Optolong have released a new broadband light pollution filter, the L-Quad Enhance. Can it help with bright city skies? Let’s put it to the test…
Filters, filters, everywhere…
We astrophotographers sure love our filters. OSC imagers, like me, benefit hugely from a particular type called dualband filters. These allow us to take striking photos of narrowband targets — that is, objects in space that give out most of their light in specific wavelengths, most commonly Hydrogen-alpha. They also neatly bypass a lot of light pollution that plagues our city skies. I recommend an Optolong L-Ultimate filter for narrowband targets, and I also enjoy mixing in Askar D2 data. But what about broadband targets? Stars, galaxies, reflection nebulae… these give out lots of light across the spectrum, so aren’t suitable for use with the type of filters mentioned above. Broadband light pollution filters to the rescue! In theory at least…
There are lots of broadband light pollution filters available. They’re designed to block light from artificial sources like street lights, while allowing the photons that we want — i.e. from that galaxy you’re trying to image — through to your camera sensor. Lots of city astrophotographers use a broadband light pollution filter as standard, but I don’t necessarily recommend this. Back in June 2021 I ran a light pollution filter shootout, where I tested three of the most popular broadband filters available at the time. Read the whole article for details, but the executive summary is that none of those filters actually helped more than they hindered. Since running that test, I’ve simply relied on long integration times to brute-force my way through light pollution when imaging broadband targets. And you know what? It’s worked just fine. Check out my photos of The Pleiades and The Andromeda Galaxy for examples. They were both taken from a Bortle 8 city centre, using an OSC camera, with no filter. So, it’s fair to say I have a healthy scepticism of light pollution filters.
Optolong L-Quad Enhance
In late 2023, Optolong announced a brand new broadband light pollution filter: the L-Quad Enhance. Disclaimer: Optolong very kindly sent me this filter for this review. They’re also allowing me to keep it, free of charge. This generosity hasn’t impacted my review, however!
Here’s a graph of the filter’s bandpasses, if you’re into that kind of thing:
The marketing blurb is full of claims about enhancing contrast and maximising colour balance. I’m more interested in real-world tests though, and I want to answer one question specifically: does this filter offer an improvement for me when shooting broadband targets, or am I better off with my current approach of not using any filter at all? Optolong do actually say that the L-Quad Enhance isn’t recommended for Bortle 8 or 9 skies (the worst levels of light pollution). It was brave of Optolong to encourage this review considering my skies are Bortle 8…
To test the Optolong L-Quad Enhance I used my beloved Askar 130PHQ telescope, and ZWO ASI2600 MC pro camera. The camera actually broke mid-way through the review, which delayed things by a few months. See ZWO RMA: Sending a Broken Camera from the UK to China for Repairs to read more about that saga!
I’ve tried to be as fair as possible, although without two identical imaging rigs side-by-side, this is rather tricky! I chose two targets, then obtained an equal amount of good quality data using the L-Quad Enhance, and no filter. I then integrated these and then used PixInsight to perform the same simple processing steps on both, in order to produce images that could be compared side-by-side.
Target 1: The Rotten Fish Nebula
My first target was The Rotten Fish Nebula, which is very challenging from light polluted skies. I integrated 10 hours of no filter data, and 10 hours of L-Quad Enhance (L-QEF). Use the sliders in the images below to compare.
The L-Quad Enhance data looks better to my eye. The colours are more pleasingly neutral, and there’s a bit more contrast. It’s not hugely striking, but noticeable (especially when viewing the originals). Let’s take a closer look…
This close-in crop nicely shows the better colours of the L-Quad Enhance. It’s also helped bring out some detail in the faint background galaxy.
More good news for the L-Quad Enhance in this crop too. The brightest star looks better defined, and two small glowing red clouds near the top (pockets of hydrogen gas perhaps) are hard to see with no filter, but are quite prominent with the L-Quad Enhance.
I ran the two integrated stacks through PixInsight’s SubframeSelector tool in order to get some statistics to compare. The L-Quad Enhance wins in each of the three key categories I chose. The image has more stars, tighter stars, and better signal-to-noise ratio.
| 10 hours of integrated data | No filter | L-Quad Enhance |
| Stars (higher is better) | 5322 | 7263 |
| FWHM (lower is better) | 2.41 | 2.30 |
| SNR (higher is better) | 2.54 | 4.39 |
You can read more about my Rotten Fish Nebula photo on its dedicated page here.
Target 2: M81/M82
So far it’s looking good for the L-Quad Enhance. Will it become an essential part of my toolkit? Let’s proceed with target 2, the galaxies M81 and M82. 4.5 hours of data went into each of the integrated stacks.
The L-Quad Enhance version looks visibly better when viewed full resolution on my monitor. Hopefully it shows in the slider view above. It seemed to reduce gradients in the unedited stacks too, and the colours just look nicer, which has surprised me given other filters I’ve tested previously created nasty colour casts that were difficult to process out. Maybe Optolong’s claims are accurate. Let’s zoom in…
The colours look better in the L-Quad Enhance, and the filter also seems to have brought out the areas of red hydrogen gas in the galaxy’s spiral arms, particularly at the top. This is a pleasant surprise. Will we see the same in neighbouring M82?
