Dew Zapping and Grounding Danger
Subject: Dew Zapping --part 1 of 3 
From: Mike Hamann <mikehamann jps.net>
I have used both a Kendrick dew shield and an Orion Dew Zapping in this area, and if I had to cast a vote
for either (or both), the Dew Zapping wins hands down.
First, I suspect I live in one of the wettest, but darkest places in the US -- the Northern California coast,
right next to the Pacific Ocean, halfway between 'Frisco and the Oregon border. Both myself and another observing buddy have
used C-8 and Ultima 2000 alt-az mounted scopes here. The dew shield is sometimes good for 60 - 120 minutes from setup to dew-down
(literally!), and so we both went to the Orion 20 watt, 12 v.d.c. Dew Zapper because the dew shield could only keep the corrector
plate dew-free for a limited time.
It seemed like when things got interesting, the dew would start its buildup. The time it took for the dew
was dependent upon the relative humidity, which is almost always 90% plus. The Dew Zapper never failed either one of us. The
only question in my mind is, "Does the 20 watts of heat applied to the corrector plate periphery degrade the image?"
The answer hasn't concerned me yet -- it was either get rid of the water (dew) or don't observe. I never noticed any obvious
degradation, but I was only using the scope visually -- no astrophotography/imaging involved. After the Dew Zapping is applied
to the periphery of the corrector plate, the Zapper barely feels warm to the touch; on the other hand, the Zapper on its own
(not attached to anything else) is quite hot to the touch.
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Subject: Dew Zapping --part 2
From: Bill Arnett <billanineplanets.org>
..I have seen plans for a homemade zapper somewhere on the web...
Mine is at: <http://obs.nineplanets.org/dp/dewpower.html>
Note: a new browser window should open over this one.
And at the bottom are links to a few others. Let me know if you find any more I should add to the list.
---------------------------------------------------------------------
Subject: Dew Zapping --part 3 of 3
From: Don Holcombe
After reading Philips Blanda's account of wrapping his AP 130 in foil, I coerced the wife into fabricating a Mylar wrap for
my 10" LX200. The material was purchased to make a 'Desert Storm' cover, but I had enough extra Mylar for this project.
I've attached a pic much like his description.
Subject: Dew Controller Schematic
From: Michael Sangster
My dew controller schematic is on my web site: http://members.aol.com/lx10user/astro/images/dewc.jpg
See the section on Dew. Note: should open a new browser window over this one.
Subject: Dew Zapping & LX200 Grounding Danger --part 1 of 2
From: Michael Gregory
If you are going to work outside with any device connected to the power grid, I would strongly recommend using a ground-fault-interrupter.
These devices (which are not all that expensive anymore) disconnect the power source if more than 1 thousandth (typically)
of an Ampere leaves the positive phase of the source and does not return to the negative side of the source. This is really
a life-saver if your dew zapper were to develop a fault allowing a current from the OTA through you to the earth (ground).
The 0.001 Ampere current would not kill you, and the GFI would disconnect the circuit when the fault was sensed.
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Subject: Dew Zapping & LX200 Grounding Danger --part 2 of 2
From: Chris Frye <cfryeix.netcom.com>
Date: Oct., 1998
For those of you unfamiliar with the Orion Dew Zapper, it is a rubberized band that is wrapped around the OTA near the corrector
plate. The AC model has a green ground wire coming out of the orange rubberized band. This wire must be connected to one of
your OTA accessory screws to prevent possible electrical shock. It has a 10 foot heavy duty line cord attached to it. The last
thing I want to do in the dark is to loosen one of my OTA's accessory screws and attach that green ground wire. I thought,
why not attach it permanently and make a provision whereby I can detach the 10 foot long AC line chord. Here is what I did:
I attached the Dew Zapper around the OTA and grounded the green wire to one of the OTA's accessory screws (don't use the anodized
one Meade supplies, it's an insulator, use a 6-32 x 1/4" stainless steel screw form the hardware store. I cut the AC line
cord about 5 inches from the "Dew Zapper" and used a male and female 3 pin microphone connector to be able to detach
the 10 foot line cord for OTA storage. The Radio-Shack numbers are: 274-010 & 274-011.
