Filters control the wavelength of light that reach a detector. In astronomy, there are many different uses for filters:

• color imaging at standard red, green, and blue wavelengths;

• isolating wavelength ranges for accurate scientific photometry;

• isolating specific spectral lines from astronomical sources;

• cutting light from mercury, sodium, and neon lamps; and

• reducing chromatic aberration in telescopes and lenses.

Filters are colored or coated glass plates that transmit some wavelengths of light while blocking other wavelengths; their role is to select what you see. To make color images, for example, we divide the spectrum into the same three ranges of wavelength that cone cells in the human eye sense using red, green, and blue filters. The red filter passes light from about 600 nm to 700 nm in wavelength and (ideally) blocks all other wavelengths. The green and blue filters likewise pass the 500- to 600-nm band (green) and 400- to 500-nm band (blue). From three separate filtered images, we can reconstruct full natural-color ones.

For the urban observer, light-pollution rejection (LPR) and short-cutoff filters perform yeoman duty blocking light pollution and improving the quality of star images in telephoto lenses and refracting telescopes.

The filters that astronomers use for scientific photometry resemble those used for color images, differing mainly because they block and pass somewhat different wavelengths. Filters that isolate spectral lines have long been used by professional astronomers, but since the advent of astronomical CCDs, amateur astronomers have discovered the beauty and power of hydrogen-alpha to reveal marvelous nebular details even on Moon-bright nights!

4.6.1 Filter Types

There are two basic types of filters: dyed-glass, and interference. In a dyed-glass filter, one or more metal ions are dissolved in the glass; they absorb some wavelengths while passing others. Interference filters are also called dichroic filters, and work by constructive and destructive interference in multiple thin layers deposited on the filter surface.

Dyed-glass Filters. There are three types of dyed-glass filters: short cut, long cut, and passband. Short-cut filters block short wavelengths below a cutoff wavelength while passing longer wavelengths. Long-cut filters block wavelengths above a cutoff wavelength while passing shorter wavelengths. Bandpass filters transmit a band of wavelengths and block shorter and longer wavelengths.

Short-cut dyed-glass filters generally exhibit sharp cut, high long-wavelength transmittance, and excellent short-wavelength blocking. The classic Wrat-ten #25 red filter is a fine example and a fine filter. Long-cut dyed-glass filters are almost always problematic, with a broad cutoff, poor blocking, and low short-wavelength transmittance; the classic deep-blue Wratten #47 is typical in passing long-wavelength infrared better than they pass blue light!

Bandpass dyed-glass filters seldom exhibit sharp wavelength cutoffs, so they have broad passbands and tend to "leak" a fair amount of energy outside the passband. Even in the main passband, dyed-glass bandpass filters seldom transmit more than 50% of the incident photons, so much precious light never reaches the CCD. The classic green Wratten #58 is a good example. Before interference filters became popular, the Wratten #25, #58, and #47 were the standard filter set used for color-separation imaging.

Dyed-glass filters are available as glass disks, or mounted in 1.25-inch and 2-inch threaded filter mounts.

Interference Filters. Dichroic filters are made by evaporating many fine layers of different dielectric materials (such as magnesium fluoride) onto the surface of a disk of optical glass. Interference filters are painstakingly designed, and they work by selectively reflecting photons having wavelengths that the designer does not want passed. The design is realized by adjusting the thicknesses of the dielectric layers. Dichroic filters have a characteristic "shiny" appearance because they reflect the wavelengths that do not pass.

Interference filters often have steep passband profiles, high transmittance in the desired passband—and wavelengths outside the design passband can be almost entirely blocked. A skilled designer can produce a wide range of filter transmittance curves, from wide-band filters well suited to color imaging to narrowband filters suitable for Ha imaging. Compared to dyed glass, interference filters are amazing—a dyed-glass filter will have a bandpass of 100 nm with a peak transmittance of 50%, whereas an interference filter will transmit 90% of the incident photons in a passband less than 10 nm wide.

For astronomical imaging, an interference filter with its passband centered on the wavelength of an astrophysically important element (such as hydrogen) yields a picture in the light of just that one type of atom. Meanwhile, only the tiniest sliver of broadband sources (such as Moonlight) gets through the filter, and the wavelengths of city streetlights are fully blocked.

Despite their technical elegance, two characteristics of interference filters can and do cause some problems for the digital imager:

1. Tilting an interference filter causes the wavelength passed by it to shift to longer wavelengths than the design wavelength.

2. With fast optical systems, off-axis light impinging on the filter at a sufficiently steep angle may lie outside the filter passband.

