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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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I did not find much information about making a dew shield when I searched the CN ATM posts, so I decided to post instructions for the method I devised. I forgot to take pictures while I was making the dew shield, so keep in mind that the plastic during the actual construction will be white, not painted black, as shown in the photos. Here is the telescope with the prototype dew shield installed:
I made my dew shield out of a sheet of a material called “Plas-Tex PolyWall”. It is a 4’ x 8’ plastic sheet 1/16” thick. It has a rough side and a smooth side. You can learn more about it here . I purchased the plastic for another home project, and just used the leftover portion. The 4’ x 8’ sheet cost @ $15.00. You could make 8 dew shields for 6 inch scopes with a full sheet, so the material cost for one is only about @ $2.00 if you have another use for the material. (Or need 8 dew shields!)
I first decided to pre-form the plastic sheet, so I cut a piece larger than I needed, and rolled it into a tube smaller than the diameter of my scope. I tied a few bungees around it and left it for a week. This succeeded in pre-forming the radius to approximately what I needed.
You can see from the photo above that my lens cell has a series of 7 splines around the circumference, so I decided to just use a push-on friction fit. I cut a strip of manila folder long enough to fit around the lens cell, wrapped it tightly around the lens cell and taped it together. I slid it off, then cut across it, leaving me a strip of manila having a length exactly the circumference I needed. I used this strip and a good carpenter’s square to lay out a rectangle 11” wide , and as long as my strip of manila on the curled plastic sheet. I then cut the rectangle out, using a utility knife with new blade, and the aluminum carpenter’s square. I placed the edges tightly together forming a tube, and taped along the seam with a strip of 1.5” wide electrical tape. (any good tape will do-it will be removed) I then ran a strip of tape down the inside too, to help hold it together.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Next, I repeated the manila strip trick, this time wrapping it around the first layer of plastic tube I had just created. I cut another strip of plastic, this one sized to fit around the first strip exactly, with no overlap.
Now I mixed enough 10 minute cure epoxy to run a thick line around each edge of the tube, and several lines running the length of the tube, spaced evenly around it. I wrapped the larger strip around the tube, pressed them tightly together and adjusted for a good flush fit. I then clamped the seam with a pair of 1” x 2” strips, and 2 clamps. (see photo) I learned the hard way that it is important to use a glue such as epoxy, that does not require evaporation to dry. You will see the seam separation in the photo showing my prototype which was caused by poor glue selection. This was not an issue in my second attempt.
Photo showing prototype and final versions:
When the epoxy had fully set (about 8 hours) I removed the clamps and placed the dew shield on the lens cell. It was a good fit, but I wanted a more firm friction fit, so I ran a strip of duck tape around the inside edge, which tightened up the fit beautifully.
Next I filled and smoothed the seam with a little epoxy, to give a more finished appearance. Once that cured, I painted the inside flat black, and the outside with 2 coats hammered metallic black Rustoleum paint.
In the pictures I show both the prototype, and the final version. The prototype was only made with one piece of plastic instead of two. It was wound in a spiral form, as shown in this diagram.
The final version used 2 sheets, as described above. The seams are flush because I did not use the spiral curl. The seams of the 2 layers in the final version are offset from each other by 180 degrees. You can see the seam separation caused by glue failure toward the bottom of the prototype. This was not an issue in the final version.
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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StacyJo
professor emeritus
Reged: 07/08/04
Posts: 639
Loc: Oakland, CA
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I was going to suggest getting some black poster board, some tape, and voila! Instant CHEAP dew shield...but youse did me one better 
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Stacy
"Why yes, there is a difference between photon deprivation and PMS"
AKA Konstellation Kitty
M42Gal
Stellarvue Nighthawk II
Orion Starblast 6" Newtonian - newest addition to the family!!!
7x50 Orion Scenix Binoculars
Old 3" Jason Comet Chaser
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KerryR
sage
Reged: 12/05/07
Posts: 329
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I made mine from aluminum flashing from the hardware store. Very light, which was a concern for my front heavy C6-R
I made it 10" deep for my 6" refractor, the circumference is 1.25" longer than the circumference of the objective cell, to allow for overlap. I cut it with a razor knife by scoring deeply, then flexing the score to break it.
I sanded, primed, and sprayed it black. I use Velcro running along the seem to hold it closed. There is a second Velcro strip running parallel to the seem a few inches inward along the circumference. This allows the tube to be held closed when wrapped around the OTA, behind the objective, for transport, where it is out of the way, occupies no additional space, and protects it from denting/kinking.
Basically, just an aluminum flashing version of the Astrozap.
