Using fast telescopes or lenses with large and flat illuminated fields makes imaging much easier. Try tripod shots for star trail images. Start with piggyback images and short focal lengths. Constellation images are very forgiving of imperfect tracking. M31 with a 50mm lens is an impressive target. With more experience, longer focal lengths can be used. High-quality fast refractors with flat field like the Televue NP101 and imaging dedicated telescopes like the Takahashi Epsilon are best suited for use with D-SLRs. Above 300 mm focal length, an auto-guider is highly recommended.
In many situations we have to live with light-polluted skies. Because of the 8-to 12-bit resolution of the D-SLRs, they are more sensitive to light pollution than astro cameras. Filters are a big help in overcoming these effects. The Hutech LPS filter and high-quality UHC filters allow us to reveal faint nebula structures even under magnitude 4.5 skies, while star colors can be preserved. Narrowband filters like the Astronomik 15 nm H-alpha filter need very fast lenses (< f/2.8) and low temperatures for optimum results. The better your raws and the more raws you have, the less effort will be needed to achieve the best final image quality.
As we all have to live with the given sky conditions, I select the objects to fit best with the prevailing conditions:
• Under moon-lit skies, small bright planetary nebulae and globular clusters can be imaged. Narrowband filters allow nebula imaging even with a bright sky background.
• When seeing is unsteady, I use the shorter focal length refractor and lenses for extended nebulae, open clusters and star fields (see Figure 5.11).
• The best dark and steady nights should be reserved for long focal lengths to image faint distant targets like galaxies. If possible, we should try to image difficult targets near the meridian, the point of highest elevation.
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