The time has come where the warm sweaty humidity of summer observing gives way to the cooling dry air of autumn. Slowly, my observing wardrobe changes from shorts and t-shirts to layers of long pants and long sleeves, light jackets and heavier coats.
The Milky Way objects of summer also gives way to clusters, nebulae, and galaxies of the fall. The Earth’s position in its orbit around the Sun also changes the perspective of the sky. No longer are astronomers stargazing into the heart of the Milky Way, but more towards deep sky of far off galaxies.
As the backyard astronomer experiences the cooling weather, I have learned lessons of being prepared for the cooler autumn nights that eventually trasnsitions to the freezing winter.
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Never underestimate the cooling temperatures. Observing through a telescope means sitting quietly with eye to eyepiece, the lower temperatures during an autumn night can have a chilling effect. I have learned to dress in layers and to dress like its 20°colder. Coats, sweaters, sock hats, gloves or mittens, and blankets are the dress code.
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I take my telescope and eyepiece case outside from my warm house to the cool night to acclimate to the ambient temperature. Going from a warm home or car to a cool outdoor environment will require at least 30 minutes to adjust to the cooler air.
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Dewing can be a problem. As the temperature drops, optics can attract a layer of dew. There are several ways of combating dewing: dew caps for the front of the telescope, and dew heater devices to gently maintain the temperature of the optics a few degrees above the dew point. Avoid observing objects directly overhead. Once infected with dew, a brief exposure to the warm air of a hairdryer may help, but then the optics will have to acclimate to the ambient temperature all over again.
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Beware of fogging, and condensation. It is easy to fog over eyepieces and finderscopes by inadvertently breathing on them. Don’t!
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In early autumn, the bugs and insects that bite and sting may still be a problem. As the weather gets colder, some remaining insects may seek a warmer environment in eyepiece cases, telescope cases, and telescopes and mounts. Check all equipment before packing it in for a night.