Care must be taken when painting aluminum. You must sand it and prime it with primer made for aluminum, or the flat black paint may flake off. With it sanded and primed, the paint has stayed flawlessly in place-- hasn't scratched off, and hasn't flaked off despite the abuse of repeated rolling and transport.
Kerry
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Kerry
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Quote:
I made mine from aluminum flashing from the hardware store. Very light, which was a concern for my front heavy C6-R
I made it 10" deep for my 6" refractor, the circumference is 1.25" longer than the circumference of the objective cell, to allow for overlap. I cut it with a razor knife by scoring deeply, then flexing the score to break it.
I sanded, primed, and sprayed it black. I use Velcro running along the seem to hold it closed. There is a second Velcro strip running parallel to the seem a few inches inward along the circumference. This allows the tube to be held closed when wrapped around the OTA, behind the objective, for transport, where it is out of the way, occupies no additional space, and protects it from denting/kinking.
Basically, just an aluminum flashing version of the Astrozap.
Care must be taken when painting aluminum. You must sand it and prime it with primer made for aluminum, or the flat black paint may flake off. With it sanded and primed, the paint has stayed flawlessly in place-- hasn't scratched off, and hasn't flaked off despite the abuse of repeated rolling and transport.
Kerry
I considered using sheet metal, but I was concerned about sharp edges. Did you roll the edges or anything?
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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Wes James
Carpal Tunnel
Reged: 04/12/06
Posts: 2462
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My usual concerns with working with plastics such as this are problems with adhesives. Rarely have any luck with epoxying plastics. The website's comments on applicable adhesives are:
Quote:
trowel grade, non-flammable
Parkland SS-95 polyurethane adhesive
Henry 444 FRP panel adhesive
Super-Tek PanelSet FRP
Liquid Nails FRP-310 latex FRP adhesive
A good presentation on making a dewshield, people do need to pay attention to the use of proper adhesives for the material they're using.
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
"When your work speaks for itself- Don't Interrupt" -Gamble Rogers
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2257
Loc: Courtice, Ontario
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What about using ploycarbonate and Weld-On #4?
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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Brooklyn
scholastic sledgehammer
Reged: 07/24/08
Posts: 870
Loc: Central New Jersey
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Hey guys, the astrozap dew shield actually has an inner lining of FELT to absorb any moisture that is on its way to the corrector plate.
http://www.dewshield.com/flexible.html
What did ya use to absorb moisture on the inside of the dew shield?
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Meade 8.25"(209.55mm) LX-90 EMC (SCT)
Albert Einstein =>
“Intellectual growth should commence at birth and cease only at death.”
“If you can't explain it simply, you don't understand it well enough.”
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KerryR
sage
Reged: 12/05/07
Posts: 329
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For my aluminum shield I sanded the edges to dull them, and rounded the 4 corners by snippin a small 45deg. triangle off and sanded untill round.
Aluminum is very easy/quick to sand. I used an electric sander on the faces for paint prep, and hand sand paper on the edges/corners. The edges on mine won't cut or scratch anything, though I wouldn't let it hit the obejcive. Of course, I wouldn't let ANYTHING hit the objective if I could help it!!
I think the biggest advantage is lightness. I'm not sure how much of a weight savings it is over an astrozap shield, though. And, by the time you buy the aluminum flashing, primer, paint and sandpaper, I don't think you save that much money... I made mine because I didn't want to wait for shipping, and I was planning on observing under 'dewy' conditions that night...
Kerry
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Kerry
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randtek
professor emeritus
Reged: 06/18/05
Posts: 694
Loc: Central Indiana
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Quote:
Hey guys, the astrozap dew shield actually has an inner lining of FELT to absorb any moisture that is on its way to the corrector plate.
http://www.dewshield.com/flexible.html
What did ya use to absorb moisture on the inside of the dew shield?
Currently mine isn't lined with anything. It's a good idea for an easy upgrade though! 
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Randy
"The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself.
Therefore all progress depends on the unreasonable man."
George Bernard Shaw
SkyQuest XT10
Homebuilt Surplus Shed 102mm f8.8 refractor (still under construction-OTA done, mount in work)
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KerryR
sage
Reged: 12/05/07
Posts: 329
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I didn't line mine either. I hear that felt likes to shed fibers on to the objective. Maybe Astrozap applies some sort of fixatif to prevent this, though.
It'd be cool to put flocking in it...
I don't think felt would be a necessary addition if the shield is deep enough. I think the formula is 1.5x the diameter of the objective for a minimum length. At 3x, the objective will never dew up. Well, maybe not 'never'. But, the longer the better.
I made mine 10" because that's how much room I have between the lense cell and the upper tube ring, and, as mentioned, I store the shield wrapped around the OTA, just below the cell.
Kerry
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Kerry
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