Additional notes:
Orion supplies a fabric wrapped AWG-18 cable capable of carrying 15A. This cable is very heavy and I don't think it will do
tracking any good. The "Dew Zapper" is rated at 25W (about .2A at 110V), this means we can use very thin and lighter
wire. On the other side of the 274-011 connector I used 10 feet of 2 strand speaker wire, to which I adjoined another strand
using micro wire ties. Speaker wire insulation is good for 250V for some reason, so it's safe. I only wish the microphone connectors
were smaller, together they weigh about 8 ounces.
Subject: : Dew Formation & Its Prevention --part 1 of 4
From: John Hopper <JohnLX200aol.com>
michaelschwarz.org writes:
>I have heard and read this in a couple of places and the statement confuses me. It says that dew forms on a scope when
the scope cools off faster the air.
>How does this happen? What causes the scope to cool off faster than the air. I thought the air surrounding the scope
is what determines the temperature of the scope. If it cools off faster than the air, how does the scope know when to stop
cooling off?
It's from radiational cooling. The clear night sky has an equivalent "black-body" temperature near absolute zero, sucking
heat out of you and your telescope in the same way that the high black-body temperature of the sun pumps heat into you: directly
through (mostly infrared, but really a distribution of wavelengths including visible in the case of the sun) radiation rather
than contact with air or anything else.
This is why frost can form on blades of grass when the air temperature and ground temperature are both above freezing. It
is also why the surface of the moon heats up and cools off, in the absence of air. Of course in that case, there's also conduction
through the soil too.
The good news is, it's also why a dewcap helps. It provides an air-temperature body rather than sky-temperature body for your
optics to look at over the majority of the scope's 180 degree "view" of the sky, cutting down the section of sky with a direct
look at your optics. By heating the dewcap just the right amount, it's also possible to have it radiating the same amount of
heat to the optics that the optics are radiating to space, hence leaving the optics at air temperature.
I won't get into all the equations, but the amount of radiational heat transfer between two objects is proportional to the
difference between the fourth power of each of their absolute temperatures, all other things being equal such as distance,
geometry, orientation, surface properties, etc. The three surface properties which also heavily determine the amount of heat
transferred are: reflectivity, absorption, and emissivity.
--------------------------------------------------------------------------------
Subject: : Dew Formation & Its Prevention --part 2
From: Michael Sparks <sparksmdzoomnet.net>
John Hopper's explanation is very good. I would like to add an additional comment to his.
Dew formation occurs on any object which has a temperature lower than the current dew point of the surrounding air. In other
words air can only absorb so much water (humidity). The amount of water it can absorb per unit of air varies by temperature
(relative humidity). The dew point is the temperature at which the air can no longer hold the water it has absorbed due to
cooling of the air. As a result you get dew, fog and frost. As the night air cools to its dew point objects in the night air
also cool to that temperature. As the air moves around the object the water condenses on the object. The water in the air likes
a solid surface or particle to condense on. That is why you see the dew on your scope before you see fog in the air.
--------------------------------------------------------------------------------
Subject: : Dew Formation & Its Prevention --part 3
From: Peter Dietrich
Every material has a special property called specific warmth capacity (direct translation of German expression) when theory
of heat is considered, which is defined by J/(G*K) Where is joule, is gram and is Kelvin.
In other words: joule per gram and Kelvin, e.g.: 1 J/(G*K) means you need the energy of one joule to heat up one gram of material
for one degree centigrade or for one Kelvin.
You certainly have made the experience that temperatures in areas near the coast are higher than in inland areas of the same
country and in the same season. The different warmth capacities (dwc) of water and soil are the reason.
