A Guide to Imaging Filters
Thanks to continuous advances in semiconductor technology and image processing techniques, amateur astronomers now have at their disposal camera technology that rivals what was available to professional astronomers less than twenty years ago. Compared to the old days of film astrophotography, modern digital astronomy cameras and DSLRs are extremely sensitive and relatively easy to use, and they create images that can be easily enhanced with sophisticated processing software.
But technology is never a substitute for getting a correctly exposed image onto the camera sensor. That's why astrophotographers must still understand how to focus the image from a telescope, track the image during long exposures, and combine and process images to maximize detail and minimize noise. They must also understand how to select and use optical filters for astrophotography. Such filters, which are placed in the optical path of a telescope in front of the camera, allow selected bands (or colors) of light to fall on the sensor. Filters are used to enhance detail and contrast in many astronomical objects. They are also essential in producing the sharpest possible color images with monochrome CCD and CMOS cameras.
Selecting filters for astrophotography is sometimes a little confusing for new imagers. This article helps you understand the basics of astronomy filters for astrophotography including color filters, broadband and narrowband filters for light-pollution reduction, and spectral line filters for maximizing detail in images of emission and planetary nebulae and supernovae remnants. Once you read through this article, you will have a better understanding of which filters to choose for many types of celestial objects.
All semiconductor sensors in digital cameras produce inherently monochrome images. But some astronomy cameras and all DSLR cameras are engineered to produce color images by using a matrix of red, green, and blue filters, called a Bayer filter, in front of the sensor, along with some clever processing. Color digital cameras can work well in astronomy imaging of planets and deep-sky objects, especially for beginning imagers. What such cameras offer in convenience and ease of use, however, they can take away in image sharpness and color fidelity in many astronomical objects.
To get the sharpest possible color images, many experienced imagers prefer using monochrome cameras without a Bayer filter in front of the sensor. Instead, they take several images of a celestial object, each with a separate color filter placed in front of the camera. Each image is combined and processed with software to produce a single true color image. The color filters themselves are designed to conform to one of two standard systems designed by professional astronomers long ago for measuring the brightness of stars.
Most amateur astronomers and imagers use the LRGB color system with monochrome astronomy cameras. A sharp and detailed monochrome image is made with the Luminance ('L') filter that encompasses the full range of the visual spectrum from about 400-700nm. Then color information is added with separate exposures with a Red (R), Green (G), and Blue (B) filter. The filters are typically inserted and swapped out using a manual or electronically controlled filter wheel. The resulting individual images are processed and combined into a single image. These filter sets can be used for planetary imaging and deep-sky imaging.
Figure 2-2 shows the bands of an LRGB filter set. This color system includes a small gap between the green and red filters to minimize the effect of narrowband light pollution from mercury and sodium street lamps. The filters do pass light at the wavelengths of H-beta (486 nm), OIII (501nm) and H-alpha (656nm) which is commonly emitted by many nebulae and supernova remnants.
Each of the filters in an LRGB set block ultraviolet (UV) and infrared (IR) light because, while the camera sensor is sensitive to this light, many telescopes fail to bring this light to a tight focus at the same plane as visible light. This can result in bloated star and less than sharp images. Some LRGB filter sets include another clear ('C') filter to aid in focusing.
On the other hand, some imagers wish to capture planetary detail ONLY in the infrared. In this case, an IR pass filter is placed in front of a monochrome camera instead of a color filter.
A UBVRI filter set provides an alternative to the LRGB system. This system passes light in the ultraviolet (U), blue (B), visual (V), red (R), and infrared (I) wavelengths. It is less widely used by imagers, though it is an essential system for precisely measuring the colors and brightness of stars. Baader and Optolong offer UBVRI sets of filters mounted in a 1.25" and 2" cells.
While most color filters are mounted in 1.25" or 2" threaded cells, some manufacturers, including ZWO, also provide unmounted filters in 31mm and 36mm diameters for direct insertion into filter wheels or filter holders.
Some Recommended LRGB and UBVRI Filter Sets:
- Baader Planetarium has 1.25" and 2" LRGB filter sets with and without clear focusing filters. See the Baader LRGB filter sets at this link.
- Optolong also manufactures LRGB filter sets in 1.25" and 2" cells as well as 31mm and 36mm round unmounted filters. See the Optolong LRGB filters at this link.
- ZWO has both a standard LRGB filter set and a premium LRGB filter set.
