By Ron Keating

Every telescope needs some form of dew shield, but Schmidt-Cassegrain and Schmidt-Newtonians generally do not come with dew shields. Using dew heaters without a dew shield is like trying to heat a home with the doors wide open, you end up applying far more heat than you should and will still have problems. Since the dew shield reduces the area of night sky that is sucking the heat from the corrector plate, the longer the dew shield the more effective it will be. If the dew shield were long enough dew heaters would not be needed, but unfortunately as the length increases so does wind induced vibrations. If dew heaters will be used then the dew shield can be made about the same length as the diameter.

It is very easy to make your own dew shield. The instructions below will assist you in making a light weight dew shield with a space-age material called Reflectix®. This is the best material I have found for a dew shield. It is extremely lightweight, holds its shape well yet rolls up compactly for storage. In addition, it is an excellent insulator so it can cover the heater strip to reduce heat losses to the air and allow more of the heat to soak into the telescope where it is needed. If built as directed below, the dew shield does not need to be removed to install the dust cap. The materials should all be available at Home Depot or other building supply stores:

Reflectix® - light weight insulation material, looks like aluminized bubble wrap. (Home Depot SKU# 884412).
3M Super 77 Spray Adhesive - fast drying spray adhesive that sticks well to Reflectix®, can also serve as a primer for painting (Home Depot SKU# 364533).
Velcro® - 2" wide industrial strength Velcro®, available at Sam's Club and Home Depot (SKU# 847003).
Krylon Ultra-Flat Black - spray paint for inside of dew shield (after priming with Super 77)
Craft Foam - 1/16" thick foam rubber, Wal-mart carries 8x10" sheets in many colors.

Constructing the dew shield:

1. Cover the telescope with a plastic garbage bag to prevent glue from sticking to it while the dew shield is being constructed.
2. Cut a 3" wide strip of Reflectix® long enough to wrap around the telescope tube (not the corrector plate casting) and wrap it around the telescope tube 1.5" behind the corrector plate casting. Use a small piece of tape to hold it in position. NOTE: This leaves enough space for the typical 1.5" wide heater strip to fit around the telescope tube just behind the corrector plate casting. If your heater is not 1.5" wide then adjust accordingly.
3. Cut a 5" wide strip of Reflectix® and glue it on top of the 3" strip with spray adhesive. NOTE: The 5" strip covers no more than 1/2" of the corrector plate casting, leaving enough room to install the dew shield while the dust cover is still in place.
4. Now cut a piece of Reflectix® for the main dew shield long enough to go around the 5" strip and overlap itself at least 10". This strip should be wide enough that it extends past the end of the corrector plate casting by at least the diameter of the dew shield (ex., 10" SCT with 11" diameter dew shield needs a 16" wide strip).
5. Decide what orientation of the dew shield's seam will make installation most convenient, then spray adhesive on the 5" strip and stick the large piece of Reflectix® to the 5" strip. IMPORTANT: Do not allow adhesive to get on the seam or within a few inches of the seam because you will have to cut the 3" and 5" strips at the seam in order to remove the dew shield from the telescope.
6. After the adhesive dries, carefully cut through the 5" and 3" Reflectix® strips to remove the dew shield from the telescope. Use spray adhesive to glue the ends of the 5" strip to the main dew shield.

7. At this point it is suggested to paint the dew shield or glue flocking paper to the interior. Painting will result in the lightest possible dew shield, but paint does not stick directly to the Reflectix® so you must first spray a coat of Super 77 spray adhesive and allow it to dry. You may then spray flat black paint onto the interior. It is recommended that the exterior be left natural aluminum color.

8. After painting is complete, it is recommended that the inside surface of the 3" strip of Reflectix® (step 2) be lined with a soft material such as craft foam or felt to prevent scratching the telescope tube. Form the dew shield into the same diameter as it will be on the scope and use spray adhesive to glue the foam or felt in place.
   
    

9. To hold the dew shield in place, at least a 1 foot long strip of Velcro® should be applied to the dew shield overlap area about 1" from the rear of the dew shield. This will apply tension where the 3" strip fits around the telescope tube.
10. To keep the front opening of the dew shield at full diameter, place a small strip of Velcro® in the overlap area about 1" from the front opening.
     
11. Test the fit of your dew shield by putting your dew heater strip on the telescope around the tube and just behind the corrector plate casting. Then put the dew shield on the telescope so that the 3" strip is behind the heater strip and 5" strip contacts the corrector plate casting sealing the heater strip. Pull the dew shield tightly around the telescope and fasten the Velcro. The dew shield should not slip and should hold its shape without blocking the light path.

Other Considerations:

If you would like the dew shield to roll up as compactly as possible, the 3" and 5" strips can have vee notches cut in them. Start with the side farthest from the overlap and attempt to roll the dew shield into a tight roll. Notice that the strips of Reflectix® do not bend together with the main dew shield. The solution is to cut notches in the 3" and 5" layers so that they will not interfere. Cut the first notch about 1" from the end, the next notch 1.5" from that, the next 2", the next 2.5", and continue cutting notches with each succeeding space 1/2" wider.

If your telescope has side rails or a dovetail bar, the heater strip should be passed through the narrow gap between the tube and the rail. It may not appear wide enough, but the heater will pass through. Start by working the thin end part of the heater strap through then gently slide the heater back and forth along the rail as you slide it through. Once in place, leave it there, there is no need to remove it after each use. Also, when building the dew shield, cut the 3" and 5" strips of Reflectix® to fit between the rails. If need be, add several more 5" strips to build the dew shield diameter up so that the main dew shield does not have to be notched.

It is very important that as air tight a seal as possible be made where the dew shield meets the telescope so that no pathways allow air to enter the interior of the dew shield from the bottom. If air leaks exist, a chimney effect will take place where cool air enters at the base of the dew shield and warm air inside the dew shield rises out the top.

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