Water has a higher dwc than soil, so water heats up slower than soil does, but that also means that water loses warmth much
slower than soil. In fact, the water of the sea warms the coast during night.
And now back to the scope. Metal has a much lower dwc than gas. From what was mentoined above, we can tell that metal heats
up very fast (good heat conductor) but also cools down very fast (faster as air).
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Subject: : Dew Formation & Its Prevention --part 4 of 4
From: Philip Freeman
I know this is getting beat to death, but I'll interject a few ideas (maybe it will help, maybe not). The corrector plate
will reach a steady state temperature when the heat flowing into the plate = heat flowing out of the plate (Qi = Qo). Heat
flow is Q = heat flow by conduction + heat flow by convection + heat flow by radiation.
Heat flow by conduction is done from the plates direct contact with the OTA. This is proportional to the difference in temperature
between the plate and the OTA. Heat flow by convection is done between the plate and the air. This is also proportional to
the difference in temperature between the air and the plate.
Heat flow by radiation is done between the plate, the air, the ground, and outer space. This however is proportional to the
difference in the fourth power of temperature, so the heat transfer between the plate and outer space DOMINATES this term.
You can set up the equations (found in a basic heat transfer text book) and solve for the temperature of the corrector plate.
Because of the radiation term, it is not only possible, but likely, that the equilibrium temperature of the telescope will
be below that of the air. Yes, the air is _still_ putting heat into the plate, but that heat is just being radiated out to
space. Heat hasn't stopped flowing, it's just reached equilibrium. If this equilibrium telescope temperature is below the dew
point, dew will form, even if the air temperature is still above the dew point.
So, what can be done to increase this equilibrium temperature is two fold, and well known. 1) increase one of the other terms
in the heat equation. A dew heater will raise the conduction term. A gentle breeze will raise the convection term. 2) Reduce
the radiation term. Use a dew shield. The dew shield now blocks the telescope optics from radiating out to space. It can be
as simple as a piece of cardboard. Now the radiation term becomes radiation with air, ground, space, and dewshield, with space
still dominating, but having less impact because there is less "view angle" now between space and the telescope. Trace rays
from the telescope's corrector plate to space without intersecting anything, before the dew shield you could trace rays all
the way around. This is a full hemisphere of view angle. Now with the dew shield you have only a tiny patch of space that is
part of the view angle. The corollary to this is: when not at the eyepiece, point the telescope at the ground. Now radiation
with space is zero (no view angle to space) and you can prolong the onset of dew.
Subject: Dew Prevention Inside Corrector Plate --part 1 of 3
From: Gene Horr <genehorrtexas.net>
Date: Dec 2002
Alan Voetsch wrote:
> Occasionally this time of the year I get a layer of dew on the inside
> surface of the corrector. Using a hair dryer takes forever and causes such
> a heat increase that star images go crazy.
Try a blast through the rear opening with no heat. Just getting the corrector to ambient will dry it out.
> I would like suggestions on how
> to permanently remove this problem. I use a dew-zapper on the outside and
> for the most part that solves that problem.
A dewshield will help a lot. The less exposure to the open sky the lower the radiation. That plus a heat strip on a low setting
should work fine. I work at close to 100% humidity almost year round and this works for me.
> What about inside the OTA? A couple 'Dri-Z-Air' packets?
The OTA is far from air tight. If you want to get really fancy a fan blowing air into the OTA would keep the corrector warmer.
But IMO the best solution is a long enough dewshield along with your heat strip.
----------------------------------
Subject: Dew Prevention Inside Corrector Plate --part 2
From: Kevin Wigell <kwemailtwcny.rr.com>
I've never had problems with moisture build-up inside my OTA, even when other LX200's right next to me have. Here's what I
do:
When I have the OTA inside (where the relative humidity is presumably low), I often leave the cover off the rear cell, with
the cell pointed down (so that dust is less likely to find its way inside). A few hours of this should allow dry air to permeate
the inside of the tube. Then, before I take it outside, I either put the plastic cover on or put the glass "UV" filter
on. Then, once I'm outside, I minimize the time that the OTA is open to the atmosphere. If I'm changing eyepieces or putting
on a camera or whatever, I don't leave the OTA open to the outside air any longer than necessary. I figure this keeps the dew
point of the air inside the tube below the ambient temperature.
