University Lowbrow Astronomers

TO DEW OR NOT TO DEW

by Doug Scobel
Printed in Reflections:  October, 1996.

You’re out at your favorite dark sky site.  The night is perfect.  A cold front has just gone through, leaving the air cool, clear, and wonderfully transparent.  There are so many stars that you have trouble making out the constellations.  “Awesome!”, you say to yourself.  You begin observing, and cannot believe how much you are seeing.  Suddenly you blurt out “Someone give me an Old Milwaukee - it just doesn’t get any better than this!”.  But, after a while you begin to notice that you can’t seem to see much through your finder, and the images through the eyepiece start looking a little fuzzy.  No, a LOT fuzzy.  Something is horribly wrong.  What’s going ON?!  OH NO!!  DEEEEWWWWWW!!!!!!!

Sound familiar?  It does to me (ask Doug Nelle or myself about Astrofest a couple of years ago).  Very few things can frustrate the dedicated observer like dew (except maybe mosquitos, but that’s another story).  It can creep up on you and spoil an observing session big time.  But, with a little preparation and inexpensive equipment, you can keep that nasty wet stuff away and observe all night if you wish.

Keep it Covered!

The first thing to remember is that dew only forms on surfaces that are exposed to the open air.  So if you are not using it, keep it covered.  Keep your eyepieces in their case while not being used.  Cover your finder while you’re not using it.  Chris Sarnecki puts a knit cap over his Telrad while not in use and it stays nice and dry.  Also, be careful that your breath does not get near the eyepiece while you’re observing.

Dew Caps/Shields

Other things are in use all the time and cannot be covered.  Things like SCT corrector plates, Newtonian secondary mirrors, and refractor objectives.  But you can cover them sort of.  Dew shields are simply an extension of the telescope’s tube beyond where it normally ends.  It reduces the degree to which the corrector or lens is exposed to the sky.  A dew shield can be made of most anything - plastic, waterproofed cardboard or poster board, metal, or foam rubber.  You could even tape on a paper bag in an emergency.  The thing to remember is that the greater the insulation factor of the material the better.  That makes foam rubber good; metal bad.  Also, it is important that it extend AT LEAST one tube diameter’s worth beyond the end of the telescope.  The drawback is that dew shields usually don’t work forever - they simply put off the inevitable, especially under damp conditions which are frequent in Michigan.

Electric Dew Chasers

If you really want to keep the dew away indefinitely (or at least all night), the best way I know of is to provide a little electric heat where you need it.  You can buy such devices commercially, but you may wish to construct your own for a lot less money.  All you need is a voltage source (a battery) and something to provide a little electrical resistance to in turn provide some heat.

First you need to know how much heat is enough.  I have found that about one watt is more than enough for finders and eyepieces.  If you have an SCT or a refractor, then you’ll probably need more for the corrector plate/objective lens.  But, once you have determined your desired power, how do you figure out how much voltage and resistance you need?  We’ll do a little high school algebra to figure it out.

First, we’ll start with Ohm’s law

E (volts) = I (amperes) times R (ohms),

and the power equation

P (watts) = I (amperes) squared times R (ohms).

Since we have two equations and two unknowns (I and R), we’ll get rid of one (I) and solve for the other (R).  Solving for I in each equation yields

I = E / R

and

I = Sqrt(P / R )

Next, we’ll set them equal to each other,

E / R = Sqrt(P / R )

Square both sides,

E2 / R2 = P / R

And viola!

R = E2 / P

There you have it.  Simply settle on a voltage and power, and you know how much resistance you need.  For example, suppose you want to get two watts out of twelve volts.  The equation gives

R = 122 / 2 = 72 ohms

But how much power do you really need?  Not much.  All you want to do is keep the heated surface from falling below ambient temperature.  Too much heat will distort optics and cause air currents that will deteriorate your images.  I have found that using one watt on my finders is more than enough.  If I forget to turn them on until after the lenses start to dew up, they’ll be clear after five or ten minutes of operation.  I don’t really know what you’d need for say, an eight inch SCT corrector plate, but I suspect three or four watts would be plenty.

Practical Matters

I decided that on my scope, I wanted to supply one watt at twelve volts.  Putting those values into the equation gives 144 ohms.  Since the NiChrome wire I used was around 150 ohms per foot, all I needed was about a foot of it to provide a watt.  Once around the finder objective, twice around the finder eyepiece, and a foot serpentined around my Telrad window is all it took (all wiring was done in parallel).

I used NiChrome heater rope I bought at Astrofest for a dollar a foot.  It’s nice because it’s insulated.  If you cannot find NiChrome wire, you can also use resistors.  Just figure out how much resistance you need, and wire a few in series so that they are spaced evenly, and add up to the total resistance.  Be sure to take the power rating into account when deciding which ones to buy.  The wire/resistors can be taped, glued, or wrapped wherever needed.  Naturally, be careful to insulate well any bare wire.

I use a 4 amp-hour, 12 volt, rechargeable, lead-acid gel cell to power them.  All three loops together only draw about a quarter of an amp, so I can easily run it continuously all night if I need to.  I use the small battery because it rides along on the scope.  If you don’t need an “on board” battery, a full size deep cycle lead-acid battery is ideal.  Besides powering your anti-dew system, you can run fans, drives, and most anything else off of it.  Another good choice would be the batteries made for those motorized “Power Wheels” cars made for kids.  They are designed for physical abuse and repeated deep discharge/recharge cycles and should prove to be quite durable.  You could also use a couple of six volt lantern batteries, but that may get expensive since they are not rechargeable.  Keeping a couple on hand for an emergency might be a good idea, though.  You might even be able to make a battery pack from a few “D” size rechargeable NiCads.

To prolong the life of any lead-acid type battery, don’t discharge it all the way unless it is designed to be (i.e., unless it is a deep cycle battery).  I know, because I have killed one gel cell battery already.  (If you use a gel cell, be sure to recharge it with the kind of charger that turns itself off when the battery is fully charged.  Gel cells are very susceptible to over charging.)  Conversely, NiCad batteries NEED to be fully discharged before recharging.

I have also added a dew shield to the end of the main tube, as the secondary mirror would sometimes dew up.  The shield is made of Kydex, and is held in place with hook and loop fasteners (Velcro TM) for quick and easy set up and take down.  I have never had a problem with the secondary dewing up since I began using it.  It also has the added benefit of preventing ambient light entering the field lens of the eyepiece, increasing contrast.  I have also seen telescopes with heaters behind their secondary mirrors to keep them dry.

To Dew or Not to Dew

So, now you know what to do to not dew.  With a little bit of inexpensive equipment, dew will never again ruin your observing sessions.  If only it was so easy to take care of those pesky mosquitos!  But that’s another story.

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Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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