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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14711
Loc: Cumberland, R I , USA42N71.4W
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I've noticed this before, and it came to light again last night.
I was observing last night intermittently from about 7pm to about 1am. During the course of that time, temperature dropped from the mid-upper 40s to the mid 20s. And I suppose when I first set the binoculars up at about 6:30-7pm, they were still at about 65° room temperature. So throughout the course of the evening, the binoculars dropped from 65° to the mid 20s, a 40 degree temperature change.
Two or three times during the night I had to refocus the Garrett 22x85. Something that is never necessary during the warm months. So, worthwhile to keep in mind, it pays to get your gear outside to acclimate. Not so much of an issue with small gear, but for big gear it is an issue. Temperature changes do have an affect on all that mass, especially when the focal length is about a half meter long.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Erik D
Post Laureate
Reged: 04/28/03
Posts: 3196
Loc: Central New Jersey, USA
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The GO SS 22X85 is of aluminium construction, close to 16 inch(~406 mm) in length.
40 deg F delta = 22.2 deg C.
Coefficient of linear thermal expansion for Al is ~23/1,000,000 per deg C
A 40 F/22.2 C temp drop would result in ~ O.2 mm contraction of binocular housing.
Dimensional displacement of the objective spacing, prisms and eye pieces are additional factors to consider.
Coefficient linear thermal expansion for steel is approximate 1/2 of aluminium.
ERik D
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