Optolong L-eNhance Narrowband Filter
The Optolong L-eNhance is a not-so-secret weapon in the war against light pollution. The enemy. The result of urbanization and un-optimal street lights. Every night when a lot of us try to view or take images of the night sky, we need to fight the pollution.
To that end narrowband filters are a great addition to the arsenal. The L-eNhance falls in more specifically as a multi-bandpass filter.
In this article I’ll be sharing my thoughts and feelings about this filter after having used it for several months. It was loaned to me from First Light Optics for review, and that’s exactly what I’m about to do.
So as mentioned above, this is what’s known as a Multi-Bandpass filter. You may be familiar with Hydrogen-Alpha filters and in fact my daily driver Ha filter is the 7nm from Baader. So what is the deal with the multi-bandpass?
The L-eNhance lets through 3 wavelengths of light. These are Hydrogen-Alpha, Hydrogen-Beta and Oxygen-III. The main ones being the Ha and OIII really. Hydrogen-Beta is often forgotten about and really can only flex its muscles on several targets as David Knisely lists on CloudyNights.
- Hydrogen Alpha is more red at 656nm
- Oxygen-III is more green/cyan at 500nm
- Hydrogen Beta is more blue at 480nm
Useful For Colour Cameras
It’s down to the bayer matrix, or Colour Filter Array (CFA). I want you to imaging your camera’s pixels. 4 of them in a block. With a standard RGGB pattern, you have a red block, two green blocks and a blue block.
I used this filter with the ZWO ASI 533mc Pro colour camera, which has a bayer pattern of RGGB.
With conventional filters like a dedicated Hydrogen-Alpha filter, it’s widely agreed that only the red pixel allows light through. But with the L-eNhance, all four blocks allows light through. This greatly increases resolution, detail and signal to noise ratio.
Also the fact that all the colours are reaching the sensor means more “natural” looking photos. I think the star colours in this Bubble Nebula look quite nice considering it’s a narrowband filter.
Finding the bandpasses for the L-eNhance was weirdly difficult. I couldn’t see it listed on any retailer websites I looked at. It wasn’t on Optolong’s website. I asked people in the trade to no avail. I even asked Optolong to no avail.
In the end I settled on making a little scale using the transmission chart to try and get a rough idea.
- Hydrogen Alpha is about 12nm
- Oxygen-III & Hydrogen-Beta is about 30nm
As you can see in the picture – my scale is a bit rough. I fully admit this. However it seemed to work. It seems accurate and I can believe it. I have since seen it cited as 10nm Ha and 24nm Hb/OIII. Which also seems close to the truth.
The transmission chart states “over 90%” also. But once again, using the scale on the transmission chart, I measured it about 94% Hydrogen Alpha and 95% OIII & Hb. I don’t have a way to measure this properly (yet?). But just from using it, I definitely feel its a sensitive filter that lets a lot of light through.
There is one type of target the Optolong L-eNhance strives on: emission nebula.
Above is an image of NGC2244 – the Rosette Nebula that was captured with the ZWO ASI 533mc Pro and the Optolong L-eNhance. This was then stacked in Deep Sky Stacker and actually had very little processing further to that. Also it has no calibration frames. Just to give you an idea what sort of results you could expect.
Emission nebula are this filter’s bread and butter. It excels at them. If you turned this filter towards a star cluster, a galaxy or a reflection nebula, you’ll be left disappointed. Again down to the light that is emitted. Galaxies, for example, have very little Ha and practically no O-III or Hb.
So picking your targets is important with this filter. Just like with any narrowband filter. Take some time in a planetarium program like Stellarium and find emission nebulae that works well for your set up. You’ll then be in the best position to use the filter.
Like everything, the L-eNhance can’t be perfect (though it’s painfully close).
The main drawback is one that plagues all multi-bandpass filters. Moon glow. As is often the case with deep sky astrophotography – the Moon is not your friend.
When the Moon was about 60% full I begun to notice significant glow appearing in my images. This only really manifested in the Oxygen-III. This is because O-III isn’t as good as Hydrogen Alpha at resisting Moonlight.
Let’s have a look at some images.
