Dew shields do they work

[todd8137]

My sct 9.25 is a dew magnet when out side dew shield and heaters are a must many a great night is killed,on the hair dryer front be carefull these will dry stuff on to the glass and is hard to get of.

Pat

[Luke]

I'd recommend heater strips if you can squeeze in the cost. They are not cheap but I'm so glad I got some, you can spend hours and hours imaging and with dew heaters it's one less thing to worry about going wrong! (unless like me you forget to turn up the heat enough one time!...)

[Delboy_Hog]

Seems mad doesn't it... We have to wait for scopes to cool down to ambient to get best performance. And then we warm the front to keep dew off!

I think the mad part is that we're out in the dark in these cold damp conditions in the first place!

However, this does raise an interesting point, and while I guess the answer lies in the number of people who are pro-heated strips here, I curious - would having a source of heat not create that awful shimmer that you get from having anything warm in the line of sight? Or do the heated strips not get so toasty as to cause this problem?

I recently got an Astrozap dew shield for a birthday prezzie and it makes a huge difference, though, as has already been pointed out, spending a long period of time on the same object, especially if it's high in the sky, will probably still encounter dew problems.

[SniffTheGlove]

Thanks for the calculation. I am no scholar, but I'm beginning to realise that it's not hard to build these things yourself with basic electricity knowledge. Already looking at DIY parts.

Speaking of dew and frost. I actually noticed something this morning (since Stockholm got its first hit of morning frost today). My car windows were all frosted over. Except a small patch underneath an aerodynamic part I have above my back window. This indicates that frost really do seem to fall from above, which in turn seems prove the concept of a dew shield.

Here is an Excel file that I have found on the Net with some modifications I have made to calculate heater strips in Parallel or Series. Hope this helps. Just put in the dia of where you want to put the heater and it will show in the greyed out area the specs.DewDesignHeaters.xlsx

[Cath]

Dew shields certainly do work! .. they do on my little SCT anyway. Without the dew shield the front corrector plate (the glass lens at the front) dews up in minutes, since I bought the dew shield the only part that now dews up is the eye piece - so far.

[Moonshane]

dew heaters should be set to be literally just a tiny bit above ambient. the metal should not be warm to touch just used enough to 'take the edge off'.

[fredster]

dew heaters should be set to be literally just a tiny bit above ambient. the metal should not be warm to touch just used enough to 'take the edge off'.

I set mine very low and leave them going long before i go out. To the touch it feels like they are not working, but the lack of any dew tells a different story.

[Delboy_Hog]

dew heaters should be set to be literally just a tiny bit above ambient. the metal should not be warm to touch just used enough to 'take the edge off'.

I set mine very low and leave them going long before i go out. To the touch it feels like they are not working, but the lack of any dew tells a different story.

Ah, thank you, yes that makes sense.

[Stargazer33]

Any sort of dew shield does make an enormous difference to the amount of time you get before dew formation on the corrector plate on MCs and SCTs. I was really surprised when I made my first one for my Skymax 102mm. I now have much bigger one for my C8 together with an anti-dew strip and they keep the corrector clear for as long as I need. I am only 6-10 feet away from a large fish pond and on concrete slabs too!

[jimmyjamjoejoe]

Any advice for a dewshield for a 200p? I tried a camping mat jobby but it keeps getting deformed and oval at the open end, ever so slightly cutting into the light path (that is, i can see the edges of it by looking into the focuser with no ep in).

I remember seeing someone on here who had the same problem and had reinforced it with some kind of ring fixed in at that end, i just cant remember what they used...

[Moonshane]

some have used embroidery rings I think?

[jimmyjamjoejoe]

Ha! That was it! Thanks mate

[ajohnson]

I'm looking to buy a MAK or SCT so have found this informative I know from day one I'm going to have a dew problem so this is being factored into the purchase price.

Someone asked about a 200P, I'm assuming this is Newtonian. I have a 300PDS and like most newts the secondary is vulnerable to dew (not as bad as a MAK/SCT etc) I have built a dew heater for my Secondary, it's basically a zigzag coil of resistance wire stuck to the pack of the mirror, on low power I usually stay dew free all night. The Primary also has a heater pad, used once so far on a particularly humid night. As noted above they have to be used proactively, I put my scope out before setting up at home, if it's a bit damp when I drop it onto the mount cradle I connect up the heaters.

These Items seem expensive, but bear in mind that even if you buy one you are looking at maybe £100 tops (less than half that to make one). On smaller scopes this is a big proportion of the price but bear in mind that it could be the difference between spending hours outside looking at the stars or packing up 20 - 30 minutes after getting set-up. We all buy the best scope we can afford be it £100 or many thousands (eek!) for me sorting out dew was a higher priority than buying eyepieces. OK I don't have a big or good collection of eyepieces, but those I have I can use all night!

[ajohnson]

I'm looking to buy a MAK or SCT so have found this informative I know from day one I'm going to have a dew problem so this is being factored into the purchase price.

Someone asked about a 200P, I'm assuming this is Newtonian. I have a 300PDS and like most newts the secondary is vulnerable to dew (not as bad as a MAK/SCT etc) I have built a dew heater for my Secondary, it's basically a zigzag coil of resistance wire stuck to the pack of the mirror, on low power I usually stay dew free all night. The Primary also has a heater pad, used once so far on a particularly humid night. As noted above they have to be used proactively, I put my scope out before setting up at home, if it's a bit damp when I drop it onto the mount cradle I connect up the heaters.

