Key Takeaways:
- Astrophotography filters are either narrowband (for nebulae) or wideband (for galaxies and star clusters).
- Narrowband filters isolate specific light wavelengths; wideband filters let through a broader range.
- Filter size depends on your camera sensor size; larger sensors need larger filters.
- Monochrome cameras use single-wavelength narrowband filters; OSC cameras often use multi-band narrowband filters.
Astrophotography is a hobby with a huge learning curve, and one aspect that can muddy the waters is the dizzying array of filters on the market. What is narrowband? What size do I need? Which are good for monochrome and one-shot color? Let’s cover some of the basics.
There are two types of filters for astrophotography: narrowband and wideband (some say broadband, but I think this is too similar to the term for high-speed internet). Wideband filters cover large swaths of the optical light spectrum, and while they are most commonly used for star clusters and galaxies, they can be used for any kind of target. For monochrome cameras, these filters include luminance, red, green, and blue (LRGB) filters, as well as light-pollution filters such as the Optolong L-Pro and Astronomik CLS-CCD, which are also applicable to one-shot color (OSC) cameras. Also, you can use a light-pollution filter as the L channel for monochrome cameras.
Narrowband filters pass only small slices of the optical spectrum centered around emission lines, which makes them excellent for imaging nebulae under light pollution. The three most common narrowband emissions are Hydrogen-alpha (Hα) at 656.3 nm (red), Oxygen-III (OIII) at 495.9 and 500.7 nm (blue-green), and Sulfur-II (SII) at 672.4 nm (deep red). There are also other less commonly used filters, such as Hydrogen-beta (Hβ) at 486.1 nm (blue). For monochrome cameras, filters that pass only one of these wavelengths are the most useful so the images can be individually colored. If you use Hα, OIII, and SII, you can make Hubble-palette images by assigning SII to red, Hα to green, and OIII to blue.
For OSC cameras, filters that transmit multiple bands are best to get some light on all the red, green, and blue pixels together. Dual-narrowband filters are the most common, such as the Optolong L-Ultimate and Antlia ALP-T Hα & OIII, which have 3nm and 5nm bandpasses, respectively, around the Hα and OIII wavelengths. At a higher cost are quad-band filters, such as the Optolong L-Quad and Antlia Quad Band, which pass Hα and OIII as well as SII and Hβ. On an OSC camera, it’s difficult to distinguish Hα and SII since they are both red, and OIII and Hβ, which are both in the blue, but because these filters pass more wavelengths you can get more light on targets such as planetary and emission nebulae, and supernova remnants.
Determining what size filters you need can be difficult for beginners. There are several common sizes available: 1¼”, 31mm, 36mm, 50mm, and 2″. Those with sizes in inches are mounted filters, which means they come in a threaded housing you can easily screw into a filter wheel or onto the end of some nosepieces or extension tubes. The metric-sized ones are generally unmounted, which can only go into a filter wheel.
The driving factor behind which size filter you need is the size of your camera sensor. Full-frame cameras, such as the ZWO ASI6200MM Pro, are 43 mm long on the diagonal, so 1¼”, 31mm, and 36mm filters will block the corners of the sensor and cast a shadow. On the other hand, 4/3-size sensors like on the ZWO ASI1600MM Pro are only 22 mm along the diagonal, so even 1¼” and 31mm filters will cover the entire sensor. Even though I don’t have a full-frame astrophotography camera yet, I invested in 2″ and 50mm filters (and their corresponding filter wheel size) from the start to avoid having to buy a whole new set of filters later, but larger filters and filter wheels can be significantly more expensive than their smaller counterparts.
Having the right filter for the right job is an important component of creating a good astrophoto. Choosing the type and size for your camera and target will help you capture all those incredible celestial objects that lie buried in the deep sky.
