For the many of us who live near cities, light pollution makes quality astrophotography difficult. Fortunately, there are many filters that alleviate the problem. But which one is best?
Types of filters
Astrophotography filters fall into two major categories: wideband (or broadband) and narrowband. Until recently, narrowband filters were only useful with monochrome cameras since only single-band narrowband filters were available (H-alpha, oxygen-III, etc). But now, new multi-band narrowband filters are on the market, such as the Optolong L-eXtreme and the Radian Telescopes Triad. These bring effective narrowband imaging to one-shot color cameras.
To understand the difference, one needs to look at the spectra – or the graphs of separated wavelengths of light. Light pollution filters, such as the Astronomik CLS-CCD and Optolong L-Pro, pass most wavelengths of light, but block regions of the light spectrum where city lights tend to emit.
As you can see, the CLS-CCD filter passes blue and green light, blocks yellow and orange light, and passes red light. It also blocks ultraviolet (UV) and infrared (IR) light, which is helpful for non-DSLR astrophotography cameras that do not have UV/IR filters built in. (The CCD version of the CLS filter can also be used on DSLRs and cameras with built-in filters). Even though greater numbers of cities are adopting full-spectrum LED lights, high-pressure sodium lamps still prevail enough to make light pollution filters work wonders for city dwellers.
Multi-bandpass narrowband filters for one-shot color cameras are increasing in popularity. They combine multiple narrowband filters into a single filter, which allows for one-shot narrowband imaging that covers two, three, or sometimes four of the major emission lines. The Optolong L-eXtreme has 7 nanometer widths on both the H-alpha and OIII emission lines, making it a duo-bandpass filter.
The Optolong L-eNhance filter is similar, except that the OIII bandpass is much wider to include one of the hydrogen (H-beta) lines – 24nm around OIII, and 10nm around Ha. The wider bandpasses allow more light to pass, but also let through more light pollution from sources like the Moon and full-spectrum LEDs. Because of the wider bandpasses, the L-eNhance is better-suited for fast (<f/4) systems than the L-eXtreme, but the L-eXtreme cuts out more light pollution and moonlight.
When to use each type
Narrowband filters are not ideal for every type of target due to the specific wavelengths of light they pass. H-alpha and OIII emissions are primarily present in most kinds of nebulae – except for reflection nebulae, like the Pleiades Cluster or Messier 78 – but only make up a small portion of the light from galaxies. While adding H-alpha data to galactic images can enhance them, you will generally not get much from a narrowband filter on galaxies.
Using the Optolong L-eXtreme filter, the author was able to capture the dim (magnitude 14), diffuse planetary nebula Jones-Emberson 1 from her Bortle 7 backyard. (Bortle is a measure of light pollution brightness, ranging from Class 1 dark skies to Class 9 inner-city skies).
Another use of wideband light pollution filters is for the luminance channel on monochrome cameras.
Using light pollution and narrowband filters doesn’t just make imaging from the city possible. They enable astrophotographers in light pollution to capture quality images. Wideband light pollution filters like the Astronomik CLS-CCD are great for galaxies, star clusters, and as the luminance channel for monochrome cameras. Multi-band narrowband filters like the Optolong L-eXtreme are great for one-shot color cameras on emission nebula targets. Happy imaging!
MSRP: Optolong eXtreme $230
Astronomik CLS-CCD $199