Here we have the Wizard Nebula. Shot from my garden with the L-eNhance. This was during a 68% illuminated Moon (shot 3rd February 2020). As I mentioned in the top right corner, particularly on the O-III image, there’s significant Moon glow entering. It wasn’t just restricted to the Wizard Nebula either. I split this image using AstroPixel Processor – more on that later.
Most of the photos you’ll see here have a gradient in them from the Moon. It’s a pest, but you can always split the Hydrogen Alpha and the O-III/Hb apart (as seen above) to give you greater control.
Using this approach, it’s possible to use the L-eNhance as a dedicated Hydrogen Alpha filter. Though as I discovered in the Astronomik 12nm Hydrogen Alpha review, 10-12nm isn’t narrow enough to resist a very bright Moon either.
No. For major Moon light, I’ll be differing you over to a dedicated Hydrogen Alpha filter. The narrower the better for resisting Moonlight and Light Pollution.
The Optolong L-eNhance – at the time of this review – is only available in 1.25″ and 2″ versions. This is absolutely fine for a lot of us. It fits onto many eyepieces, it fits on many cameras. The 2″ threading is extremely common in many coma correctors, field flatteners and nose pieces. So it just screws straight on and you’re away. It’ll even fit into 2″ filter wheels if you’re that way inclined.
However this is a downside for those who enjoy using their DSLR cameras. This filter isn’t too harsh for a DSLR and I am confident to say it’s possible. But if you enjoy using your superfast widefield lenses like the Rokinon 135mm f/2 for example, then you’re being left out. With no clip in version available for you, there’s no way to attach this. Short of taping it to the front of the glass, but that doesn’t seem like a good idea to me.
I would really enjoy seeing Optolong make a clip version for the DSLR shooters, so that they can also enjoy this filter.
L-eNhance In Use
I used the L-eNhance on a variety of emission nebula. Each came out with wonderful colours, clarity and contrast. As well as the Moon glow as mentioned above. But I still maintain it was a fun experience. However at first I found that my images were very blue straight out of the camera. This is sorted in stacking and calibration though. It blocks a good portion of light pollution and is versatile. As you’re using all colours, you end up with a colour image that is usable. The blue cast was only due to the Debayer preview in Astrophotography Tool and didn’t cause any problems processing the data.
So really, if you see this colour cast in your own images using the filter. It’s no cause for concern.
You are also able to split the Hydrogen-Alpha and Oxygen-III channels into their separate grayscale sections using a program such as AstroPixel Processor, which then gives you more versatility in making a bi-colour image, for example.
On the right you can see my Wizard Nebula. This is a somewhat synthetic Hubble pallet style I was able to create by splitting the data, and then recombining it in a way I wanted.
My Favourite Thing?
If there’s one thing I had to say is my favourite about the Optolong L-eNhance it’ll be this:
I noticed NO halos! None, not even on a 10 minute exposure there on IC63 with the bright star Navi. Zooming in with a light bit of post-processing still doesn’t expose any halos hidden in there. I am absolutely thrilled with this. As halos for me are a complete deal breaker.
Of course I wasn’t able to test every camera and telescope combination going – but with the ASI 533mc Pro and my Sky-Watcher Evostar 80ED, I didn’t get any. Even on this 10 minute exposure of the Ghost of Cassiopeia – IC63. With a bit of editing in Photoshop, mainly to adjust the levels and curves. I still wasn’t able to see any halos.
This makes me a very happy astrophotographer.
So in summary:
This filter performed really well, through my Bortle 5/6 skies it performed admirably, I regularly used 5 minute sub exposures with my ZWO ASI533mc Pro. But it’s entirely possible to use this filter with a DSLR as well as the narrowest is 12nm on the Ha channel, which is gentle enough with a DSLR.
The lack of halos, absolutely glorious. The speed of being able to make a narrowband bi-colour image also, wonderful.
I’d love to see a DSLR Clip variant though for imagers who enjoy using their high quality lenses, and I’d like them to disclose what the bandpasses are.
The filter, at time of review, has an RRP of £145, though currently seems to be on offer for £129 for the 1.25”, £185 – or £166 on offer – for the 2” from First Light Optics.
Use this filter correctly, away from major Moonglow, and you’ll get some solid results. I’m not here to tell you how to spend your money, but I really feel like adding the Optolong L-eNhance to your colour camera arsenal – or using it as a super luminance for your mono images – would make a fine addition to your kit.