3. The reflective surface of the filter facing the reflective surface of the CCD chip may cause ghost images and haloes around bright objects.

These problems must be taken into account, but they are not severe. The first only requires that you take care to mount the filter perpendicular to the incoming light. The second is an issue only with optical systems f/4 and faster, and can be handled by using a filter with sufficiently wide passband to include the wavelengths of interest. The third is the most difficult to cure, but it occurs only when you try to image faint objects that are near very bright ones.

Interference filters are available in 1.25-inch and 2-inch threaded mounts. Bear in mind that interference filters do have delicate surfaces, and are subject to attack by moisture and chemicals. With care, however, they can and do render excellent service for years.

4.6.2 Filters for H-Alpha and Other Emission Lines

When an atom is struck by a photon with sufficient energy, its outmost electron jumps to a higher energy state, and then eventually falls back. When it does, it emits a photon at one of several wavelengths. These wavelengths are characteristic for each atom. Filters that selectively pass only the strongest of these spectral lines for certain atoms are called emission line filters.

Emission line interference filters are available for many of the bright nebular emission lines. These have exceptionally narrow passbands, ranging from 10 nm down to around 4 nm. In the visual spectrum, nebular emission lines include:

• doubly-ionized oxygen (OIII) at 500.7 nm wavelength,

• hydrogen-alpha (Ha) at 656.3 nm wavelength,

• ionized sulfur (SII) at 672.6 nm wavelength.

Because they block all other wavelengths, narrow-band filters increase contrast, bring out fine detail in objects where it is present, and allow deep imaging from bright-sky locations. It is important to find a good compromise between competing needs. Rejecting unwanted light to get maximum contrast calls for a narrow bandpass, yet accepting all of the light from a fast optical system calls for a wider bandpass.

It is possible to make vivid pseudocolor images by joining OIII, Ha, and SII images into a color image, using the shortest of the wavelengths (OIII) as blue, the middle wavelength (Ha) as green, and the longest (SII) as red. Many of the Hubble Space Telescope's nebula images use this particular color-coding scheme— and it works equally well for amateur astronomers.

4.6.3 Light Pollution Rejection (LPR) Filters

Filters used to block the emissions of low-pressure and high-pressure sodium- and mercury-vapor lamps are called light-pollution-rejection (LPR) filters. Visual observers have long known that viewing through LPR filters enhances the contrast of faint objects, helping them stand out against a black sky background. LPR filters are usually designed to pass the Ha and OIII emission lines that are strong in emission nebulae. LPR filters designed for the needs of CCD imaging can really improve the images taken under moderately light-polluted skies.

LPR filters are effective from urban areas because they cut background sky light. The high background light level limits exposure times by creating a "plateau" on which the signal from the target sits. As a result, stars, nebulae and galactic cores reach saturation during relatively short exposures. The LPR filter attenuates the wavelengths from man-made sources, dropping the background and leaving more "headroom" for the celestial object being imaged.

They are available in 1.25-inch and 2-inch-barrel threaded sizes, SCT rear-cell threads, and threads for standard camera lenses.

Figure 4.7 Filters are an essential component In the CCD imager's toolbox. The image above shows the Helix nebula imaged with a wide-band red filter; the image below shows the same object imaged with a narrow-band Ha filter. By suppressing sky light, the Helix stands out against a dark background.

4.6.4 Blue-Block and Violet-Block Filters

Many refractive optical systems suffer from chromatic aberration, in which all wavelengths do not focus at the same point. This is true of simple achromatic objectives, ED-glass objectives, and apochromatic objectives to varying degrees. Refractors that are designed for visual observing can seldom accommodate the range of wavelengths that the astronomical CCD "sees" clearly.

In imaging with refractors, when an observer focuses using short exposures, the image appears sharp because the user is focusing primarily on the electrons from the red photons to which the camera is more sensitive. Not seen are the electrons generated from blue and violet photons at the short end of the spectrum. After a long exposure, however, every bright star is surrounded by a halo of out-of-focus blue and violet light. For color imaging this is especially troublesome because every bright star ends up with an obvious blue halo.

One solution is to add a filter that selectively blocks photons from the short end of the spectrum, usually below 420 nm wavelength (violet light). With the out-of-focus light excluded from the optical system, star images look small and sharp, and don't have the tell-tale "blue-bloat" halo. Minus-violet and minus-blue filters are available in 1.25-inch and 2-inch threaded mounts as well as cells threaded to fit many camera lenses.

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