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Best length for a dew shield

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#1 1_old_dog

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Posted 08 October 2008 - 08:15 AM

Hi guys
I didn't want to hi-jack the previous thread on dew shields, so I thought I would start a new one and ask this question.

What is the magic number, in inches past the corrector for a dew shield to work properly?

From what I have read on this forum, in order for a dew shield work properly it should be at least 3x the diameter of the objective in length.

Following that rule I construct my own shields. My present shield is 32" long and it's 24" past the corrector and it's completely flocked, I also have a small piece of flocked material over the rear cell. Lots of dew forms on the surface of the shield and scope but it hasn't shortened my viewing time.

I've done many many all nighters using a shield only and I've not had any issues with dew. I always keep my eyepieces in my pocket, or in my closed case so dew on those is never a problem.

So IMHO and very limited experience, maybe? the store bought dew shields are too short for them to work properly, and this is why ya'll are only getting an extra hour or so of viewing time before you have to fire up the dew heaters and strips.

Being a minimalist I think the fewer power sucking gadgets we have to use the better it is. Maybe it all comes down to where a person does his viewing as to what works and what doesn't work. I don't know.

Just trying to get my head around it, so thanks for any input in advance.
OD

#2 NeoDinian

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Posted 08 October 2008 - 09:42 AM

Simple. We've had this same question several times now.

1.5x the diameter of the OTA... :)

#3 1_old_dog

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Posted 08 October 2008 - 09:54 AM

Hi
I don't mean to sound impertanent, but where does this 1.5x info come from?

Reason I ask is because I've made many different lengths in my quest for the ultimate dew shield and that measurement "simply" doesn't work very well.

What am I missing?

thanks
OD

#4 TONGKW

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Posted 08 October 2008 - 10:17 AM

When the relative humidity is high, I find the dew shield length needs to be at least 2x the diameter of the telescope and it is better to have it longer. Even so, under the most severe condition, I find dew heater is necessary for viewing to be able to continue.

K W TONG
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C8 + CG-5GT, TSA102 + HEQ5 PRO, MK67 + Voyager Mount, WO ZS 80FD + Mizar K mount, NexStar 6SE, WO Megrez 72 + Kenko Mount, Mini Borg 50, C5, PST

#5 KerryR

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Posted 08 October 2008 - 10:40 AM

The 1.5x the objective diameter is a number that came out as the minimum usefull length for preventing the onset of dew. I beleive it was attained by experimentation.

The 1.5x won't prevent dew all night under heavy dew conditions, though. It just delays it significantly compared to no shield.

The longer the dew shield, the longer the time before dew forms on the corrector or objective, up to the point of diminshing returns, which I believe is 3x the objective diameter. Tube Newtonians just about never dew up, so I guess that proves that if you made the shield long enough, you wouldn't dew up at all.

The idea is twofold: 1) hold a column of air with limited ability to move away over the objective, which acts as an insulator. This only works if the air in the column is cooler or the same as the ambient air-temp; Otherwise, the air warmed by the objective moves up and out of the tube. 2) Limit the angles to which the objective is exposed to the night sky. The clear sky is a massive super-cold heat sink to which heat radiates; loose enough heat via radiant loss and the object in question drops below the dew point (the temperature at which the air can not hold it's water in solution). Limit the amount of sky that can 'see' the objective, and you limit the amount of heat that radiates to the sky. 1.5x is a safe minimum to limit exposure to the sky.

Normal length dew shields (1.5x objective diameter-- common in commercial dew shields) won't hold off dewing all night on most nights where dewing could be a concern. A combination shield and heater is best with 'short' sheilds when you want to observe for more than a few or so hours. But, as I mentioned, newts don't dew (often), so obviously one could make a dew shield long enough to prevent dew on nearly all nights. The longer the shield, the harder it is to address anchoring issues and balance, especially on fork mounted SCTs. Transport also becomes an issue. So, most common shields strike a happy medium of 1.5x; light, easy to mount, easy to transport, and REASONABLY effective for dew prevention.

Your results can and will vary. If you don't want to heat, I'd use the longest shield that suits your needs. 3 or 4 times the objective seems like it'd do that.

You can also point a box fan at the objective/corrector. At first take, it seems like this would accellerate cooling. But, the moving air actually adds heat to the objective as it radiates it's heat away-- that's why you don't see dew on breezy nights. This is how fruit growers prevent frost (same as dew but the dew point is below freezing) on plants-- get air moving in the field with small fires (smudge pots) and/or fans...

