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.
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.
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.
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.
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.
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.
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.
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.
. Buy it from Clark Rubber. Comes in a good sized roll.