Doing this, I've never had a hint of moisture inside the OTA, even while other LX200 users right next to me are complaining
about the same. Not only is the moisture build-up annoying, but over time it could promote mold. However, it will likely work
only if the telescope is in an air conditioned room.
If your scope is permanently mounted outside, I don't see anything wrong with putting some desiccant (see part 3 below) inside
with the tube all closed up. Just remember to take it out, or it may end up somewhere where you don't want it!
The reason my method works is that I do most of my observing in the spring through fall (due to near-constant clouds in the
winter where I live - central New York State), and my house has central air conditioning. So, with the A/C on, the absolute
humidity (i.e., dewpoint) inside would lower than outside. You are soooo right that the method I described would probably not
work well in the winter, when the absolute humidity indoors is almost certainly higher than outdoors.
----------------------------------
Subject: Dew Prevention Inside Corrector Plate --part 3 of 3
From: Gregg Ruppel <ruppelglSLU.EDU>
I use a method similar to what you have described. Use a standard 35 mm film canister; this fits snugly into a 1.25 inch eyepiece
holder. Punch or drill some small holes in the bottom of the canister. Go to a craft store and get some Drierite (anhydrous
calcium sulfate). This stuff is blue and turns pink when saturated with water vapor; it's sometimes used to dry flowers etc.
You might also search for it online. Put a piece of coffee filter or similar porous material in the canister and then fill
with Drierite: <http://www.drierite.com/>, they have online ordering.
Drierite is supplied as granules but contains some dust. The filter material helps prevent any dust from getting into the
OTA. Put the cap on the canister and insert into the 1.25 inch adapter whenever the scope is not in use. When the Drierite
turns pink you can put it in the oven to drive the water off, but it is not terribly expensive so I just refill the film canister
with fresh Drierite. If you can't find Drierite you could try to put some small packs of silica gel in the canister but it
doesn't seem to be as effective. Note: another source is: <http://www.sigmaaldrich.com/>
In either case, be sure to order "indicating" which is pink when dry, blue when wet.
Editor's note: for sources of heat tape, see Miscellaneous Topics-->Telescope
Storage.
Subject: Homemade LX200 Dew Zappers --part 1 of 7
From: Scott Baker <scottb80cts.com>
Date: Feb., 2001
I don't like the idea of the resistors so I built my own dew zapper using NiChrome wire, for my 10" LX200. I've been
meaning to put this up on a web page, but just haven't gotten around to it yet. Anyway, here's how I did it.
I first decided that 20 - 25 Watts was about the maximum wattage I'd need on the worst night. I also decided that simplicity
is better, and that a simple On/Off switch was all I needed to turn it on, no rheostats or electronic drive circuits. I also
decided that running it at 12 Volts was good if I ever wanted to power it from the car. I next measured the circumference of
my scope (about 36"). I then calculated the resistance needed to achieve 20 - 25 Watts at 12 Volts (Ohms law) R = Voltage
squared (12*12) divided by 22 Watts = abut 6.5 Ohms.
I then went shopping for NiChrome wire. I need a wire that would give 6.5 Ohms in three feet of length. NiChrome wire is rated
in Ohms per foot, so I was looking for approximately 2 ohms per foot. What I found was 4 ohm per foot wire, which is fine,
I just needed twice as much. This worked out very well, since now I could use two pieces of wire in parallel and two separate
switches to turn them on. One switch on = 11 watts, two on = 22 watts. I then connected an insulated wire to one end of each
of the two NiChrome wires (three wires in one connection). I then took some tubular nylon strap and had my wife sew up the
center of it, creating two insulated tubes. I then fed one piece of the NiChrome wire down one tube and the other NiChrome
and the insulated wire down the other tube.
