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Dew Advice

Started by plunkett, 01/15/2005 07:02AM
Posted 01/15/2005 07:02AM Opening Post
Hi All:

I'm sure this has been asked before, but it's one of those perennial "winter" questions... When I come in from a cold night, should I put the lens caps on my refractor and Mak-Cass? I remember seeing something about leaving the caps off and letting the optics equalize, which I have been doing for a while, but saw something recently (SOMEwhere) that said it's better to "cover up". Which do you advise???

Posted 01/15/2005 11:02AM #1
You want to keep the cold lens or mirror or other optical surface from contacting the warmer, and therefore moister air. Absolutely put the lens cap on at the very least! Some wrap the OTA in plastic bags but the main thing is to keep moist warm air from contacting the optical surfaces!

After the equipment has nearly reached indoor ambient temperature, take the cap off to assure complete drying, add a desicant pack in the eyepiece diagonal to suck any residual moisture out of the tube for complete piece of mind.

Enjoy those cold clear nights with no worrys!!
Posted 01/15/2005 11:17AM #2
If possible, take the scope into the garage or some other cooler and drier place before taking inside the house. This helps a lot to avoid the condensation.

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 01/15/2005 12:38PM #3
This was recently discussed on the Talking Telescopes forum. The advice from the experts over there was to leave off the caps when bringing in the scope and eyepieces.

Hugh Bartlett
Oakland, CA

"Praise the Lord for the expanding grandeur of creation, worlds known and unknown, galaxies beyond galaxies, filling us with awe and challenging our imaginations." 2007 Reform Siddur
Posted 01/16/2005 04:52PM | Edited 01/16/2005 04:55PM #4
Let me first offer a mini-lesson on humidity so that will help you understand the basics of condensation. Humidity is measured as a percentage of how much water vapor the air can hold at a given temperature. At 100% humidity, water starts to condense out of the air. As air cools, it can hold less water vapor because there is less heat energy available in the air to keep the water vaporized. Now, imagine you have a closed vessel containing air at 50% humidity. Now imagine dropping the temperature of the vessel. As the air cools, it is capable of holding less and less water vapor before it condenses out, whereas the total amount of water in the cooling air remains constant. Thus, the relative humidity starts to rise. As the temperature continues to drop, the humidity in the vessel will eventually reach 100% humidity and the water will start to condense out of the air onto cold surfaces. More and more water will condense out as the temperature keeps falling.

Now imagine that we drain off the condensed water and heat the air in the vessel back to its original temperature. Since much of the water was aready removed as condensation, there is now much less water present in the vessel than used to be exist. Thus, the relative humidity level in the vessel once it reaches its original temperature will be much less than the 50% it started with. This implies that capping the telescope prior to bringing it indoors will keep moisture away from the optics (much of the moisture already condensed out of that air when it was colder). As the trapped air heats, it will dry out considerably. This is why the air in a heated house gets very dry without a humidifier, and why grandma used to put a pan of water on top of the radiator.

Let us now itlook instead at what happens if we bring the telescope into a warm house without capping it first. We have a very cold mirror/lens that is suddenly exposed to warm air at perhaps 50% humidity. If we now cap the telescope trapping the warm, moist air of the house, the cold telescope (especially large heat sinks like optics and metal) will remove energy from the air and moisture trapped by the covers, thereby causing the moisture to condense on the cold surfaces. Further, because the telescope is now capped, this condensed moisture cannot escape as vapor as the telescope gradually warms back up. Once the telescope is brought into the house, the harm is already done and it is too late to cap it.

You should also be able to see why capping a telscope indoors, then taking it outdoors (or into a cold garage for storage) can result in condensed moisture being trapped inside the telescope.

Cooling air presents a condensation problem, not heating air. If we cap a telescope after bringing it indoors, the trapped warm air inside the telescope begins to cool and its relative humidity level begins to climb. But if we first cap (i.e. seal) the scope prior to bringing it indoors, the cold air from outside begins to warm and dry out.

I hope this helps.