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Dew Shield Over Dew Shield for Refractor


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Hello, everyone!

I saw a pic on the Yahoo TEC Group that showed how one fellow had placed an Astrozap flexible dew shield over his TEC-140's dew shield.  It appeared from the pic that the Astrozap dew shield significantly extended the original dew shield, to almost twice its original length.  I suppose that the purpose of this was to decrease the tendency of dew to form on the objective lens.

Has anyone here put a flexible dew shield over his refractor's original dew shield?  If so, does it really help prevent dew from forming on the objective?  I thought I would ask here, before I spent the funds to buy a flexible dew shield to extend the existing dew shield of my 130mm refractor.

Thank you.

William

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The effectiveness of a dew shield depends only on its length. Extending it (with cardboard or anything else) will have some effect in reducing dewing. It's just a case of having it as long as possible without it sagging into the light path or becoming inconvenient. The solid tube of a Newtonian reflector is in effect a big dewshield that minimises the primary mirror's direct exposure to clear sky - which is what causes dewing.

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I use an additional dew shield on my refractors and I guess it works but at the end of the day it is only a measure to extend the amount of time you get before the lens dews up. With out having 2 identical refractors sat side by side, one with and one with out the extended dew shield I can't say 100% if it has any significant effect on prolonging the inevitable. For this reason I also have a DIY dew heater and tape which again isn't any guarantee as if it is a particularly damp night dew will settle like it or not.

I guess what you have to ask is, "is it worth me getting a longer dew shield if it going to allow me an extra hours observing ?" and I would say yes!!

Is a commercially made dew shield worth it? I'd say no as they are often over manufactured and heavy things with no real advantage over a simple DIY dew shield. There are many threads going over the different ways to make one so why pay £20-30 for one. I use this stuff myself. PLAIN BLACK CLOSED - LD45 PLASTAZOTE POLYETHYLENE FOAM 6mm from http://www.rhnuttall.co.uk/contact/ in Birmingham.

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  • 2 weeks later...

Hope not too off topic, but...

I have the 127mm ES APO triplet and it too has a nice dew shield. I fought Dewing a lot when I had my c8 and ended up getting a zapper. For the refractor I just got one too. These things are long enough as it is , and since there is already power for the motors, an extra cord really isn't as cumbersome to use for me as an extra long refractor. Portability in my small car was also a consideration.

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I use an Astronaut over my dew shield too. Because... why not?

But especially in the daytime it helps protect against pollen and dust, and during public events it protects against fingers and snot.

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It is very possible to make a very lightweight slip over dew shield for well under ten pounds, in pet stores they sell cat feed mats that you put under moggies food and water bowl, plus you will need some black flocking to line the inside, cut to size roll around original and glue plus tape the join (min 1 inch overlap) flock the inside, if you then want to push the boat out and get some self adhesive fake carbon film and do the outer side to make it look posh

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I'm not sure how dew shields work, I guess they either trap relatively warm air in contact with the OG, or they limit radiative cooling of the OG. Personally, I've not been troubled (yet!) by dewing, even though I've had frost on the tube before now, but it did occur to me that perhaps one way to extend the dew-free time is to stop the dew shield cooling off for as long as possible. OK, a dew heater will do that indefinitely, but requires another power source, so I wondered whether just thermally insulating the outside of the dew shield before taking the 'scope out doors will delay any dewing. Steve Nicholls had a dew-shield 'cosy' made, but I don't think the weather's been cold enough since to test it out, so the jury's out at the moment.

Ian

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