Dew shield

[geordie85]

I looked everywhere I could for a black one but couldn't find one.

Sports direct do a blue one for £6. It's pretty much the same one as go outdoors have but it's £9 there

[Stu]

Maybe a bit obvious to others but why does a 200P need a dew shield?

Isn't the meter long tube in front of the mirror adaquate - they sort of come with their own dew shield built in. :confused:

On a refractor and an Mak/SCT I can see a shield in the form of foam tubing being useful but a why something that ends up as a 1.5 meter tube on a reflector ?

It prevents dew on the secondary and also cuts down stray light entering the tube and improves contrast.

[cjdawson]

When deciding the size for your shield.  The rule of thumb is that it should be 1.5 * as long as your optics are wide.  So if you have an 8" scope, you need about 12" of shield, if you have a 10" scope you'll need 15" of shield.  Don't forget to allow extra length to be able to fasten it to the scope.  Also make sure that it's ridged enough to be able to keep it's shape as if it flops, it'll block your scopes light path.

[Itabot]

I looked everywhere I could for a black one but couldn't find one.

Sports direct do a blue one for £6. It's pretty much the same one as go outdoors have but it's £9 there

No, I've had no luck, will have  a look in Sports Direct....thanks.  

Suppose sky blue will be ideal 

When deciding the size for your shield.  The rule of thumb is that it should be 1.5 * as long as your optics are wide.  So if you have an 8" scope, you need about 12" of shield, if you have a 10" scope you'll need 15" of shield.  Don't forget to allow extra length to be able to fasten it to the scope.  Also make sure that it's ridged enough to be able to keep it's shape as if it flops, it'll block your scopes light path.

noted......Thanks 

[omo]

If you drop FLO an email stating the outer circumference of your scope at the objective end then they should be able to tell you which one is suitable for your 200p. They didn't have one in stock to fit my Intes Mak so had to ordered me one in from the suppliers. Another classic example of FLO's awesome customer service     

Exactly what i did last week concerning a 200p dob, got a very quick reply from Martin at FLO 

And ordered a moonlight  and astrozap for a 7" & 8" MAK SCT today 

[Bathurst]

When deciding the size for your shield.  The rule of thumb is that it should be 1.5 * as long as your optics are wide.  So if you have an 8" scope, you need about 12" of shield, if you have a 10" scope you'll need 15" of shield.  Don't forget to allow extra length to be able to fasten it to the scope.  Also make sure that it's ridged enough to be able to keep it's shape as if it flops, it'll block your scopes light path.

I was searching the forums for this answer so just want to say Many Thanks for that I've looked at FLO but can't justify the price when I can make one so cheaply with a yoga mat!

[miafey]

Will a dew shield be sufficient? Or is a heated dew strip needed? 

[cjdawson]

From my experience (8" sct)  you'd benefit from having access to all three dew busting methods...

1. Dew shield

2. Dew heater

3. 12v hair dryer.

The shield is a given and will be all you need in the spring and summer. 

The heater on lower power, should be all that's required to get you through the winter, high power will get you through those few freezing nights in mid winter.

And the hair dryer will sort you out on those odd nights when you get caught out.

I decided for my dew heater to make my own, they're not hard to make.  I've realised that my heater elements are extremely powerful, so I only need to put them on at a low setting.  Each element is capable of drawing about 3amps of power and turning all of that into heat.  I think it works out that each of my heaters is capable of about 32watts.  I tend to run them at about 10% power so they normally only need about 3 watts to keep the dew off the corrector plate. The reality is that I'm pulling about 600mA for my heaters under normal conditions, maybe less.  (I need to check with a calibrated ammeter)

[Charic]

Would a fan alone creating an airflow keep the dew off your mirrors, I understand how the heating works, but does this not affect your viewing at higher magnifications. Surely there will be an issue with the mirrors being warmed from one direction?

[cjdawson]

With my heater, the element is a ring of nichrome wire wrapped in heatshrink, then threaded through a copper tube.  The tube is bent into a ring that goes around the corrector plate (The ring site just in front of it)  So the heating effect is from the whole ring, not just a part of it.

I think a fan would make the problem worse not better. A fan will cause airflow that will take away the heat from the scope causing it to fall below the fewpoint.  That will mean that dew will start to form on the scope.  The fan would continue to blow, so the dew would most likely gather and turn into a drop.

