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is a dew shield necessary?


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I recently posted a topic in the astro louge titled 'How to make someone feel small'. I wont repeat the details here but the shop owner in question told me I dont need a dew shield for my Celestron 114eq, is that correct please? I was prepared to buy one because I don't want my viewing ruined, but obviously I wont if it's not necessary.

Many thanks.

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Since you have a newtonian it's not so important because your primary is at the bottom of the tube, but having one will protect your secondary from moisture and can enhance contrast a bit if there is any stray light.

Dew shields are mainly used when you have an exposed optical surface like a refractor or SCT.

Martyn.

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I have a newtonian and I use a dew shield. It helps contrast a treat with all these street lights around.

Obviously the sales person took a great deal of time to understand your viewing needs and conditions.

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As nightfisher has said, make one on the cheap from cardboard before spending 30+ pounds for a sheet of ABS plastic with velcro stitched to it...talk about a racket...

If you have streetlights around, it would help. Also depending on conditions, as has been mentioned, it could be useful for protecting the secondary mirror.

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I think that the points to consider about dew shields are as follows:

very cheap to make

reduces impact of local light pollution (local street lights/lit windows)

increases contrast generally

prevents dewing on secondary mirror

it's rare for newtonian secondaries to dew up in my experience (especially for solid tube scopes) but for the sake of a couple of quid and a little time, why risk losing a good night's observing? once you realise you should have made/used a dew shield it's too late. a bit like an oil light on a car - if it comes on, the damage is done (although thankfully with your scope you can just let it dry off!).

I'd basically ignore any of the advice you received from the shop you visited recently. we have no ulterior motive to sell you another product, will embrace all comers to the hobby and offer advice based upon our experience - and all free of charge!

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As nightfisher has said, make one on the cheap from cardboard before spending 30+ pounds for a sheet of ABS plastic with velcro stitched to it...talk about a racket...

Necessity is the mother of invention! I keep meaning to make dew shields for my binos with two suitably sized black plastic flowerpots!
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A dew shield isn't necessary on a newtonian, but depending on the conditions you might need one if the secondary mirror dews over pretty fast.

in other words, a dewshield IS necessary on a newtonian :smiley:

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My experience with a 114mm starblast was that a dewshield was not necessary. It will probably help improve contrast by keeping stray light out the tube though. By contrast, I found a dew shield was necessary with a 12" dob. I think the difference is that the secondary on the starblast was largely covered by the bit that holds the mirror so it never presented much area to the night sky. On the 12" the secondary was large, presented a large area to the night sky and cooled quickly. Without a dew shield the mirror would mist up after a couple of hours.

I have often wondered if gluing a think layer of cork on the back of the secondary of the 12" would have stopped it cooling so fast and so prevented dewing. On the other hand, would it have kept it too hot and caused tube currents? Also what would you do if you ever wanted to recoat the mirror? I was never brave enough to try.

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You'll want one, but it's far better to make one yourself.

I've left my scope out for 90 minutes, nearly 2 hours (too long, I know!), when I finally went out, the view was completely misty through the eyepiece, and cleared after about 20 minutes. It probably was the eyepieces too, which I left out, but still, dew can wreck views completely. Mind you, it taught me never to leave the scope out for that long when it's rained that day, even if it's clear.

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I put one on my Newtonian. Now my Telrad and finderscope always dew over first :)

Telrad isn't really a problem because you can wipe it with a soft cloth and not worry about fragile optical surfaces. I need to make one for the finder really, sometimes I need it to refine the target finding capability of the Telrad.

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Newtonians definitely do dew up on the secondary, sometimes very quickly! Obviously it depends on the humidity, but also on the altitude of your target - the higher the altitude, the more chance of the dew "raining" into the OTA. If in doubt, I always pop the (£1.50 home-made) dew shield on.

I've left my scope out for 90 minutes, nearly 2 hours (too long, I know!), when I finally went out, the view was completely misty through the eyepiece, and cleared after about 20 minutes. It probably was the eyepieces too, which I left out, but still, dew can wreck views completely. Mind you, it taught me never to leave the scope out for that long when it's rained that day, even if it's clear.

Too long?? I regularly use mine all night!

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Newtonians definitely do dew up on the secondary, sometimes very quickly! Obviously it depends on the humidity, but also on the altitude of your target - the higher the altitude, the more chance of the dew "raining" into the OTA. If in doubt, I always pop the (£1.50 home-made) dew shield on.

Too long?? I regularly use mine all night!

Left it facing the same position for two hours whilst I was inside, rather different than observing when you're moving it about a lot :angel:.

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