Sunday, December 23, 2012

Insulated DIY Dew Shield for Telescope v1.3

(Click on any image to enlarge.)
I live in the temperate rain forest of coastal Oregon. Dew is a big problem. This doesn't make me an expert on dew, it just gives me more incentive to try to make a better dew shield. Honestly, I haven't had the opportunity to try this design out, but I thought it promising enough to share as is. The idea was to create a lightweight insulated tube with a protective outer shell and a light absorbing cloth or felt interior. My design uses high density polyethylene (HDPE) for the outer shell ($7.95 for a 2'x 3' sheet at Tap Plastics); fleece for the interior fabric ($4/yard or less); a can of expanding insulating spray foam ($7). The total material cost for my 6" scope was about $20, weighs in at about a pound and should have an R=value of around 2.

UPDATE: 12-24.2012:  I took the scope outside at 6:00 pm.  Interior temp: 68 F; Exterior 47F with 80% humidity. The only thing that was visible was the moon and it mostly a glowing disc behind Earth's clouds.  3 hours later the outside of the shield is damp, but inside clear as a bell.  Not the worst dew I've encountered, but a pretty good indication that the basic design is sound.

 STEP 1: Frame

You will need a frame or scaffold to support the inner wall. I conveniently had four. 1/2" threaded rods, but wood dowels or something else would do as well.
The rods are located so that their outer edge is tangent to what will be the inner diameter of the dew shield.
Four supports seemed enough for a 6" tube. More would probably be needed for larger diameters .
The length of the frame should be generously bigger than the length of the tube you are making. In this example I am making a 16" long tube 4-1/2" of which will be on the optical tube leaving 11-1/2" in front of the scope. The rule of thumb, I gather, is 1.5 times the diameter. So this design is nearly 2x.

STEP 2: Inner Form

I used part of my sheet of HDPE to  make the inner form.  This part stays in only until the foam hardens, then it is removed.  Other things could be used; a cardboard tube of the right diameter or sheet metal, etc.

Here I've simply stapled the inner tube in place.

Step 3: The Liner

I chose fleece for 3 reasons:
  1.  I thought its many fibers would be good light absorbers.
  2. The fabric is resilient, workable and cheap.
  3. It is itself an insulator.
I understand that felt is often used and this method does nor preclude that.

Whatever material you choose fit it tight around the inner barrel BUT DO NOT ATTACH IT TO ANY PART OF THE INNER BARREL. Recall that the inner sleeve will be removed.

 STEP 4: Spacers

Attach spacers the height of the foam insulation  (how thick your dew shield will be).  Some sites show spray in foam with an R-value of R4/inch.
In this example I'm using approximately 1/2" spacers for a value of  R2
Glue the spacers on in a staggered pattern.


STEP 5: Outer wall

Secure the outer wall tight against the spacers.  I tried  gluing it, but eventually added 3 rivets.  One could also simply duct tape it.
Tape the cloth ends to protect them and keep them out of the way.

STEP 6: Foam

Stick the expanding foam straw in all way and pull the trigger and pull the straw back foam almost all the way to the edge.  Repeat around both ends of the tube

STEP  7: Curing

Because the foam is sandwiched between HDPE it may take a day for the center to cure while the ends will be done in an hour.  Don't be fooled;  let it cure.


STEP 8: Remove the inner liner

One you are sure the foam is  sufficiently cued, take the tube off the scaffolding and push the inner sleeve out.
Removing the inner tube will also expose the central area to more air enabling final curing and more importantly, its final weight.

STEP 9: Trial Fit

Pull the fleece back over the outside.  Test fit it on your scope.  One end my work better than the other.
If the fit is to tight you can compress the foam and/or remove some of it.
If the fit is too loose you can pull back the fabric from the foam at one end and layer is some additional fabric to reduce the diameter.
In fact this second method could be used to create tubes of larger inner diameters to support longer lengths or cure  vignetting issues.

STEP 10:  Finishing

Trim the excess fabric back and then stretch and tape it to the barrel.  Most people will be able to achieve superior aesthetic results.

STEP 11: Cut outs and other features (Optional)

As you can see in the photo I had to cut out a notch for my Celestron tube mount.  One can imagine fitting more elaborate mounts and/or fittings into the design before foaming.  These are left as an exercise for the reader.

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