The whole point of a heating element isn't to warm the scope, but to create a warm layer of air that will stop the scopes optics from falling below the dew point.

[Charic]

Colin, my bad, I was thinking  Newtonian set-up, I did not read your signature, your requirement is slightly different!

Although I'm  still interested  in the viewing experience with your type of scope with the dew strip in operation. As you say, you need to warm the corrector plate to just above dew point, but too much heat can cause convection currents inside the scope, which will degrade the image. In this case what do we blame, the device supposed to cure the effect or the weather/seeing conditions?

I'm not criticising the technique, just trying to better understand 'how it works' and what the end user experiences.

[cjdawson]

Colin, my bad, I was thinking  Newtonian set-up, I did not read your signature, your requirement is slightly different!

Although I'm  still interested  in the viewing experience with your type of scope with the dew strip in operation. As you say, you need to warm the corrector plate to just above dew point, but too much heat can cause convection currents inside the scope, which will degrade the image. In this case what do we blame, the device supposed to cure the effect or the weather/seeing conditions?

I'm not criticising the technique, just trying to better understand 'how it works' and what the end user experiences.

First things first, I didn't think for a second that you were critising the technique.  Questions and practical experience are the only way to really understand a topic sometimes

The whole thing with using a dew heater is to get the most out of the scope on that night.  The key point is the amount of power that you put into the heater.  The idea isn't to heat the scope up, so much as to stop it from cooling below the dew point.  This key temperature is what will make the difference between perfection and opaque mist messing up your image.  The dew point is that magic temperature at which water vapour in the air will condense on a surface - 0.1 of a degree above means no dew, 0.1 below and you'll have problems.

The whole purpose of a dew heater is to enable you to keep the critial part of the optics above the dew point.  In reality, 1/2 a degree won't make alot of difference. So this is where the dew heater comes in.

The simple heaters (like mine) have a knob that you turn, that knob is responsible for setting the power level. So I can have it anywhere between low and high.

Some expensive heaters have thermometers and humidity sensors built in so they can predict the dew point. Those types don't need a control know, just and on off switch.

either way, the idea of a dew heater is to put just enough energy into the heating element to stop whatever it is heating from falling below the dew point.  The closer it can get the bettter, but never below.

In practical use, I've never seen a convection current caused by my heater.  It doesn't make that much heat.  Actually unless I put the heater on at full power, it does not get warm to the touch.   This is because the little bit of heat that is generated is conducted into the optics by the metal plate which holds the corrector plate in place.  (the heater is pressed up against it)

I have the same setup on my Refractor, but this time, it'll be conducting through plastic (I haven't seen how this operates yet as I've only just installed it)

On a newtonial, the dew strap would tend to go around the main tube rather than directly connected to the optics.  The idea of that heater is to stop the bottom of the tube from falling below, and therefore keep the primary mirror dew free.  Again, you don't need alot of power do accomplish this, just enough to stop the decline in temp.  So again, I'd expect that the heater would not create any convection currents.

When I said there won't be any convection currents, what I really mean is that there won't be any more currents, and they certainly won't be any worse than the currents that were already present as the scope was cooling to the dew point.

As for blame, well I don't like to blame, it's not really productive (it's a personal thing, just don't like the blame culture in any form).

So looking for points of failure.

1. open tubes (newtonian), corrector plates (Cat), lens (refractor)  can all be a major flaw when it comes to dew prevention.

To sot them, try covering the tube - with pcv sheet for an open tube.  Or a dew shield for both the cat and refractor.  (the PCV would work, but you'd need something to support it)

2. very humid weather.

The higher the humidity, the more likely things are to end up getting wet.

You can't really do anything about the weather.  but you can do things for the scope to protect it from dew.

As I said, I prefer to have a combination of things, using both a dew shield to lessen the heat radiating off the optics - this will stabilse the image quicker, and mean that the scope takes longer to dew up.

Then add a dew heater, with the shield I can run it at a lower power than I would have had to otherwise.

And finally have a 12v hair dryer at the ready just in case the heater was set too low - whilst this will create little hotspots and mean the scope will be a bit off perfect for a few minutes whilst everything evens out, it does mean that the observing session can continue when it would have been called off to dew.