I then sewed some Velcro to the tube end where the three wires come together, thus sealing that end. I then took the other
end, where the wires exit the tube and put them into a small plastic box that holds the switches. This box also has a wire
coming out the other end for my 12 volt input. Velcro on the box allows me to strap it onto the scope. I know this sounds a
little confusing and a picture is worth a thousand words, so If you'll email me, I'll take some shots of the dew zapper, and
email them to you. This is not a difficult job and can be done by anyone that can use a soldering iron. The toughest part is
finding small quantities of the wire without spending a fortune. I got 50' roll of 26 gauge (4 ohms per foot) from <http://www.omega.com/>
for $10.00 plus shipping and handling.
Now before anyone jumps on me for having heating elements in a meltable tube, let me say this, at 22 watts, maximum output,
the two pieces of wire never get hot enough that you can't hold them in your hand. It gets real warm, but not even close to
the melting point of nylon. This zapper works great, was cheap and fun to build. I even put two red LED's on the box to show
which element is on, not necessary, but looks cool.
---------------------------------------
Subject: Homemade LX200 Dew Zappers --part 2
From: Bruce Johnson
Since dew removers with NiChrome is being talked about, I might as well throw in something as well. Now, this is strictly
for permanent mounted scopes with AC available.
I took a broken toaster and used the wire from that, (NiChrome) and made a strip that would develop 20 watts off of 12.6 volts.
Then, I bought a dimmer switch that is used for dimming house lights for $3.00, and I also took an old Radio Shack 12.6 volt,
3 amp transformer. I just wired the dimmer switch in series with the transformer primary (wouldn't work at 12.6v). I laid the
strip of heat wire along a stretch of duct tape and covered it with another strip of duct tape. The duct tape goes around my
10" LX200 (OT) and I now have a dew zapper that can give continuously variable heat from 0 to 20 watts.
Before I decided to try the dimmer, I had made a small box with two switches in it. The switches were the kind with the 'center
off' type of toggle switch. By using the five combinations of the two switches, I had a setup that could give me five different
heat settings, from 7W to 20W. I just used the switches to short out more and more of the NiChrome wire, which then developed
more and more power.
---------------------------------------
Subject: Homemade LX200 Dew Zappers --part 3
From: David Sanders
American Science & Surplus (http://www.sciplus.com/) has heat rope
for sale. Their part number is 1490 and cost $1.95 each for a 27" long piece 3/16" in dia.
---------------------------------------
Subject: Homemade LX200 Dew Zappers --part 4
From: Robert Cadloff <bomosympatico.ca>
There's also Watlow (watlow.com), who make a huge range of heaters of
various types. Their silicone flexible heaters are perfect for the job. I managed to get a 1" x 30" sample. Here
is the direct link to a pdf brochure:
<http://www.watlow.com/literature/specsheets/files/heaters/colflx0202.pdf>
Note: should open a new browser window over this one.
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Subject: Home-Made Dew Zapper --part 5 
From: Wendel Burkhardt, Date: Feb 2001
Here is an update on a dew heater, that I have now completed, that is made from NiChrome wire from an old heating blanket.
I have tried building one several ways including using a potentiometer to allow variable resistance and have found an approach
that what seems to work best. Here it is:
For my dew heater I wanted several power settings. The levels that I eventually selected were 6.5W, 9W, 13W, 18W, and 28W.
The NiChrome wire from the dead heating blanket that I used had a resistance of 0.33 ohms/ft. In the heating blanket, each
heating "wire" contains 2 separate NiChrome wires imbedded in insulation. the insulation is flat and about 1/8 inch
wide and 1/16 inch thick. The two wires are insulated from each other, similar to what the old flat TV antenna cable was like.
To get the lowest setting, I needed a 75 ft. length of NiChrome wire. Higher power settings required shorter lengths of NiChrome.
