[AZ-Observing] Re: Dew Shield
- From: gene lucas <geneluca@xxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 25 Mar 2005 15:57:10 -0700
Jeff,
The aluminum flashing idea should work well and will certainly be more
economic than the corresponding commercial dew shield units. All
conductive (metal) objects radiate heat to the sky at night (presuming
it is clear) and thus stay colder than the surrounding air. If colder
than the "dew point", moisture will condense. The dew shield works by
reducing the sky angle that the front of the telescope is exposed to. I
prefer using some sort of insulating material such as stiff plastic, as
the metal will continue to chill the air in front of the telescope. In
moist conditions, even a paper grocery sack formed into a tube can be
pressed into service to reduce dewing.
At the SAC meeting tonite I will share with you (and another others
interested) some details of the anti-dew heater I installed on my Meade
SCT years ago. It uses a coil of "nichrome" wire (actually thin gauge
stainless steel aircraft "safety" wire) in plastic insulating tubing.
It draws about 10-15 Watts of power (about 1 amp) at 12 VDC and keeps
the dew off the corrector plate by warming the air slightly. Actually
it will evaporate any dew if it has already formed.
Gene
(17250)
Jeff Hopkins wrote:
>For those with SCT's, even though Arizona is mostly very dry, there
>are wet times and there are times when there is dew. Dew is a real
>killer for photometry with a SCT, but can also degrade visual
>observing. If you have looked at commercial dew shields you may have
>suffered sticker shock. They vary from $35 to over $150.
>
>Rather than spend my limited financial resources on a commercial unit
>I decide to make my own. For under $1 you can have a sturdy,
>lightweight and effective dew shield.
>
>To make one or several, go to a hardware store, e.g., Home Depot, and
>purchase a small 12" wide roll of aluminum flashing. This can make
>several dew shields. Measure the front circumference of your SCT.
>Just use a string if you wish. Layout the string/measurement on a
>strip of the flashing. Mark it about an inch larger. Use a paper
>cutter (board with big knife cutter) to cut the flashing and you will
>get a nice square even cut. Now wrap the flashing around the front
>end of the telescope and mark where it over laps. Make sure there is
>about a half inch over lap. If too much, trim on the paper cutter.
>Now comes the hard part. Clamp one end so the diameter is that of the
>telescope for a snug fit. Drill an 1/8" hole near the bottom of the
>overlap and use an 1/8" pop rivet with washer to secure it. This will
>be the top end of the shield. Now do the other end, but make sure the
>hole is up an inch or so from the bottom so you have a tight fit on
>the telescope at that end of the shield. Drill and pop rivet again.
>Do at least one more in the middle of the overlap or if you want,
>make a total of 5 evenly spaced holes and pop rivets. Now spray the
>inside with flat black paint. Spray the outside whatever color you
>want or leave plain. If 12" is not long enough (12" should be fine
>for AZ), just add another section on the first. This will produce a
>ridge and light weight low cost dew shield. I've been using one of
>these for a while and it works well.
>
>Jeff
>
>
>
>
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