I recently bought an Optolong L-Enhance Filter (see photo below). Note that you can’t look through the filter as it has a mirror-like finish, so you only see your reflection.

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This is a tri-band filter which effectively isolates the H-alpha (656.3 nm), H-beta (486.1 nm) and Oxygen III (500.7 nm) emission lines (with a maximum transmission at these wavelengths of 90%). Since it blocks out other wavelengths it also acts as light pollution filter, and can be used even when there is a full moon. The downside of using the filter is that your sub-exposures need to be around three times longer than if you simple use an IR/UV filter.

I’ve been trying this filter out recently to see how it performs.

The first image below is the Heart Nebula in Cassiopeia (IC1805). I took 30 images, at 3 mins each, using a ZWO 071MC Pro cooled camera (with gain =90 and sensor cooled to 0 C) attached to a William Optics 81GT IV refractor, with a 1x flattener, and the L-Enhance sensor. Stacking was performed in DSS, and further image processing was done using Nebulosity 4. Note that there was a full moon low down in the sky when I took this image. This was the first time I have tried the Heart Nebula and I was fairly pleased with the result. Next time I’ll have to try it with a 0.8x flattener/reducer to try to get a wider field of view. Note that I only used dark frames for calibration, no flat frames. I was autoguiding using ASI Air Pro, and typical RMS guiding figures were in the range 0.5-0.8″ (with no dithering). Total exposure time for image below was around 1.5 hours.

A full size bitmap image is available at: https://drive.google.com/file/d/1oYUdCnmc5mzc-lgDzWmKfDONmJ6GyDyR/view?usp=sharing

Another target I have taken images of recently (using the L-Enhance and also just a UV/IR filter) is the North American Nebula (NGC 7000). The image below is one where I used the L-Enhance filter. For this image I used the same camera and set up as above (gain = 90, sensor cooled to 0 C) but in this instance used 2 minute exposures. Total exposure time for image below was around 2 hours.

A full size bitmap image is available at: https://drive.google.com/file/d/10ESLVi_0zF97f9FqJeLcw0ScuzCSjdPz/view?usp=sharing

As a comparison, below is an earlier image of NGC7000 when I just used a UV/IR filter. Same camera and telescope set-up as above, but in this case I used 1 minute exposures and a gain of 240. Total imaging time for image below was around 40 mins.

A photograph of my set-up from the other night when I took the photo of the Heart Nebula is shown below:

In summary, the L-Enhance filter looks like a great buy for using on emission nebulae – I’m looking forward to trying it out on the Orion and Horsehead nebula later in the year. Also, I like the fact you can use this on targets even if there is light pollution or a full moon up. I bought the 2″ inch version of the filter, which wasn’t cheap (at around £170, including postage, from First Light Optics) but it looks like it is worth the money.