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Yup you have a newtonian, so the best thing to do is get a Dick smith digital thermometer and put one end on the side of the primary mirror.
A few of us "actively" cool our primary mirrors and at times we can get the mirror below ambient. Either way, if there is a difference in temperature, then air will move.........causing you to lose detail in a big way
For high magnification imaging, then less than .5 degree difference is a must......and to be honest even observing jupiter or splitting double stars really works soooo well when less than .5 degree difference.
Get to know your local temperatures over the night.... you might find you consistantly only drop 10 degrees over night......down here in tassie, we can drop 15+ degrees and quite quickly.....hence the need for actively cooling the mirror
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