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Homemade Dew Prevention--Mylar?

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8 replies to this topic

#1 Phillip Creed

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Posted 29 September 2009 - 05:58 PM

I was at this year's Black Forest Star Party and as transparent as the skies were, I had problems with dew. Specifically, my 120ST was a real problem child, even with the dewcap that's included with the scope.

I got to thinking (dangerous as it may be; it happens)...what would happen if I just bought one of those reflective Mylar "emergency blankets" you see in sporting goods stores like Gander Mountain and cut out a layer and wrap it around the scope? Has anyone tried this?

Clear Skies,

#2 GlennLeDrew


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Posted 29 September 2009 - 06:24 PM

If you ensure that there is an airspace between 'scope and 'blanket', you should expect to retard the rate of cooling of whatever's being shielded. If the blanket is in contact with the tube, etc., I'd think its efficincy as an insulator will be compromized.

Even so, unless an extension is added to your dewcap, the IR flux radiating from the objective into the cold night sky will be as great as ever, and hence dew formation *may* be delayed for a bit. The best bet, in lieu of active heating, is to extend your dewcap by about double its current length.

Instead of a mylar blanket, how about a thin layer of some foam-based material. This is easier to work with in that it's perfectly OK to have it in contact with the 'scope. And it will probably better hinder dew formation on the objective than would mylar, whether with or without a dewcap extension.

#3 PitchHitter



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Posted 29 September 2009 - 08:04 PM

The thin mylar depends on reflecting back the thermal energy from a body.While it may slow some heat transfer from your scope, what I used was a mylar water heater blanked. It is double mylar with an insolation or air pocket sandwiched between. In extremes a dew heater is needed. Here are plans for the most efficient one you can find: http://www.intergate...r/dewheater.htm

#4 davidpitre



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Posted 30 September 2009 - 07:46 AM

Following with Glen's suggestion, I've found that integrating foam insulation surrounding the circumference of the objective with the dewshield can delay dew formation.

#5 StarStuff1



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Posted 30 September 2009 - 08:57 AM

Quite a few years ago when i was a "sophomore" amateur astronomer I attended a star party in the NC mountains. It was very humid and, of course, I had brought along batteries and a hair dryer to fight dew. A few feet away a guy set up a brand new AP refractor and wrapped a couple of layers of bubble wrap around the dew shield. It looked kinda odd but the guy said it would keeep the objective dry for over an hour. I asked him what would he do then? His answer was that he had plenty more bubble wrap dry and warm in his car. Dang if it didn't work.

#6 jayscheuerle



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Posted 30 September 2009 - 09:49 AM

You can buy dark-room mylar (black on one side, reflective on another) on ebay. I'm planning on using it for a shroud. - j

#7 G Smith

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Posted 04 October 2009 - 09:21 AM

Slow down a bit folks and consider the problem of Dew. There are mixed ideas in the above posts. Dew forms on a surface because it is COLDER than the air. There is a problem with scopes because they are in Radiation communication with the clear sky. It is beastly cold up there. The ground is warm, and so is the underside of trees. Consider the air neutral.

So, you do not need nor want Insulation, nor do you want to prevent Radiation to the ground. What you do want is to prevent Radiation from the Top Side of your scope to the cold sky, and you do want Radiation from the warm ground to your scope underside. So, just cover the top half of
your scope with Mylar film, which is readily available on the internet. I believe mine was intended for indoor gardening. The prevention of radiation is very efficient.

I have a steel tube reflector, so a few small magnets keep it in place. For a non-magnetic tube perhaps a rubber band on each end and a cord wrap in between would do fine. Or you could epoxy a magnet inside at certain
spots to attract an outside magnet.

Please get a Radio Shack non-contact thermometer that senses Infra-Red. You will be amazed at the indicated temperature difference when you point at a cloud, the clear sky, the ground, and the underside of foliage. It has
endless other uses in finding weak insulation in the house wall, frozen spot of a water pipe, a hot semiconductor on a circuit board, proper pancake skillet temperature, etc. You will want to cover any highly reflective surface with dark masking tape, or whatever to get a good reading.
Otherwise you will only get the temperature of a reflected object.

Good Luck, Gene

#8 Kight


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Posted 03 December 2012 - 12:57 PM

This may be a dumb idea, and it may not even work, but I'm wondering if some kind of hand warmers (the kind you shake and they last a couple hours) would work to place maybe under the scope. I'm trying to keep the dew off for as cheap as possible. Still the heater strips my be cheaper in the long run. I have an xt8i reflector.

#9 Steve Drapak

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Posted 03 December 2012 - 01:41 PM

I think the cost of the throwaway heaters might end up being more expensive eventually. The other issue is you might get hot and cold spots. I know there are reusable hand warmers that you can put in a microwave to recharge if you really want to try it.

A while back I picked up some NiChrome wire from eBay that can be plugged directly into a voltage source to generate heat. Just wrap it around the scope anywhere you want with tape or velco etc. I had it on a guide scope and even put it underneath a corrector plate on one telescope.
The amount of heat it gives off depends on the length of the wire and the rated wire resistance. Shorter wires get hotter (less resistance).
Different niChrome wires have different resistance per given length, so you need to figure out how long a wire you need and then determine the appropriate resistance. I know there's websites out there that can be googled with the details for resistance calculations.
Hope that helps.

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