A Guide to Imaging Filters
✅ A Guide to Imaging Filters |
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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. |
Figure 2-2: The passbands of a Baader LRGB filter set. Credit: Baader Planetarium. |
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. 3. Broadband Light-Pollution Filters for Astrophotography 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 h-ydrogen atoms and oxygen ions and red light emitted by h-ydrogen 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. 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 3-1). 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. Guide to Imaging Filters |
Figure 3-1: Broadband light pollution filters such as the Lumicon Deep-Sky Filter transmit the widest passband of all light-pollution filters. These filters pass light from H-beta, OIII, and H-alpha. Image courtesy of Lumicon. |
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. 4. Line Filters for Astronomical Imaging 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: H-ydrogen Alpha (656nm). The most commonly used line filter, the H-alpha filter passes red light emitted by ionized h-ydrogen and brings out the fine, delicate detail in emission nebula and supernova remnants. The filter is also useful for bringing out HII (ionized h-ydrogen) regions in nearby galaxies. (SAFTETY NOTE: H-alpha filters for imaging CANNOT be used for observing or imaging the Sun. H-alpha solar filters also pass 656nm light from the Sun. For this reason, these filters are sometimes call "Night Sky" H-alpha filters. H-alpha imaging filters; they have far narrower bandwidths and they also come with associated hardware that is designed to reduce the Sun's light to a safe level.) H-ydrogen 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. 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 h-ydrogen. When used with monochrome cameras, images of SII emission at 672nm is often assigned a false color. 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. Guide to Imaging Filters |
Figure 4-1: Clockwise from upper left, an image of the Rosette Nebula taken through narrow line filters including H-alpha, OIII, and SII. At lower left, all three images are combined using the colors of the Hubble palette. |
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. |