Image Clarity

Do images in the new scope seem a little less sharp than expected? That usually does not mean the CAT has bad optics. SCT optics especially are pretty consistently good these days whether they come from Meade or Celestron. The problem is most often collimation, the alignment of the secondary and primary mirrors. See Chapter 9 for collimation instructions. Do not feel bad if the new scope arrives out of collimation. Given the bumps a scope must take on its way from the factory or dealer, it is amazing when one arrives in collimation. What does miscollimation look like? Stars, even those near the center of the field, tend to look more like little comets than points.

A close runner-up to collimation is cooldown. If the scope has not been allowed to acclimate to outdoor temperatures, to equilibrate, there is no way it will deliver good images. On some evenings, a half hour or even 2 or 3 hours is not enough time for a complete cooldown. When the temperature is falling steadily, a larger-aperture CAT, especially an MCT, may never cool off. "Shimmering," or moving images, are the hallmark of incomplete cooldown. The Moon or a planet will tend to waver in a large-aperture SCT and bounce around in the field of a small-aperture one. The star test (discussed in the next section) can reveal cooldown problems.

Closely related to cooldown is seeing. A scope can be perfectly equilibrated but still deliver poor images if the atmosphere above the observing site is disturbed. Poor seeing is typical for many locations in the wintertime, when the jet stream is roaring overhead. How do you tell if the seeing is bad? Take a look at the sky. Are the stars twinkling madly? If so, do not expect good seeing. For a more scientific forecast of seeing conditions for a particular location, try the Clear Sky Clock at This weather tool for amateur astronomers gives seeing predictions in addition to transparency and cloud cover forecasts for specific locations.

If seeing is poor, forget doing high-magnification planetary observing, but it may still be possible to have some fun by sticking to low powers and the deep sky. Higher magnifications exacerbate problems with seeing and cooldown. Star clusters, nebulas, and galaxies are also degraded by poor seeing, but far less so than planets. Generally, smaller-aperture telescopes are less affected by seeing than larger-aperture ones. A 5-inch SCT, after all, is looking up through a smaller diameter column of disturbed air than a 10-inch.

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