Since, the wire is doubled, I used about 37 feet of cable, enough to do 14 wraps of my 10" OTA and then used a crimp connector
to join both wires at one end giving me a total length of 75 feet. For the other power settings, I spliced into one side of
the heating blanket cable, at the appropriate lengths, to get shorter lengths of NiChrome wire. At each splice, I used crimp
connectors to slice in a supply wire.
Prior to wrapping the OTA, I put a wrap of duct tape around the OTA, adhesive side out. I wrapped 2 layers of the heater cable
on this, 7 wraps to a layer. I used the duct tape to cover the wrapped heater elements. With a few more layers of duct tape
on it, it is pretty strong and holds its shape well.
To supply the power to heater cable, I used some extra CAT 5 Ethernet cable that I had available. This cable had 8 24 gauge
wires in it, which are thick enough to handle the 2.3-2.5 amps that the heater would draw at maximum power. I ran the Ethernet
cable back to a control box where I connected the wires to a 6 position rotary switch I purchased at an electronics store.
For a power cord, I used a cigarette lighter connector that I purchased at Radio Shack.
I find it works very well, is easily removed from my scope, and the control box doesn't heat up, which is the problem I found
when I tried to use a potentiometer to vary resistance. The duct tape protects the OTA very well and provides a very good enclosure
for the heating elements.
I plan to write this up in a bit more detail, with pictures, and put it on my web site when I have a few spare minutes.
---------------------------------------
Subject: Home-Made Dew Zapper --part 6
Dew Controller information and Instructions on Making a Dew Strip
From: David Moody <blckbeltbellsouth.net>
Date: Feb 2002
Previously, some of you had expressed interest in a dew controller that Ron Keating, a member of the Pontchartrain Astronomy
Society, had designed and built. This dew controller can maintain a user adjustable temperature above ambient, has 8 total
outputs - 4 temperature controlled and 4 set to 40% activation (this can be adjusted internally). It is compatible with Kendrick
heating strips (Or you can make your own like I have been doing. If any of you are curious, see my postscript).
Here is a link to a picture of the controller:
<http://personal.atl.bellsouth.net/b/l/blckbelt/interest.htmimages/DewBuster.jpg>
Ron calls this the "DewBuster". He told me to let you know about his web site: <http://personal.atl.bellsouth.net/b/l/blckbelt/interest.htm>,
you may contact him at <ron_keatingyahoo.com>
if you are interested in ordering one of these units. He said that the price, fully assembled, is $150. This is an excellent
controller that can reduce your battery drain, carry a higher load than a Kendrick, and will easily work with the more complex
dew control requirements (say for example, if you want to control dew on your Schmidt/Cass corrector plate, Telrad, eyepiece,
diagonal, finder lens and finder eyepiece).
Ron has put a great deal effort over the past year designing and refining this controller, testing it on several different
types of telescopes. Several members of the club now have these controllers and are actively using them in the field with solid
results.
I have no financial interest in this, but Ron is a terrific guy with a great deal of enthusiasm for this hobby. I have been
constantly frustrated with the dew problems down here in southern Louisiana and he has, with something that started out as
a small project, made a product that works really well. If this is something in which you are interested, drop him a line.
So you want to make a dew heating strip? It is very easy to do as long as you have a soldering iron and associated
tools, and a little patience. Ron showed me how to make these, and I learned a few other things in the process. I know there
are several other ways of doing this, but this way just turned out to be the most convenient for me.
The basic design for this homemade dew strip is a parallel circuit of 330 ohm 1/2 watt resistors (I would try to buy these
in bulk instead of from a place like Radio Shack, because you will need several. Radio Shack charges $.79 for 5 whereas I can
buy them in bulk for about $4-5 per 200). The general rule that seems to work best is to have one resistor every 1/2".