These Items seem expensive, but bear in mind that even if you buy one you are looking at maybe £100 tops (less than half that to make one). On smaller scopes this is a big proportion of the price but bear in mind that it could be the difference between spending hours outside looking at the stars or packing up 20 - 30 minutes after getting set-up. We all buy the best scope we can afford be it £100 or many thousands (eek!) for me sorting out dew was a higher priority than buying eyepieces. OK I don't have a big or good collection of eyepieces, but those I have I can use all night!

Just an addition here, a few weeks ago I partially flocked the 12" Newt. It's flocked around the primary and secondary the centre section is blackboard painted (topic for another thread).

However, in the process I dropped out the secondary, what a mess! The constantan wire on the back was OK but the insulation materials made of dense PE foam and silver backed polystyrene had suffered. Bearing in mind these are usually used with under floor heating I was bit shocked to see they'd melted and burned right through (explains the odd smell). I have replaced them with 5mm HD card (same as the primary heater). Card is almost as flammable but with a significantly higher melt / ignition point. Basically if have the heater up enough to burn the insulator I will also be cooking the mirror foil off the secondary anyway! 

The new insulation works well so my lesson is this - select your insulation material carefully when building your own heaters.

I did not buy an SCT or Mak in the end I decided wide field was better for the objects that I go hunting, maybe I will review this later.

[BGazing]

interesting topic. i am sure they work, but as my mak is entering the autumn, i begin to wonder whether a simple passive shield will do, or whether i should plop for the flexi heat one and kill two birds with one stone.

ep heater seems like a must for winter, irrespective of whether one observes from serbia or england.

[cjdawson]

I'm a little late to this party, but have some useful information.  The scope that I'm working with is a Meade LX-90.

Dew.

1. Above freezing, dew will form on the scope as water, it'll be misty, then droplets will form is allowed.

2. Below freezing, Frost will form, if left this will become a layer of ice.

If you want to experiment and see the effects, just look at car windscreens.   This is exactly the same thing that happens.

How to clear it.   There are bascially 3 ways.

1. Hair dryer - I'm not kidding, use warm air to gently evaporate the moister.

pro's  Can get rid of dew once formed.

con's

Mains hairdryer = 240v in a outdoors!   Heat could damage the optics.

12v hair dryer = pulls lots of power, lower temperature, so less chance of heat damage.

Will leave hotspots all over your optics, so they'll need to cool again to get a stable image.  (not good for photography)

2. Dew shield

Pro's No power required

Con's cannot sort out dew if already formed on the optics.

They slow the onset of dew forming, but a long time.  This alone can be enough for some observing sessions.

3. Dew heater.

Pro's actively heats the options, or the air in front to prevent the heat radiating away

con's needs a power supply

I made my own dew heater several years ago, here's version 1

http://astronomy.cjdawson.com/projects/DewHeater.html

and here's version 2

http://astronomy.cjdawson.com/projects/PowerBox.html#NewHeatingElement

When I first made my heater, I did the maths needed and found that "at full power" the heating element could draw 2.67amps at 12v.   Whilst this sounds like alot of power, this happens when the heater is running at 100% power output.  This is actually enough to clear frost from the optics.    That's alot of power available.  In reality I have found that I run the heater between 10% and 20%,  This works out that at 20% power 0.534 Amps will be drawn, or 534mA.

No one has said that you have to only have one of these options.  There's no problem with having all three options available to you - this is what I do.  Bascially I put a dew shield on my scope for normal use.  Then have the heater, running at 10% to keep the dew away, and finally, can keep a 12v hair dryer in my kit for that one time when the heater is set too low, and didn't prevent the due.

[gonzostar]

Thankyou for posting this info it is very useful to know

[davekelley]

On 11/10/2012 at 09:13, Charon said:

Dew is terrible out here this time of the year and I was forever having problems. So I bought a Astrozap Flexi-heat Dew Shield from FLO, end of dewing problems.

But the expense!   The dewshield seems reasonable in price but do you need the dew controller in order to power up the shield?  they are not cheap!

 

[Mr Spock]

Cooling a scope down only to heat it up again seems most illogical.

An alternative is to stop the heat exchange process by blocking the radiation. Once your scope has cooled sufficiently, wrap it in insulation. This will stop the tube radiating heat. The effects will be two fold: firstly, for an SCT, the lack of tube radiation will have a similar effect on the corrector plate if combined with a long dew shield; secondly the lack of radiation will stabilise tube currents and reduce seeing issues.

Aluminium bubble wrap is very inexpensive and can easily be layered and cut as a wrap-around connected by velcro.

[johninderby]

But doesn’t wrapping the scope in bubble wrap stop it from cooling further as the temperature drops?

[Mr Spock]

No, it just stops the excess.

[maw lod qan]

I made dew heaters from nicad wire, duct tape and a PWM to control the temperature. Hook it all up to a 12V lawnmower battery that has a small solar trickle charger keeping it full.

I do have to be careful  to keep the PWM turned down low or it will get very hot.

Even run the small fans to keep the secondary clear off the battery.