This year Santa brought me a new Optolong L-Extreme narrow-band filter to use with my also-new ZWO ASI533MC Pro one-shot-color camera. I recently had the chance to try out this new combination for the first time, choosing the Rosette Nebula in the constellation Monoceros as my first target.

The Optolong L-Extreme filter is sensitive to H-alpha (red, 656.3 nm) and O-III (green, 495.9nm) emission lines commonly found in nebular objects in space, while blocking practically all other colors. The bandpasses for each of these two emission lines is 7nm. Many astro imagers choose this filter as an effective blocker of light pollution as well as moonlight, and in that role it is very effective. Light pollution is not that much of an issue where I live (I have Bortle 4 skies) but moonlight can be an issue for anyone. Other imagers have reported success imaging deep sky objects with this filter even under full-moon conditions!

The bandpass for the L-Extreme filter

One of the reasons I chose this filter is that it reduces the light from stars in images while allowing nebulae to be captured. I had previously tried to image several nebula (the Heart Nebula, for example) and the sheer number of stars in the resulting image overwhelmed the nebula I was trying to capture. Under those conditions, even star reduction techniques in post-processing are only marginally effective. The narrow-band filter is much more effective for capturing the nebulosity without an overwhelming presence of stars.

The image above consists of 20 5-minute exposures using my ASI533 MC Pro camera with the L-Extreme filter, with 20 dark frames for calibration. The image was taken through my Sky-Watcher Esprit 100mm f/5.5 refractor on an iOptron GEM45 mount. I used N.I.N.A for managing the imaging process, plus PHD2 for guiding. Processing was done using Deep Sky Stacker and Affinity Photo, plus the Astro Flat Pro and Denoise AI plug-ins.

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