16 replies to this topic

[AnotherMoneyPit]

Thanks in advance.

 

I've read a lot of comments here saying you need both (in areas / times / seasons where due is a problem), but still don't know why.

 

Wouldn't the shield just be extra weight and surface area (that the wind could act on) you don't necessarily need with a heater?

 

Is the only reason for the shield to reduce the power requirement for the heater? If my power budget allows it, would running just a heater make more sense?

[Stellar1]

I would guess that if you were to just use a heat strip in certain areas the dew would eventually overwhelm the heater. A dew shield is the first line of defence against dew, you use heat if and when you need.

[BrentKnight]

Depending on conditions, dew will still likely form even with a dew shield.  A dew heater will prevent it and the shield will allow it to work more efficiently.

 

The dew shield also provides some protection from nearby ambient lighting...

[daveco2]

Dew can get on an optic two different ways.  The first way is by forming droplets in the atmosphere above the telescope and then dropping out of the sky like rain.

The second way is condensing out of the atmosphere when it contacts the cold optical surface.

 

A dew shield will help with the first method, and a heater strip will help with the second method.  

[AnotherMoneyPit]

Depending on conditions, dew will still likely form even with a dew shield.  A dew heater will prevent it and the shield will allow it to work more efficiently.

 

The dew shield also provides some protection from nearby ambient lighting...

So assume I either don't care about the added protection from ambient lighting, or my concerns regarding added weight / surface area outweight concerns about ambient light.

Is there a reason the rule you just mentioned can't be reversed?

Why can't the heater be the first line of defense? Or maybe the only line of defense in at least some circumstances?

Will it simply not work at all without a shield? Or does it introduce other problems that a shield alone would not?

[AnotherMoneyPit]

Dew can get on an optic two different ways.  The first way is by forming droplets in the atmosphere above the telescope and then dropping out of the sky like rain.

The second way is condensing out of the atmosphere when it contacts the cold optical surface.

 

A dew shield will help with the first method, and a heater strip will help with the second method.  

That makes perfect sense! Muchas gracias.

[BrentKnight]

AnotherMoneyPit...

 

What type of telescope are you working with?

[WadeH237]

Why can't the heater be the first line of defense? Or maybe the only line of defense in at least some circumstances?

Will it simply not work at all without a shield? Or does it introduce other problems that a shield alone would not?

Hi, and welcome to Cloudy Nights!

 

To understand the answer, it helps to know a bit about how dew forms.

 

The short answer is that dew forms when the temperature of the air is too low to maintain water vapor in gaseous form.  When that happens in the sky, you get clouds.  When it happens near the ground, you get fog.  And when it happens at the surface of a solid object, you get dew (or frost).

 

So why, then, can you get dew or frost when the air is perfectly clear?

 

The answer is that the space is cold.  Really cold.  Like 270C below freezing cold.  If you expose an object to the night sky, heat from that object will radiate from warm to cold.  This is called night sky radiant cooling.  That process can easily bring the surface temperature of an object to well below the temperature of the ambient air.  Once the surface of the object gets cooler than the dew point, dew or frost quickly forms.  If you can shield the object from the sky, it helps to prevent this process.  This is why dew is rare on cloudy nights (since the clouds act as an insulator).  It's also why frost will form on a car windshield in the driveway, but not on a car in a carport, even though they are the same temperature.

 

So a nice, zero heat solution would be to block your scope's view of the sky.  Of course, you can't observe when you can't see the sky.  So a compromise solution is to extend the tube past the end of the scope.  Depending on the where the scope is pointed, this can help quite a bit.  For example, if you are looking at an object that is 45 degrees up, the dew shield will block the sky at the zenith, where most of the radiant heat goes.  But since you can't completely cover the scope, dew can (and does) still form when conditions are right.

 

So what if you could keep the temperature of the front of the scope just a few degrees above the ambient air temperature?  It turns out that this is a very effective way of preventing dew, but it does lead directly to your question.

 

To answer that, know that warming your optics causes a few problems.  First, if you have heat rising from the lens, or corrector plate, or meniscus, or primary mirror (depending on what scope you have), it will disturb the air directly in front of the scope.  This makes the image at the eyepiece less sharp than it should be.  And second, heat actually changes the shape of the optic, introducing distortion.  Also, if heat causes the optic to expand.  Depending on how the optic is mounted in the scope, it could stress it, causing further aberrations (we call this one "pinched optics").

 

So a complete solution is generally composed of 3 parts:

 

• A dew shield as the first line of defense.  A good rule of thumb for the length of the shield is about 1.5x the diameter of the scope.  So if you have an 8" scope, you would want a dew shield that's at least 12" past the front of the scope.
• A dew heater as a second line of defense, to prevent heat loss that the dew shield doesn't block.
• A smart controller for the dew heater.  The ideal situation is to have some mechanism that keeps the temperature of the front of the scope to just a tiny bit above the ambient temperature.

I have a couple of dew prevention systems that incorporate all three of these elements in different ways.  The difference in my systems is the controller.  I have an older dew controller that works with two temperature sensors.  One of them is in contact with the front of the scope, near the lens.  The other is held off the scope, so that it can measure the ambient air temperature.  The controller works by comparing the two temperatures and adding just enough heat to keep the lens above ambient.  The second system only uses single temperature sensor, for the ambient air temperature.  But it also has a hygrometer to measure humidity in the air.  It then does some math to calculate dew point and uses an algorithm to vary the current to the dew heater to keep it at an appropriate temperature.

 

Both systems have been 100% effective for me at preventing dew, no matter how humid the night.  For purely visual use, the older controller is ideal.  I have the newer one because it integrates with my imaging automation.  But for each system, there have been plenty of mornings where the scope was literally dripping wet with dew, with the lens completely clear.

