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Figure 11.7. Didymium glass (Hoya Intensifier filter) cuts through skyglow from sodium-vapor streetlights. Each image is a single 3-minute exposure of the field of Zeta and Sigma Orionis with a Canon Digital Rebel (300D) at ISO 400 and a 180-mm lens at f /5.6. Note reflection of bright star caused by filter (arrow).

Figure 11.8. The Horsehead and other faint nebulae captured under light-polluted skies 10 miles from central London. Stack of 20 3-minute exposures and 23 2-minute exposures with unmodified Canon EOS 30D camera, 10-cm (4-inch) f /8 Takahashi apochromatic refractor, and Astronomik CLS broadband light pollution filter. No dark frames were subtracted. (Malcolm Park.)

Figure 11.8. The Horsehead and other faint nebulae captured under light-polluted skies 10 miles from central London. Stack of 20 3-minute exposures and 23 2-minute exposures with unmodified Canon EOS 30D camera, 10-cm (4-inch) f /8 Takahashi apochromatic refractor, and Astronomik CLS broadband light pollution filter. No dark frames were subtracted. (Malcolm Park.)

narrow-band nebula filters used by visual observers are usually disappointing for photography; they block too much light.

Most telescope dealers sell several brands of interference filters; one of the most respected vendors, offering a very broad product line, is Lumicon (www. lumicon.com). Another is Astronomik (www.astronomik.com).

Interference filters are not always available in sizes to fit in front of camera lenses. Instead, they are sized to fit 11-inch and 2-inch eyepiece barrels and the rear cell of a Schmidt-Cassegrain telescope (see Figure 5.5, p. 54). Those threaded for 2-inch eyepieces will fit 48-mm camera-lens filter rings; the thread pitch is not the same, and only one thread will engage, but that is enough.

One last note. Not every filter that works well on an eyepiece will work equally well when placed in front of a long lens. The long lens magnifies optical defects. I have only had problems with the optical quality of a filter once, but it's definitely something to check.

11.4.3 Imaging with deep red light alone

Even near cities, there is comparatively little light pollution at wavelengths longer than 620 nm. Accordingly, if you use deep red light alone, you can take pictures like Figure 11.8 even in a light-polluted sky.

A modified DSLR is helpful but not absolutely necessary. What is important is the right filter, either an Astronomik interference filter that passes a narrow band around Ha, or a dye filter that cuts off wavelengths shorter than 620 nm or so. The common Wratten 25 (A) or Hoya R60 red filter is not red enough; it transmits some of the emissions from sodium-vapor streetlights. Suitable filters include Wratten 29, B+W 091, Hoya R62, Schott RG630, and the Lumicon Hydrogen-Alpha filter. For more about filters and their nomenclature, see Astrophotography for the Amateur.

11.4.4 Reflections

Reflections from filters are more of a problem with DSLRs than with film cameras because the sensor in the DSLR is itself shiny and reflective. The sensor and a filter parallel with it can conspire to produce multiple reflections of bright stars. The reflection risk seems to be much greater when the filter is in front of the lens than when it is in the converging light path between telescope and camera body. To minimize the risk, use multi-coated filters whenever possible. This is only practical with dye filters; interference filters are inherently shiny.

Part III

Digital image processing

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