So, the first thing you need to do is determine how many you will need. Measure the outside diameter of whatever you are planning
on heating, say, a corrector plate. If you have a 10" SCT, the outside of the rim is a little over 37". That means
you will need 74 resistors for that strip. The connecting wires on either side should be 16 gauge copper wire or better (that
also goes for your connecting cable that goes from the dew strip to the controller, otherwise the cable will heat up, which
is not what you want to do.). On a smaller dew strip for an eyepiece, 18 gauge should be sufficient.
I made separate connecting cables, using RCA plugs on either end, that hook the dew strips into the controller. You could
just have a long extension going from the dew strip to the controller, but I wanted a little more modular design since I alternately
change from a 80mm f/5 to a 90mm ETX on top of my 10" SCT (an LX50).
Once you have everything laid out, wrap the ends of the resistors once around the wires. Your wires should be pretty close,
say 2 or 3mm to the body of the resistor, but not any closer so it is touching the body of the resistor since you will be soldering
those wrap points. I would wrap all of the resistor leads and then commence soldering, checking the distance interval between
each resistor before soldering. When you are done and everything looks neat and satisfactory, clip the excess leads and use
a fine file or emery board to smooth the solder joints.
Put a long strip of non-flammable, heat-resistant foam on what will be the back of the strip. This will help keep the heat
directed inwards towards the corrector plate. If you are from the Tool Time School of assembly, now is the time for Duct Tape!
Actually, however you end up covering your dew heater, applying a strip of duct tape over either side of the length of the
heater and then trimming to edge will help keep your heater together with the insulation and will provide a protecting layer
from or for the outer covering. If you want to put an outer covering (a light but high wear fabric, for instance) on, this
would be the time to do it. You will then want to attach a strip of fuzzy side Velcro to the back of the dew strip towards
the connection side. Then attach the hook side Velcro to the far end of the dew strip so when you wrap the strip around the
corrector plate, the hook side will lay face down on the fuzzy side and secure the strip.
Subject: Home-Made Dew Zapper --Part 7 of 7
From: Jim Talbot, Date: Sep 2002
I have a home-made dew heater for the corrector plate of my LX200 that may be of interest:
<http://frontpage.wave.co.nz/~jtalbot/dew_heater.htm>
Subject: Dew Shield Construction & Proper Color
From: John Mahony <jmmahonyhotmail.com>
A dew shield should be your first line of defense against dew. Using a heater without a dew shield requires lots of heat,
more than enough to create your own bad seeing.
Astrozap makes lighter flexible shields, but they have a black exterior, which is the last color you want on the outside
of a dew shield. Dark shields radiate heat rapidly at night and loose their effectiveness. See <http://www.dewshields.com/> for
a very effective, very lightweight dew shield. This is the only commercially available dew shield I know of that's made with
any consideration of the basic thermal issues involved. These are made of Reflectix™ insulation, available at most
hardware stores, with a layer of black felt on the inside, so you can make one yourself if you want. A reflective car windshield
sunblocker
with a layer of black felt added also works well. Ever notice how many other radiative insulators are shiny? There's a reason!!!
At least in this case since the 16" is white you probably got a white dewshield, but I can't believe other dewshields
are almost always dark color.
BTW make sure you get the above url dewshields.com correct. If you go to www.dewshield.com (note: not plural) it will just take
you to Astrozap again.
I learned in freshman physics that just as dark objects absorb light/heat fast in the daytime, they emit it fast at night
and cool rapidly, so the typically dark colored flexible dew shields are not as effective as they could be. A reflective exterior
makes a big difference. As an alternative you can also use a reflective car windshield sunblocker. These are also extremely
light, made of insulating material, with reflective foil on the outside, and just the right size to roll into a dewshield.
About $5.