#6 1_old_dog

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Posted 08 October 2008 - 10:43 AM

Hi

I find the dew shield length needs to be at least 2x the diameter of the telescope and it is better to have it longer.

Did you find this out yourself through testing different lengths?

Even so, under the most severe condition, I find dew heater is necessary for viewing to be able to continue.


yes I agree "under the most severe conditions"
"Most Severe" being the operative words, an active system would be needed but under normal inland conditions (if there is such a thing) is an active system really needed if the length of the dew shield is increased?

thankyou
OD

#7 1_old_dog

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Posted 08 October 2008 - 11:08 AM

Thanks KerryR excellent post and it answered my simple question.

I've had a newt and it never dewed up, then I got an SCT. I had big time issues with dew at first, then the thought occured to me that if my dew shield was long enough it would prevent dew all night long. After a bunch of experimenting with different lengths I found that 3x the objective was perfect for my 8" scope.

Mine is velcroed (spelling?) and it lays flat when not in use, so transport is a non issue. Balance on the other hand is an issue, but a simple DIY magnetic counterweight system or DIY balance rail eliminates that issue as well.

I wonder why astro shops don't offer different shield lengths?
thanks again
OD

#8 NeoDinian

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Posted 08 October 2008 - 11:57 AM

The 1.5x the objective diameter is a number that came out as the minimum usefull length for preventing the onset of dew. I beleive it was attained by experimentation.

The 1.5x won't prevent dew all night under heavy dew conditions, though. It just delays it significantly compared to no shield.

The longer the dew shield, the longer the time before dew forms on the corrector or objective, up to the point of diminshing returns, which I believe is 3x the objective diameter. Tube Newtonians just about never dew up, so I guess that proves that if you made the shield long enough, you wouldn't dew up at all.

The idea is twofold: 1) hold a column of air with limited ability to move away over the objective, which acts as an insulator. This only works if the air in the column is cooler or the same as the ambient air-temp; Otherwise, the air warmed by the objective moves up and out of the tube. 2) Limit the angles to which the objective is exposed to the night sky. The clear sky is a massive super-cold heat sink to which heat radiates; loose enough heat via radiant loss and the object in question drops below the dew point (the temperature at which the air can not hold it's water in solution). Limit the amount of sky that can 'see' the objective, and you limit the amount of heat that radiates to the sky. 1.5x is a safe minimum to limit exposure to the sky.

Normal length dew shields (1.5x objective diameter-- common in commercial dew shields) won't hold off dewing all night on most nights where dewing could be a concern. A combination shield and heater is best with 'short' sheilds when you want to observe for more than a few or so hours. But, as I mentioned, newts don't dew (often), so obviously one could make a dew shield long enough to prevent dew on nearly all nights. The longer the shield, the harder it is to address anchoring issues and balance, especially on fork mounted SCTs. Transport also becomes an issue. So, most common shields strike a happy medium of 1.5x; light, easy to mount, easy to transport, and REASONABLY effective for dew prevention.

Your results can and will vary. If you don't want to heat, I'd use the longest shield that suits your needs. 3 or 4 times the objective seems like it'd do that.

You can also point a box fan at the objective/corrector. At first take, it seems like this would accellerate cooling. But, the moving air actually adds heat to the objective as it radiates it's heat away-- that's why you don't see dew on breezy nights. This is how fruit growers prevent frost (same as dew but the dew point is below freezing) on plants-- get air moving in the field with small fires (smudge pots) and/or fans...


^^^ What he said... :)

In my opinion, a LONG dew shield isn't going to help with Dew... If you still dew up with a 1.5x shield, you should look into ACTIVE dew control, rather than Passive. This of course is in addition to the shield.

#9 Dylan Gladstone

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Posted 08 October 2008 - 01:20 PM

3x the objective!? On my scope the dew shield would be longer than the scope. :)

So far I've never had the corrector fog up with the Orion brand dew shield. The problems I have with dew now are eyepiece related.

#10 KerryR

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Posted 08 October 2008 - 01:24 PM

One of these days I'll get around to making a longer dewshield for my SCT. I'd like to make it 3x the length, and see how long that holds off the dew.

With my current passive 1.5x shield, I get about 3 hours on fairly dry nights. Less if it's 'almost' foggy.

I like the idea of passive dew prevention because I don't want to have to add a power-tank for powering a heater to my transport. I don't do astrophotography, so my power requirements are currently minimal, and adequately met with the LX-90's internal batteries. (I also use a battery powered SCT cooler-- runs off of 8 AA cells.)