The red hydrogen areas are definitely more prominent when using the L-Quad Enhance. Some of that is down to the colour, but I’m looking at this closely and think there’s more structure visible when using the filter.
Running SubframeSelector on the two freshly-integrated stacks shows the L-Quad Enhance in the lead across the board again.
| 4.5 hours of integrated data | No filter | L-QEF |
| Stars (higher is better) | 1521 | 1691 |
| FWHM (lower is better) | 2.51 | 2.26 |
| SNR (higher is better) | 2.14 | 2.41 |
I actually collected 6 hours and 24 minutes of decent quality data in total using the L-Quad Enhance. I went ahead and processed this, and I think the result is perfectly fine, especially considering the short integration time. (I normally aim for 20+ hours). For more details, and to see my previous M81 M82 attempts, you can visit a dedicated page here.
What’s the verdict?
I admit it: I’m surprised by the result. I thought that I’d see a marginal benefit at most, but when processing the test images, it was just easier with the L-Quad Enhance data. I’ll be using it as standard for my broadband imaging. Note though that long integration times are still needed to get a decent signal-to-noise ratio! You can read more about that in my article How To Get Long Integration Times.
But! Before you rush out and buy one, a word of caution. The effectiveness of broadband light pollution filters like this one is very dependent on how they cope with specific sources of skyglow. The lights around me are not the same as the lights around you. (Unless you live in Bristol city centre!) So I can’t say for sure that this filter will benefit you. What I can say is that it’s the best broadband light pollution filter I’ve tested, and the only one I’ve found worth using. If you think you’d benefit too, by all means buy one but give it a good test before you commit to using it long-term, just to make sure you’re really getting a benefit.
THE END.
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Thanks for this helpful review. I really appreciated the time you put into both qualitative (sliders) and quantitative (fwhm, snr, etc) comparison results. I agree with your conclusion and appreciated the caveat you included wrt the local lighting conditions being key (and likely variable…ymmv). I’d be curious how these fare as LED outdoor lighting continues to ramp up.
For sure, LED lighting is a challenge! There’s quite a bit around me, but this filter still performed well.
Thank you for this well-done review. I just have started to compare new filters for my color cameras, ASI294MC Pro and ASI2600MC Duo. At this moment, I’m using L-eNhance filter in an area bortle scale 6 for almost all deep sky objects so far. It happened to me last week. I tried to capture M101 which is very good target from my apartment house, but it was not good result as you could imagine easily. I can say it has very low contrast compared with M42 or Rosetta nebula that I have catptured many times. Then, I have started to consider getting a new filter for galaxies from bortle scale 6 area. “Optolong L-QUAD ENHANCE” is a possibility to get a good result for my case. However, I don’t have any of good review of it here in Japan, so I started to read overseas reviews. I knew you have enough experiece to use L-eXtreme, so could you please give me some points how would you choose L-eXtreme and L-QUAD ENHANCE? I mean if you would use L-QUAD ENHANCE or not for M24 and Rosetta nebula. Do you think L-QUAD ENHANCE could over take the position of my L-eNhance filter?
Hi Kenny, thanks for your comment. Your question is a good one and I get asked it a lot, so I think I’ll write a full article giving a detailed answer. Look out for that next week!
Thank you, Lee! I’m looking forward to your new article. I have ordered L-QUAD ENHANCE though.
Excellent testing. I know there is a huge debate about these filters now that more lighting is less narrowband (Sodium/Mercury vapor). Most testing is unscientific… like “I got this filter and look at my nice picture, the filter is great!” No controlled experimentation, just anecdotes.
I’m glad you are comparing the SNR because that is really the primary metric of merit in astrophotography. I have a lot of globe style Sodium Vapor lights in my neighborhood and around the area so I’m thinking of this filter because I think I may be one of those people still in the “narrowband light pollution” camp. So despite me being a bit skeptical for purely broadband targets, I’m going to give this a shot and do a very similar experiment: Swapping between no filter and filter in the same night, same conditions, same target, same integration time…. and compare the SNR.
Thanks for the comment, I’d be interested to hear the results of your experiment!
Well, unfortunately when I tested the L-Quad it created difficult to remove colorful gradients. I think it may be related to the fact that I have to mount it on the front of the Newtonian coma corrector, and it intrudes just a tad into the light path and this may be causing some reflections of stray light. It’s not an issue with dual narrowband filters but it is with this.
So I can’t do robust testing and this filter will not work for me so I have to return it without trying to really test it well. With one night of imaging on M101 compared with the previous night with similar sky conditions, and after correcting for the gradients, I saw no advantage. If fact the SNR and subtle nebulosity was a bit worse but this is comparing two nights which isn’t a good controlled experiment. But it was enough to let me know that it’s likely if were advantages to the filter, they would be modest, but I can’t say for sure unfortunately.
Thanks for reporting back!