- Baader also has a 1.25" UBVRI filter set for those who wish to explore imaging with this range of wavelengths. More recently, Optolong has also introduced these in 1.25" and 2" versions.
Light-pollution filters are a big help for visual observers who wish to see emission and planetary nebulae and supernova remnants in urban and suburban skies. These filters improve the contrast of such celestial objects by passing only a band of visible light, especially blue-green light emitted by hydrogen atoms and oxygen ions and red light emitted by hydrogen atoms, while blocking light emitted by sodium and mercury street lamps from 540-620nm and by natural sky glow at 589nm. While they are not a cure-all for light pollution, and they don't replace truly dark skies, these filters are also an enormous aid in imaging many celestial objects with CCD and CMOS astronomy cameras and DSLRs coupled to telescopes. They can also help nightscape imagers using DSLR cameras and wide-angle camera lenses to get better images of the night sky where urban and suburban light pollution is a problem.
As mentioned in a previous Agena article on visual observing with light pollution filters,these filters fall into two main classes: narrowband and broadband. Narrowband filters have a passband of 20-30nm wide centered around the H-beta wavelength at 486nm and OIII wavelengths at 496nm and 501nm in the blue-green part of the spectrum. Since they have a smaller passband, these filters reject more light pollution and offer higher contrast on many nebulae, especially planetary nebulae.
Broadband light pollution filters have a passband of 50-60nm in the blue-green, so they reject light pollution, especially broadband light pollution, to a lesser degree. But many such filters also pass red light from H-alpha at 656nm (see Figure 3-2). This light is not useful for visual observers because the eye is relatively insensitive at this wavelength. But CCD and CMOS astronomy cameras are very sensitive at 656nm, so these broadband filters are particularly useful for imaging nebulae, especially reddish-pink emission nebula such at the Orion and Lagoon Nebulae.
The sensitivity of camera sensors in the red and infrared, as well as in the ultraviolet, has a downside however.IR or UV light is often brought to focus by refractor telescopes and camera lenses at a slightly different plane than visible light. So if IR and UV light from stars passes through a broadband or narrowband filter, then bloated and out-of-focus star images can result. That's why most serious imagers choose filters that block infrared and ultraviolet light, or they add a second UV/IR cut-off filter to do the job.
Broadband and narrowband filters can be used with monochrome cameras to produce detailed monochrome images to suppress light pollution and provide good image contrast. Or they can be used with color astronomy cameras, DSLRs, and astronomy video cameras such as Mallincamsor the Revolution Imager. Because they do not pass the full band of visible light, however, these filters can produce distorted colors in stars and other broadband light sources when used with color cameras. This is especially true of nebula filters than only pass blue and green light.
Light-pollution filters can also be used with DSLR cameras and lenses to reduce the effects of light pollution when taking wide field nightscapes with a camera lens. In this application, accurate color reproduction is important to faithfully capture the colors of stars. So some filter manufacturers have developed broadband light pollution filters than pass selected bands of light across the visible spectrum in the blue, green, yellow, and red while still excluding the light pollution bands. The L-Pro filter from Optolong, for example, offers better color correction than its CLS-CCD filter. This filter, as well as the CLS-CCD filter, are offered in standard 1.25" and 2" cells as well as clip-in format for select Canon and Nikon DSLR cameras. The well-regarded IDAS LPS filter also works well to balance color and reduce the effects of light pollution.
Some Recommended Narrowband and Broadband Filters Filter Sets:
- Lumicon Deep-Sky filter and Baader UHC-S filter both pass blue-green light from OIII and H-beta and red light from H-alpha. They are designed to not block IR and UV light. The Optolong CLS filter has similar characteristics, but the Optolong CLS-CCD filter includes IR/UV block characteristics to help imagers get sharper star images; it's one of the few filters in this bandwidth class that does so. Astronomik makes a similar CLS-CCD filter for the photographic applications.
- The Optolong L-Pro filter, as mentioned above, has better color characteristics but rejects less light pollution.
- For narrower-band applications in the blue-green part of the spectrum, the Lumicon UHC filter and the Orion Ultrablock filter work well to reject light pollution, though neither is optimized to reject UV and IR light. The Astronomik UHC filter has a very narrow band in blue green but also passes H-alpha in red which makes it useful for basic imaging and video astronomy.
