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Cooling Down


The mirror in your telescope is an amazing thing.  It is an extremely sensitive tool that is exact enough to focus star light to a single point, while being sturdy enough to resist flexing even a small fraction of a single wavelength of light.  To accomplish this, most mirrors are made of thick Pyrex glass, 2” thick is not uncommon!  This precision hunk of glass is the reason that your telescope is so heavy, and it also takes a while for it to cool down to lower night time temperatures.  It’s important that your mirror be at the same temperature as the evening sky.  If it’s not, you’ll have difficulty focusing the image because the mirror will be minutely flexed due to thermal differences.  If the mirror is colder than the ambient air temperature, you may get dewing on the mirror like steam on the bathroom mirror after a shower.  Either one of these will result in bad images through the eyepiece.

About an hour before you plan on using the telescope, get it set up and remove any caps to allow airflow down and around the mirror.  Point the telescope away from the Sun so it doesn’t heat up, and let the optics reach ‘Thermal Equilibrium’… in other words, the mirror should be the same temperature as the surrounding air.  If the mirror is a big one, 10” or larger, you may consider blowing a fan on the back of the mirror to help speed up the process.

Some evenings the temperature can change rapidly around midnight.  You may find yourself letting your optics cool down again, catching up with the night’s lowering temperature, while you get some Hot Cocoa so that you DON’T.


Introduction to the Night Sky - Part I

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