 

Oh, and there is one more reason to couple a dew shield with a heater.  It takes just the slightest puff of breeze to move air across the front of the scope.  If your only defense is a heater, blowing the heat away from the scope is not what you want.  A dew shield helps to keep the air at the front of the scope still.  This keeps the [small amount of] heat right where you need it.

 

I hope that this helps to clarify things.

[dnayakan]

I’ve always felt the primary purpose of a dew shield was the same function as a lens hood on a camera lens - to keep extraneous light from contributing to flare within the lens cell and reducing contrast. The fact that it forms a first line of defense against dew is a bonus (basically, it reduces the angle subtended to the sky to reduce radiative heat loss and any moisture forming in the slightly warmer air above the lens is likely to settle on the dew shield instead of the lens surface). A dew heater more actively combats radiative heat loss by supplying a small amount of heat to the lens cell to replace the radiative heat loss.

Cheers, DJ

[sellsea]

Seeing is a heat transfer term that defines the amount of radiation between two bodies.  The dew shield reduces radiant seeing, because the objective lens is exposed to less sky.  This means radiant cooling is slowed, but not prevented.  That's where the dew heater comes in.  Theoretically, a dew heater can prevent dew formation without a dew shield, but the dew heater's temperature would have to be increased, which could lead to other issues like convection currents and differential thermal expansion (pinched optics).

[Antny777]

Hello there I am new to CN . Is there a difference between a flexible dew sheild and an aluminum dew sheild as far as what they can do? Would one be better than the other? 

[rrpallechio]

Hello there I am new to CN . Is there a difference between a flexible dew sheild and an aluminum dew sheild as far as what they can do? Would one be better than the other? 

I bought a flexible dew shield for my 8". I didn't like it, it seemed like a hassle getting it on and off. Probably just me. So I bought the Celestron aluminum dew shield. Holds its shape, is easy to get on and off, and on cloudy family movie nights doubles as a popcorn tub

[kathyastro]

It is like asking why a house needs both insulation and a furnace.  One adds heat; the other prevents heat loss.

 

Yes you could get by with just a heater.  But because there is nothing slowing the heat loss, you would need a bigger heater that will consume more battery energy.  It would also cause thermal stresses in the optics and internal seeing problems.

 

And, just as sometimes the furnace doesn't have to come on in a well-insulated house, sometimes a dew shield is all you need on the scope.

 

The two work together.

[Redbetter]

So assume I either don't care about the added protection from ambient lighting, or my concerns regarding added weight / surface area outweight concerns about ambient light.

Is there a reason the rule you just mentioned can't be reversed?

Why can't the heater be the first line of defense? Or maybe the only line of defense in at least some circumstances?

Will it simply not work at all without a shield? Or does it introduce other problems that a shield alone would not?

A properly configured dew shield will reduce radiant cooling (to space) by several fold.  So in mild/moderate conditions, a dew shield alone will do the job.  But when dew is heavy, a heater system can account for the final fraction.  Without a dew shield, you will need several times as much energy input to do the same job, and the heater strips might not be up to the task.  If nothing else, you will have to run them harder, with a larger delta T from edge to center.  Pushing edge heating more could cause some degradation of the image.

 

When I was observing in East Texas with an SCT I only used a dew shield.  I never had any strip heaters (still don't.)  With some forethought I could generally keep the corrector from dewing or frosting up.  Some of that involved taking breaks and pointing the scope at warmer things during the breaks.  

 

A home made Reflex insulation wrap/dew shield has worked pretty well on the Mak here in heavy dew conditions in the backyard during the wetter months.  (I used black paint on the inner portion that projects as a dew/light shield.)

 

For the 20" Dob I finally resorted to a secondary heater with built in controller, since dew or frost can be an issue at times.  Fortunately, it is rarely needed at my higher altitude site.  I have started using a big light baffle on the back side of the upper tube assembly, which should also function as a reasonably effective barrier to the largest source of radiant loss from the secondary--it is a geometry thing.

[AnotherMoneyPit]

I hope that this helps to clarify things.

IMMENSELY!!!! Thank you so much.

 

Thank you to everyone who's chimed in, this thread has been a huge help to me and a great education.

 

Another question, since it's related...

 

Based on this thread, I ordered a dew shield, heater ring, & controller (all Celestron).

The controller has the option to mount to the OTA's dovetail forward of the mount, under the nose of the (6")SCT.

As things stand now, the ota is very much rear-heavy in the mount, pushed as far forward as it can go.

If I were to add the controller in the aformentioned position, I believe it (and the dew shield when used) would help balance this assembly.

With all of it added to the OTA as described, I'd be right at 1/2 the rated capacity of my mount (AVX). About 15 pounds total on a 30lb rated mount.

Alternatively I could mount the dew controller off the OTA, and save some weight, but then I lose most of the counter balance I was hoping to add, and the wiring isn't as clean.

What would be the best way to go?

[Mike Q]

Whether you can use a dew shield and have it work is based on where you live.  In my neck of the woods a shield only delays the inevitable.  For me a dew heater is almost mandatory.  Now I run both a shield and heater.  The shield is more for ambient light and since it covers the heater i can run it at a lower power setting.  Its a win win  

[Napp]

It all depends on local conditions whether you need just a dew shield or a dew shield and heater for SCT’s or refractors.  Here in normally high dew point Florida a dew shield alone only delays the inevitable by a very short time.  As to a dew heater alone, one night I forgot the dew shield for my 8 inch SCT.  I tried to operate with just the heater.  The heater kept the outer inch or so of the corrector clear.  The rest was dewed over.