Dew Shield (good materials and cheap!)

[04Stefan07]

Title says it all!

I have been looking into making one myself the past few days.

I need to buy a hard case from Bunnings later today so I wouldn't mind buying some parts to make a Dew Shield for my 90mm MAK.

Any advice, thoughts or ideas?

Thanks!

[Astro_Bot]

I use EVA75 foam (from Clark Rubber, but probably available at other foam places). It's excellent material - robust, matte black, light-weight, excellent insulation, holds shape well when wrapped around the end of an OTA.

I bought a whole roll and cut it myself (much cheaper in the long run), but that's becuase I figured I'd sell the dew shield with any OTA I sell, and make another one for a new OTA from the stock of foam I keep. Otherwise, Clark staff will cut what you need (for a fee). You can get an over-the-phone quote.

I've tried other materials from time to time, and EVA75 has been the clear leader so far. I have no plans to use anything else.

[Astro_Bot]

PS: You can cut EVA75 with a big pair of scissors - in fact, that's the recommended way to do it - no need for a jig saw.

PPS: The stuff I use is 6mm thickness.

[el_draco]

Very interesting. Do you know how this stuff responds to polyester resin?

[Astro_Bot]

No idea, sorry. I glued velcro patches onto it with contact glue - that worked well enough.

[multiweb]

In the past I have used flute panels. They're cheap, good insulator and easy to fold on a radius for obvious reasons. I don't like the foam/flexible stuff as it gives you an oblong aperture and that will change your star shapes.

[Astro_Bot]

Mine stays quite round enough, and I have never noticed any distortion. EVA75 is stiffer than regular "camping roll" foam.

[multiweb]

Fair enough. I first noticed that when star testing. I thought it was tube currents but it turned out to be the dewshield aperture doing it.

[Astro_Bot]

YMMV. I guess it depends on the aperture, length and material used (shape deformation).

I once got weird star test results not realising that I was looking through a tree ... <facepalm> ... but never with the dew shield.

Come to think of it, light from off-axis outside the FOV shouldn't affect the image .... in fact, that's what the dew shield/light shroud is trying to prevent. Hmmm. With a longer F/L scope, especially a Mak or SCT, the retaining ring ought to still be the extreme edge of the FOV.

[LewisM]

I have a spare dewshield from a 7.5" OTA if anyone wants it - sitting unused. Easy to modify to other OTA diameters. It is commercially made one, not homemade.

12" length, matt black textured plastic with velcro (stitched on) fastening.

[LewisM]

PS: I made all my refractor dewshields from flexible cutting boards available in Sam's Warehouse / Crazy Clarks. $2 per cutting board. I glue on the velcro, then spray them with blackboard paint. Never failed me yet, and not ONCE had a dew up on the objective.

[AG Hybrid]

As mentioned earlier. EVA 75 is what you want my friends :). Buy it from Clark Rubber. Comes in a good sized roll.

My 12" and 4" utilize them nicely. See the pics. Easy to work with. LIGHT and flat black. Just add Velcro with a self adhesive back. You can also use it to make dew shields and light shields for finders and telrads. :hi:
Also the only dew that builds up on them is this ultra thin film of moisture. Unlike an OTA which can feel like it was left in the rain.

[04Stefan07]

Thanks for the advice everyone.

I might pop out to Clark Rubber when I get the chance!

[04Stefan07]

Went to the gold old 24 hour Kmart Campbellfield last night and found one of them exercise foam mats for $4!

Will try it out when I get the chance.

[Astro_Bot]

You may be lucky and have picked up some cheap suitable foam, but in my experience, typical exercise mat (or camping mat) foam is too soft (floppy). Still, for $4 it's worth a try. :)