A friend made a nice dew shield out of silver 'bubble wrap' style insulation, with black velvet sewn onto the inside, held together by velcro. Roles up very small and very light. I like the idea of the insulation becasue it should help limit 2 stage radiant loss-- the objective/corrector radiates to the dewshield, the dewshield radiates to the sky (can be a concern if your shield is aluminum-- great conductor). Problem is, that stuff is only available in huge roles. I have seen some dashboard sun shades made of similar stuff, which might be a way to get a smaller sample. I doubt it'd make a long enough tube though.

#11 1_old_dog

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Posted 08 October 2008 - 01:47 PM

If you still dew up with a 1.5x shield, you should look into ACTIVE dew control, rather than Passive.


Jeff, please understand that I mean no disrespect, but why not use a longer shield in the first place and stay completely passive for the entire night?

Spending $150. to $200. for a dew prevention system is very cool I suppose, but is this 1.5x a good rule to espouse to beginners? Many people, like myself are looking for a simpler and more cost effective way of observing.

OD

#12 mikiek

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Posted 08 October 2008 - 01:56 PM

One problem with any dew shield is that they seem to act like a sail in the wind. It seemed odd, but my shield (1.5x) will actually jiggle the fork mounted OTA when there's a breeze. I would think that 3x would cause even more problems.

#13 1_old_dog

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Posted 08 October 2008 - 02:39 PM

Ya I can see that happenening with a really long dew shield. However, if the wind is blowing wouldn't that almost negate the need for a dew shield or active system in the first place.

OD

#14 1_old_dog

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Posted 08 October 2008 - 02:58 PM

I used vinyl flashing that comes in rolls of 36" wide by 30' long. I had to do some repairs to the siding on our garden shed and I had some left over.

It's double flocked with peel and stick sheets of Presto Felt from wallyworld and I used velcro to secure the edge. When in use it covers 3/4 of the length of my SCT. It's notched for the bottom counterweight rail and notched for the top ADM dovetail. I find it to be a very secure setup.

#15 mikiek

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Posted 08 October 2008 - 05:48 PM

I haven't been in this hobby that long, but with my SCT there are two things I have learned. ALWAYS use your dew shield and ALWAYS use your dew heater. Regardless of what you think the weather looks like. FWIW.

Learned from more than one spoiled night. :bawling:

#16 stevew

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Posted 08 October 2008 - 11:33 PM

I have always used 2X the aperture of my scope and rarely have a problem. Of course it all depends on your location, and dew point.
Steve

#17 AlexN

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Posted 09 October 2008 - 03:24 AM

I use the standard Celestron dew shield for my C11, its exactly 1.5x the objective... I use it every time I use the scope, however I have found that in a slight breeze if the breeze blows into the dew shield, you can say goodbye to any images for the night.. it shakes the mount around like crazy if it 'buckets' the wind... I must say I like the added safety of having it on there... much less likely for stray bits of flotsam in the air to land on my corrector... hard to accidentally touch the corrector in any way... which is good if you're somewhat clumsy like myself :)

I'll be moving to active heating sooner rather than later because of the climate I live in..

#18 1_old_dog

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Posted 09 October 2008 - 07:48 AM

Yes anything more than a slight breeze will shake things up with a long shield like mine. If it's breezy I go down to a 1x shield because dew really is a non issue then.

I'm also very clumsy. I like to get up from my viewing chair every once in a while and walk around and I quite often bump into the long shield. This as you would guess throws everything off.

Don't get me wrong, I think an active system would be absolutely necessary in some locations, luckily I don't live in those places.

OD

#19 exile

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Posted 09 October 2008 - 08:00 AM


Being a minimalist I think the fewer power sucking gadgets we have to use the better it is. Maybe it all comes down to where a person does his viewing as to what works and what doesn't work. I don't know.


Fellas, I doubt that anyone that has posted here and championed active dew control has local climatic conditions worse than I do - high relative humidity and calm air is the norm the year round here (how about regular assaults of mould on the inside of the corrector plate?), yet I manage to observe for long periods without the need to use exotic 'dew control' systems.

It can be as minimalist as sheets of thin black card rolled and cut to size to fit over the length of the OTA (very cheap, very light, and easy to replace). The shield can slide back flush onto the tube for travel and storage if made to fit snugly. For my 90mm Mak, this works out nicely to be about 18cm - twice the size of the aperture - and dewing is never a problem.

From experience, pray for a light evening wind. Use 2x the aperure, 3x if practical (as Tong pointed out) - if this fails to work go to bed (like they used to do in the old days) or buy a Newtonian.


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