Narrowband filters have become amateur astronomy friends a long time ago. They are useful both for visual observations and for astrophotography. These filters work as every other filter – they do not increase signal, they cut out some of it, and narrowband filters specifically cut out most of the spectrum and pass only a narrow fragment around some common or interesting light wavelengths. Narrowband filters for a long time were used mostly with monochromatic cameras – you are all familiar with magnificent views of nebulae in the lines of hydrogen, oxygen, or sulfur. Famous Hubble Space Telescope images (including Pillars of Creation) were captured using combined exposures made with these filters. But things have changed and now owners of color cameras can benefit from using narrowband filters.
First attempts were used with regular single-band filters that pass either hydrogen or oxygen emission lines. The problem with this approach was, that once we put a hydrogen alpha filter that passes only a 656nm red emission line in front of the color sensor, then only 1/4 of the pixel will collect the light. G and B pixels will not record any data, because red light is filtered out by the sensor color Bayer matrix. The situation is much better for oxygen narrowband filters because it passes the light around 500nm which is green-blue. And then 3/4 of color camera pixels will record this light either partially or fully – depending on the actual Bayer matrix implementation.
But why not pass two or more narrowband emission lines at the same time, so all RGGB color camera pixels can record light? Similar approaches were already done in some UHC, CLS, or advanced light pollution filters – but these filters were not narrowband. The emission bands were quite wide.
And soon many manufacturers started to design and produce the first dual-band filters (sometimes called also duoband). The idea was simple – the filter needs to pass narrow bands around both Ha and Oiii emission lines. Then all pixels in color cameras will be happy with collecting light. The evolution of this was designing three or more band filters. Additionally, hydrogen beta, sulfur, and/or nitrogen emission bands were added to the stack.
These filters opened some new opportunities for astrophotography amateurs who capture data with color cameras (DSLR as well). First, it is now possible to achieve decent results under light-polluted sky, but also when the Moon phase is large. The contrast of the emission nebula against the background is increased. The level of this increase is related to the width of the band the filter passes. Simple filters pass a 10-30nm wide band and these can be recognized as a kind of super UHC filters actually. More advanced (and more expensive) filters bandpass is 3-10nm wide. This is comparable to regular narrowband filters that are used with monochromatic cameras. The most expensive multiband filters pass a narrow fragment of 3-5nm around each emission line. These filters usually cannot be used with very fast optical trains, but on the other hand, they can provide the best contrast ratio under bad-quality sky.
I recently purchased a color PlayerOne Ares-C cooled camera based on well known IMX533 sensor. Its main purpose is holiday astrophotography because under my light-polluted sky (19.50 mag/arcsec2 average) imaging with the color camera is not recommended. I was however tempted to test the modern multiband narrow filters and got the Optolong L-eXtreme 1.25″ filter. This is a two 7nm bandpass filter – we can say it is a current standard. The result images from this filter are presented in this blog entry. It is also worth noting, that images captured with the L-eXtreme filter presented here were made with almost full Moon in the sky, so the conditions were very uncomfortable.
However, it’s important to note that modern multiband filters are not a complete solution to light and Moon pollution. These filters only block certain wavelengths of light, and no additional light will be captured with the filter compared to observing without it. The primary function of each filter is to reduce unwanted light. Multi-narrowband filters are most effective when used with emission nebulae, where their benefits are most apparent. Using them on galaxies or star clusters will not significantly enhance the results.
It is important to remember that the best results are achieved with cooled cameras. Since the filter blocks most of the background light, thermal noise becomes a significant contributor to the overall noise. Additionally, the use of this filter can more or less affect the color balance. Processing images captured with multiband filters requires expertise in both nebula color balancing and star color. Often, it is best to desaturate the stars and leave them white, or capture some short exposures without the multiband filter to record the true color of the stars.
Image technical data: Date: October 2024 Location: Nieborowice, Poland Telescope: TS CF-APO 80/480 Corrector: TS FF/FR 0.8x Camera: PlayerOne Ares-C Mount: AM3 Guiding: ASI2224 + 30mm SVBONY guider Exposures: depending on target 80-120 x 3 min., Optolong L-eXtreme filter Conditions: Bortle 6, Moon present
