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Mirror cooling with heat sinks: is there an easy way to cool your primary without using a fan?

TELESCOPE MAKERS CAN BE a funny bunch. Although our passion for building instruments and making optics is usually rooted in a careful, scientific approach, we also tend to put a lot of stock in hand-me-down knowledge. Some of it withstands scrutiny, some of it doesn't. So I take special note when I hear of carefully controlled experiments to investigate telescope performance.

In last May's column I discussed telescope thermals, concluding that a cooling fan for the primary mirror is usually a necessity for optimal performance. But is that the only way to chill the mirror close to the ambient air temperature and keep it there?

Among those wondering the same thing is James Stilburn of Victoria, British Columbia, whose binocular telescope was featured in last April's column. Instead of simply posing the question to an online forum, Jim did some experimentation.

"The idea was to see if a heat sink could eliminate the need for a fan, thus removing the potential for vibration and the need for a battery," explains Jim. "There was a lot of speculation on some ATM forums about how this might work, but apparently it hadn't been tried yet." So he set about running a series of cool-down trials. His test subject was a 6-inch-diameter, 1-inch-thick Pyrex mirror blank mounted in an 8-inch-diameter, Protostar lightweight, resin-impregnated cardboard tube.

To take an accurate reading on the blank's temperature, he used thermal paste and tape to affix a thermocouple probe to the front face of the glass. A second probe monitored the ambient air temperature inside the garage where his trials took place.

Jim ran several tests, the first two without a heat sink. He monitored the blank's cooling by convection alone, and then with a fan generating 27 cubic feet per minute (CFM) of air flow. He then attached a custom-made aluminum heat sink to the back of the blank with a thin layer of thermal paste, and ran three more tests. First he tried just the heat sink and convective cooling. Then he ran two tests with fans blowing air across the heat-sink fins--one trial with a low-speed fan, the other with the 27 CFM model. The results are shown in the accompanying graph.

A few surprises emerged from the tests. The most striking one is that attaching only a heat sink made matters worse. "The heat sink has about twice the heat capacity of the Pyrex blank, but its surface area is 14 times greater than one side of the blank," Jim notes. "My guess is that the convective airflow within the fins is very poor and that's why a heat sink alone didn't help."

The test results for the heat sink and low-speed fan are indistinguishable from those using the bigger fan without a heat sink, leading Jim to conclude that adding a heat sink means you can potentially get away with a smaller fan. But, as he points out, "Considering the additional trouble and expense of adding a heat sink, it's hardly worthwhile." As the graph illustrates, the heat sink and larger fan allowed the mirror blank to come within 2[degrees] C of the ambient temperature about 20 minutes sooner than with the fan alone. For me, this isn't enough of a difference to bother with acquiring a heat sink and dealing with the mirror-mounting complications that arise from such a scheme. Jim summarizes it best; "Fan 1, heat sink 0. Fun experiment though."

Gary Seronik has a collection of "cool" home-built reflectors. He can be contacted through his website, www.garyseronik.com.
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Title Annotation:Telescope Workshop
Author:Seronik, Gary
Publication:Sky & Telescope
Article Type:Column
Geographic Code:1USA
Date:Oct 1, 2012
Words:591
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