Must we make corrections

Many DSLR enthusiasts make no any guiding corrections during the exposure. In the film era, this would have been absurd, but it's possible today for several reasons. Equatorial mounts are better built than they used to be; they track more smoothly. Drift-method alignment has come into wide use, leading to more accurate polar alignment.

Most importantly, exposures are shorter. It's much easier to get 2 minutes of acceptable tracking than 20. In fact, periodic gear error being what it is, some 1-minute exposures are bound to be well-tracked even with a cheaply made mount. You can select the best ones and discard the rest. You can even work with 30-second or 15-second exposures if that's all your tracking mechanism will permit.

Figure 9.6 shows what can be achieved without guiding corrections. This was taken with a PEC-equipped Meade LX200 on a precisely polar-aligned permanent pier. It's not quite as sharp as a comparable image taken with the autoguider turned on, but it's close. And that is with a focal length of 1250 mm and considerable subsequent cropping and enlargement. For piggybacking with a 200-mm or 300-mm lens, guiding corrections are hardly needed with this telescope and mount.

Figure 9.6. The galaxy M51, without guiding corrections. Enlarged central part of a much larger picture. Stack of three 3-minute exposures, minus dark frames, with 8-inch (20-cm) telescope at f /5.6 and Canon Digital Rebel (300D) camera at ISO 400, processed with ImagesPlus, Photoshop, and Neat Image.

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