writing this article the L-eNhance retails for around $229 USD and the L-eXtreme around $309 USD for the 2" versions.
THE FILTERS
The L-eNhance and L-eXtreme filters are very similar to one another in the sense that they both isolate and allow very specific wavelengths (bandpasses) of light to pass through the filters and on to your camera sensor. The main difference is that the L-eNhance is a tri-band pass filter targeting H-alpha, H-beta, and OIII whereas the L-eXtreme is dual-band pass filter allowing only H-alpha and OIII to pass through. In other words, both filters do an excellent job at cutting through extreme light pollution resulting in a higher contrast between the light (signal) coming from the nebula and the background sky. In simplest terms the L-eNhance may be a better solution for slower optics (higher f/ratio) by allowing more light transmission than the L-eXtreme that performs best with fast optics. The L-eXtreme may also be the better choice for those working in heavily light polluted areas and/or for use when the moon is out. These filters, when paired with a color camera, can help amateurs create stunning images without the complexity and expense of monochrome cameras, multiple color filters, filter wheels, and various other pieces of equipment. Do note, though, that these filters will not work well for galaxies and other non-emission nebulae, but fortunately, emission nebulae are abundant in the universe and offer astrophotographers many imaging options to keep them entertained for years.
PRODUCT DETAILS
L-eNnhance (Tri-Band Filter, Ha + Hb + OIII)
Spectrum and Characteristics from Optolong:
Substrate: B270
Thickness: 1.85mm
Tpeak: T>90%
Blocking range: 300-1000nm
Blocking depth: light pollution line blocking >99%
Surface quality: 60/40
Transmitted Wavefront RMS: λ/4
Parallelism: 30s
L-eXtreme (Dual-Band Filter, Ha + OIII)
Spectrum and Characteristics from Optolong:
Substrate: B270
Thickness: 1.85mm
Tpeak: T>90%
Blocking range: 300-1000nm
Blocking depth: light pollution line blocking >99%
Surface quality: 60/40
Transmitted Wavefront RMS: λ/4
Parallelism: 30s
THE TESTS and EQUIPMENT
To test these filters, I've fitted them to a simple setup consisting of the William Optics Z61II scope (with a focal length of approximately 370mm) and adjustable field flattener, paired with a ZWO ASI294MC Pro cooled color camera with 4/3" sensor, and dropped the filters in via the ZWO 2 inch filter drawer. All of this sits on the Sky Watcher HEQ-5 Pro mount, driven by the ZWO ASIAIR Plus controller, guided by the Orion 50mm guide scope + ZWO ASI 120mm guide camera, and powered by a Renogy Phoenix 300 power bank. I often use this setup both at home and when traveling since it is lightweight, quick to set up, and produces decent images (despite some of the never-ending issues with optical tilt, but we'll save that for another post). To make this relatable for many backyard astrophotographers, I’ve used a fairly common setup similar to what many people start out with.
While the final images would be significantly improved by integrating more hours of data, I wanted to focus more on what you can expect out of these filters by shooting a wide view of relatively bright objects (such as the North America Nebula and NGC 7635 Bubble Nebula), under heavily light polluted skies (Bortle Class 6 - https://www.lightpollutionmap.info/), focusing on a comparison between the filters and the results of different exposure times for each.
Light Pollution Map: Pink and White areas indicate heavy light pollution, while yellow, green, and blue, and gray, respectively, indicate less light pollution.
THE RESULTS
(COMPARATIVE AND FINAL IMAGES)
At 600 second exposure times both of these filters create very clean, well-structured images, but it’s fairly noticeable that the L-eNnhance offers a bit more contrast in the gasses and showing a bit more color variation. The L-eXtreme further reduces the light pollution offering higher contrast against the background and through the image overall. One very notable difference, however, is in the handling of the bright stars: while both allow a bit of haloing around the stars, the L-eNhance has a more significant haloing effect.
Even at 300 second exposures, both filters offer similar results in terms of the contrast as noted above, and still have a good signal to noise ratio resulting in excellent image quality, easily cutting through the strong light pollution.
At shorter exposure times the noise can get pretty strong below 180-300s. The contrast also drops off pretty quickly below 120s as you can see in the images above (click the thumbnails to expand).
At shorter exposure times using the L-eXtreme filter, you can see some slight improvement in the overall contrast, revealing more of the nebula, even at exposures as short as 30s. For those using mounts that may not offer as consistent of guiding at longer focal lengths (such as the iOptron Sky Guider Pro), this is welcomed news. You will need to take and stack far more exposures to achieve bright, clean, noise-free images, but it’s all very possible. Trying to do that without these filters in the city would be near impossible.
As a final test, I pointed my scope at the Bubble Nebula - another gaseous region, but significantly smaller / further away, and much harder to pull out the subtle details on a wide view. With only a little over 1 hour of total integration time, both filters proved to be excellent options and produce similar results. Personally, I prefer the L-eXtreme that gives a richer contrast, but you may appreciate the inclusion of some of the more subtle, H-beta data as seen in the L-eNhance image (most obviously to the left of the “bubble”).
FINAL THOUGHTS
Having worked primarily with a monochrome cooled camera and RGB, Ha, SII, and OIII + L filters for the majority of my deep space imaging (aside from my Milky Way and other astrolandscape projects) I had always found color (or OSC, One-Shot Color) imaging to be far too limiting. The images were full of noise, and I was only able to shoot very short exposures unless I traveled to darker skies. However, the introduction of these highly refined narrow bandpass filters has opened up an entirely new world of deep space imagining from right here in the city. Sure, I still prefer working with a monochrome cooled camera and filters due to the enhanced image quality and granular control of the editing process, but when I want faster, simpler imaging sessions either of the L-eNhance or L-eXtreme filters make that possible and enjoyable.