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 As with all optical instruments, telescopes suffer from imperfections such as spherical and chromatic aberration, astigmatism, coma and field curvature. However, there are also numerous other issues that may limit the quality of images. Polar alignment and tracking must be set up very accurately and any errors may lead to loss of image quality. These two factors, combined with atmospheric or astronomical seeing, set an effective lower limit on achievable resolution. Even when equipped with a sensor and optics of a very high quality, perhaps resolving more than about 1 arc second, it is not possible to record this detail in an image during long exposures.

Atmospheric refraction and so-called "dewing" also have the potential to ruin images. Dewing occurs when water vapour condenses from the air onto the optical surfaces of the telescope and camera system. An object at a temperature lower the dew point of the surrounding air, which depends upon ambient temperature and humidity, will suffer this problem. When humidity reaches 100 percent, the dew point becomes the same as the air temperature.


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