- If required, separate UV/IR block filters can be threaded into broadband or narrowband light-pollution filters. These blocking (or 'cut') filters are available from several manufacturers including ZWO, Baader Planetarium, and Optolong.
To bring out the maximum amount of detail and contrast in many types of nebulae and supernova remnants, and in large spiral galaxies where large emission nebulae are visible, serious astrophotographers use very narrowband line filters that pass only a single spectral line emitted by one type of atom or ion. These filters have a very narrow bandwidth of less than 10-15nm, and premium line filters have bandwidths of less than 5nm. Smaller bandwidths pass less light and require longer exposures, but they offer better contrast and image detail. They also have superb rejection of light pollution, so these filters can be used to image nebulae and supernova remnants even in suburban and urban skies or in bright moonlight. The imager of Figure 1-1, for example, collected light through a narrowband H-alpha filter from the large city of Toronto, Canada. The filter had a narrow bandwidth to exclude most of the light pollution.
Because they pass so little light, however, these filters are not recommended for visual observation.
The most common line filters for astrophotography include:
Hydrogen Alpha (656nm). The most commonly used line filter, the H-alpha filter passes red light emitted by ionized hydrogen and brings out the fine, delicate detail in emission nebula and supernova remnants. The filter is also useful for bringing out HII (ionized hydrogen) regions in nearby galaxies.
Hydrogen Beta (486nm). Also used in improving contrast and detail in emission and planetary nebulae, H-beta filters can set off glowing regions of gas against dark nebula. Many H-beta filters, such as those from Lumicon and Explore Scientific, are for visual observing only and do not include the necessary UV/IR blocking characteristics. Recommended H-beta filters include:
Oxygen (OIII - 496nm and 501nm). OIII filters are very useful for extracting detail from planetary nebulae and some emission nebulae and supernova remnants while blocking much of the light from stars and other broadband light sources.
Sulfur (SII - 672nm). There is very little sulfur in nebulae, but the SII emission is strong and well favored for the physical conditions in many such objects. The deep-red light from SII reveals delicate detail that may be distinct from regions that emit light from hydrogen. When used with monochrome cameras, images of SII emission at 672nm is often assigned a false color.
All of the above filters have bandwidths that range from 12nm to 6nm, and even to 3nm in the case of the high-end filter sets from AstroDon. Some line filters are also available in larger 25nm bandwidths. Smaller bandwidths have the advantage of improving contrast and detail in nebulae, even in conditions of serious light pollution or moonlight.
The peak transmission of line filters is also an essential specification. It's especially difficult to engineer these filters to have both very narrow bandwidth and high transmission. Mid-range filters with bandwidths of less than 7nm have peak transmission of 85-90%. More expensive filters can have slightly higher transmission which reduces exposure times.
Most line filters are mounted in 1.25" or 2" threaded cells, and they are also provided in unmounted filters in 31mm and 36mm diameters for direct insertion into filter wheels or filter holders.
Like LRGB color filters, line filters are generally used only with monochrome cameras. The imager takes a series of monochrome images, each with a separate line filter placed in front of the camera, and each image is combined into a single color image using image processing software. Each wavelength is assigned a true color or, more commonly, a false color from a standard astronomical color palette. The most common palettes are the Hubble Space Telescope (HST) Palette and the Canada-France-Hawaii Telescope (CFHT)palette. In the HST palette, H-alpha is assigned the color green, OIII is assigned blue, and SII is assigned red. In the CFHT palette, H-alpha is assigned red, OIII is assigned green, and SII is assigned blue.
Since line filters have very narrow bandwidths, star images are often quite dim. To bring up the stars, additional images are taken through LRGB filters, and these images are combined with images from line filters to produce a final image. Some manufacturers such as Optolong and ZWO include line filters with LRGB filters for a full set of imaging filters in a number of size formats for deep-sky objects.
This guide explained the basics of color, broadband, and line filters for astrophotography. Such filters, which are placed in the optical path of a telescope in front of the camera, allow selected wavelength bands of light to fall on the sensor. Filters are used to enhance detail and contrast in many astronomical objects. They are also essential in producing the sharpest possible color images with monochrome CCD and CMOS cameras. Color filters such as LRGB filter sets are useful for imaging planets and deep-sky objects. Broadband filters can reduce the effects of light pollution when imaging through a telescope with or with DSLR cameras and lenses. And spectral line filters are essential for maximizing detail in images of emission and planetary nebulae and supernovae remnants, especially when used with monochrome astronomy cameras.