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Dew shield


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Hi all.

I want to make a dew shield for my 12"dob, I looked up early posts and a foam camping mat was mentioned. 

1)  Would this stop dew on my secondary mirror 

2)  How long would I need to make it 

3)  Is a foam camping mat a good idea (going to work).

Thanks  Dave

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I think the answer to 1 and 3 is 'yes'. At least prevent dew. In terms of length I think a guide tends to be about 1.5 times the aperture. I made one from a very cheap foil backed camping mat for my smaller newtonian and it has worked fine, even on pretty damp nights.

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Hi I have made several from the black foam mat from Ikea £2.99.

The Mat is also ribbed  on the inside

I also made them 1.5 times the tube diameter the joints are  a butt joint made with impact adhesive then an external butt strap 25mm wide glued over the joint once the initial glue has dried this gives a smooth internal finish and slides on easily  they work a treat

As for the secondary mirror I never had a problem 

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6 minutes ago, AstroGee said:

as a noob question - how do these work?

Is it a thermal-control type of insulation, a way of prevent moisture ingress by sealing, or even moisture absorption?

Thanks

I'm not sure myself I'm sure someone would be able to answer that question there very helpful on here 🤔 

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10 minutes ago, Kimboman said:

Hi I have made several from the black foam mat from Ikea £2.99.

The Mat is also ribbed  on the inside

I also made them 1.5 times the tube diameter the joints are  a butt joint made with impact adhesive then an external butt strap 25mm wide glued over the joint once the initial glue has dried this gives a smooth internal finish and slides on easily  they work a treat

As for the secondary mirror I never had a problem 

It's the secondary mirror im having the most problem with, suffer with alot of dew from where I am .when I'm observing everything looks fine, then the stars don't look sharp and harder to get in focus so I was thinking collinmation was out but when I looked through the focuser the secondary mirror is covered with dew. End of a good clear dark night😪

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10 hours ago, Dave scutt said:

Hi all.

I want to make a dew shield for my 12"dob, I looked up early posts and a foam camping mat was mentioned. 

1)  Would this stop dew on my secondary mirror 

2)  How long would I need to make it 

3)  Is a foam camping mat a good idea (going to work).

Thanks  Dave

1. Probaly yes, but will extend dew free time

2. Not sure but suggest at least 300mm 

3. Yes Camping mat is good, light and cheap.

Hope that helps

Secondary dewing this time of year is very common with a Newtonian, it is with my and others dobs we use.
I clear my secondary with a 12v hairdryer and then just observe on, but a dew shield is a good idea and simpler.

 

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32 minutes ago, AstroGee said:

as a noob question - how do these work?

Is it a thermal-control type of insulation, a way of prevent moisture ingress by sealing, or even moisture absorption?

Thanks

It is a good question  - the consensus seems to be that the main cause of dew is the cooling of the objective. If this cools below the 'dew point', then water vapour in the atmosphere will condense onto the glass. 

Without a dew shield, the main cooling mechanism is by radiation to the much cooler sky, which can occur across the full 180 degrees open to the sky. 

With a dew shield, the exposure to the colder sky is reduced to a smaller angle, and this is said to reduce the rate of cooling of the objective

I have a suspicion that there may also be an element that the dew shield creates a dead space for air close to the objective, and thus only a small amount of water vapour is available to be condensed out. 

You may be familiar with the way that condensation is worse at the beginning of a session, and that the air gets less humid as the evening progresses, I guess that if the condensation is reduced at the start of a session by the two factors mentioned above, then the overall problem is reduced as the air becomes generally less humid 

https://www.cloudynights.com/topic/57072-how-does-a-dew-shield-work/  

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Oh, and alternative to camping mats is Proplex / Correx sheeting. This is a twin-wall plastic sheeting used as floor protection by builders. It can be easily cut and will fold around the objective and can be held with sticky velcro pads.

You can get a 2.4m x 1.2 m sheet for about £5 in Wickes. and that'll do multiple dewshields. 

Lighter and cheaper than foam mats, and folds flat for storage when not in use. 

https://www.wickes.co.uk/Proplex-Black-Surface-Protection-Sheet-2400-x-1200-x-2mm/p/118670

Edited by Gfamily
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11 hours ago, Dave scutt said:

Hi all.

I want to make a dew shield for my 12"dob, I looked up early posts and a foam camping mat was mentioned. 

1)  Would this stop dew on my secondary mirror 

2)  How long would I need to make it 

3)  Is a foam camping mat a good idea (going to work).

Thanks  Dave

1) to an extent. It will help, and delay dew condensing, but it's not perfect . It is, however, cheap and easy to make, so why not do it ?

2) I've not checked my sources , in fact I've no idea where I read it, but somewhere I came across a suggestion that the max. length for a dew shield (i.e. before it started to vignette the corners of the view) was 1.5x the aperture . Ha, I've found my bookmark, there's a formula in post #6 https://www.cloudynights.com/topic/486858-dew-shield-length-how-long-is-too-long/#entry6374801

3) Foam works fine, I started with cheap camping mat , but when I ordered some closed cell foam to make a shroud for my heritage dob I bought enough of the thin black closed cell foam to make a dew shield for the little dob and one for ny 127 mak, which is very dew - prone . The thin foam works really well.

I looked at proplex, (or corriflute) which is essentially like corrugated card, but made of plastic (estate agent's signs are often made of similar stuff) but for a small diameter tube I didn't like the way it creased along the channels rather than making a smooth curve (sorry Wickes, that was me experimentally cracking the top sheet in the stack...) I also wondered if the open channels in the proplex might hold moisture.

So, I went with this stuff https://www.efoam.co.uk/closed-cell-polyethylene-foam.php 

It is closed cell, so does not absorb water, is opaque, dark and dull, so will help avoid extraneous light as a bonus.  I used 3mm thick , but for a larger diameter aperture 'scope I'd suggest going up to 5mm or more to avoid it being a bit floppy if there's a breeze. I did a bunch of experiments to find out how best to glue some velcro onto it , and the best glue I found was hot melt glue gun stuff, which surprised me as I has expected the hot glue to melt the foam ! A year on, and all the things I made are still going strong. I found the self adhesive velcro stuck to itself far better than it did to the foam., total waste of money, simple velcro + the correct glue for the job is far better.

Heather

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FWIW, if you are still struggling there is still the option of making a small secondary heater from resistors or nichrome (or you could buy one). I made one for my 200p and it works fine.

 

As for glue the suggestion above is good - I used a very sharp knife and made butted joint with superglue. So far so good.

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9 hours ago, Gfamily said:

It is a good question  - the consensus seems to be that the main cause of dew is the cooling of the objective. If this cools below the 'dew point', then water vapour in the atmosphere will condense onto the glass. 

Thanks - yes it seems to be the same as with camera gear then in terms of the cause. I am mindful when bringing gear back in from the cold, a sudden transfer back to a warm indoor environment will have the same effect. I use a plastic bag tied up until its brought back to room temperature, does the trick. (good luck trying that with a large scope though).

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  • 3 weeks later...

Foam camping mats do seem to be the weapon of choice for this although they can be made from all sorts, I made one for my Nexstar 4SE from an old tin of SMA baby milk

Dew heaters are also available for secondaries, for example https://www.firstlightoptics.com/dew-prevention/4tronix-secondary-mirror-heater-for-8-12-inch-newtonian-telescopes.html

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