Whitepeak Observatory, Tacoma, WA

Cooldown time for Refractor

Tried an experiment to see exactly how long it takes a 5" aluminum tubed refractor to cool ~25degrees to within 1 degree F of ambient.

I left my 5" inside until it was warmed to within 1 degree of ambient (66 degrees) on all points measured (Tube exterior: top middle bottom; and lens). Then took it outside (clear sky, no wind), uncapped the lens and focus tube, pointed the objective to the sky and began measurements with an infrared thermometer. The 6" f/8 achro (which lives on the pier in the observatory) was the ambient control. Observatory inside temp within 1 degree F of outside temp (Inside-outside remote thermometer). Control scope was within 1 degree of inside temp, tube and lens. Made multiple measurements (five each measurement point then averaged) at 10 minute intervals until temp of scope fell within 1 degree of ambient at all measurement points. The chart shows the results. Note: variations in relative humidity should have no appreciable effect upon heat transfer as the difference between the density of 40degree air at 0% humidity and 100% humidity is <1%. (Care to take a guess as to which is more dense--saturated or dry air? ;) )

One hour plus to ambient for cooldown was longer than I expected--and longer than conventional wisdom predicts as well it seems. I thought that the glass would radiate to the sky quicker than this, but evidently not. Interesting is that the aluminum tube cools at a faster rate the the glass yet everything catches up together a couple of degreees from the end. Anyway, an hour-- something to keep in mind for those who don't keep their scopes outside all the time.

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