Subject: DewBuster & Dew Formation --part 1 of 3
From: Mark de Regt <deregtearthlink.net>
Jun 2003
David Moody observed:
> You noticed that Rod and I have mentioned the DewBuster. That is an example
> of something a lot of folks use down here for dew heating. It applies a
> very tightly controlled amount of heat to the optics so they are only
> whatever number degrees above ambient that it takes to keep the dew from
> forming. This has two benefits: 1) It minimizes the heat being applied to
> the optics, thus minimizing its possible deforming effects, 2) It maximizes
> the efficiency of the system in terms of battery use.
David, I agree completely with your entire post, especially this part. During much of our year, on those rare occasions when
we are not clouded over, the dew is so heavy, I could not be dew-free for more than 1/2 hour, even with a very good, insulated
dew shield. Adding the DewBuster has made a great deal of difference. I have to apply very little heat, but it is the difference
between being able to stay out all night, and having to either blow-dry my corrector plate every half hour, or pack it in.
Unlike other products, the DewBuster actually has a thermostat, and carefully regulates the amount of heat applied. One cannot
feel any heat; it just keeps the corrector plate however many degrees warmer than ambient you tell it to. My scope was pointed
almost straight up all night last night; dew was collecting in pools on my observing table, but my optics stayed clean all
night.
Check the product out at: <http://www.dewbuster.com/>
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Subject: DewBuster & Dew Formation --part 2
From: Bill Dougherty <bd5572yahoo.com>
The issue of where to place the corrector heater caused a heated debate on the LX200GPS list a while back. What Bill describes
below is exactly the position recommended by Ron Keating on his Dewbuster web site. He and Bill both reason that heating the
upper end of the tube will actually discourage thermal currents inside the tube and heat the corrector evenly over its entire
inside surface, not just around the edge.
I have my heater just behind the front mounting blocks of my upper and lower Losmandy rails. Even here in the soggy Northwest,
I set my Dewbuster on low and forget it. I have no dew problems on the optics and have not noticed any tube currents either.
I suggest trying out different placements of the heaters until you find the best arrangement for your scope and conditions.
Bill wrote:
Not sure I agree with you on heating the corrector from the edge or center. I apply my heating strip to the OTA of my LX200GPS
about an inch behind the corrector. As best I can tell, it warms up the air in the tube which helps keep the corrector evenly
heated. Sometimes I will have temp temperature differential on my DewBuster set too low and dew will form. When this happens
and I turn up the temperature differential, the dew goes away uniformly across the corrector plate, not from the outer edge
of the corrector first. At least that is my impression.
I know that those far wiser than I can explain the problems with heating the corrector plate, but it seems that by putting
the strap on the OTA, vice the edge of the corrector, and setting it to the minimum required temp differential you should not
distort images. By putting the strap in the metal tube it warms up the tube, the air inside it, and the corrector evenly as
far as I can tell. Not sure about air currents in the tube. Guess I always assumed that they were minimal and would not impact
the image. Am I wrong on that?
As for power consumption, this is where the DewBuster shines. You tell it how many degrees you want it to keep the OTA over
ambient and it adjusts power accordingly. For my 8" LX200GPS I usually only have to set a 5 degree temperature differential
to keep dew away--and that is here in the swamps of Louisiana. The DewBuster comes with a sensor that detects the ambient air
temp and the temp of your OTA. It measures the difference then applies enough juice to maintain the temp differential you want.
The jump start battery I use can easily power both my scope and the DewBuster for over two nights without a recharge. When
I ran my Kendrick and scope I had to recharge the battery after one night's use. That may not be scientific, but it convinced
me that I was using less power.
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Subject: DewBuster & Dew Formation --part 3 of 3
From: Bill <gatorchaseratvci.net>
Sounds like you have it as bad in Ireland as we "dew" in Louisiana. I am wondering if you don't have a problem
with your heating strap since it's not keeping your corrector clear. I have heard that the internal connections to the straps
can
sometimes come loose and interrupt current flow. Might want to check that out. Otherwise, your Kendrick should work. The one
I had worked great down here in the swamps. Except for the amount of power it consumed I liked it. That's why I got the Dew
Buster instead.
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