Reflectix® Dew Shield
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 a dew heater without a dew shield is like trying to heat a home with the doors wide open, most of the energy is wasted rather than fighting dew. The dew shield reduces the area of sky that is sucking the heat out of your corrector plate. Long dew shields are more effective but suffer from wind induced vibrations, so the compromise is to make the length equal to the tube diameter.

 It is very easy to make your own ultra-light dew shield by following the instructions below. Reflectix® is the best material I have found for a dew shield. It is extremely lightweight, holds its shape well, yet rolls up compactly for easy storage. And since it is an excellent insulator, it can cover the heater strip so more of the heat soaks 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 following materials should be available at your local building supply store:
Constructing the dew shield:

  1. Place your heater strip on the telescope just behind the casting on the Corrector Plate.
  2. Place the dust cover over the Corrector Plate.
  3. Cover the telescope with a plastic garbage bag to prevent glue from sticking to it while the dew shield is being constructed.
  4. Cut a 3" wide strip of Reflectix® long enough to wrap around the telescope tube (not the corrector plate casting).
  5. Wrap the 3" strip around the telescope tube just behind the heater strip and temporarily hold it in position using a small piece of tape.
  6. Cut a 5" wide strip of Reflectix® long enough to wrap around the 3" strip.
  7. Using spray adhesive, glue the 5" strip onto of the 3" strip making sure it covers the heater and at least part of the Corrector Plate casting. It should not overlap the dust cover, you want to be able to remove and  install the dust cover without having to remove the dew shield.
  8. 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).
  9. 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.
  10. 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.
  11. At this point it is suggested to paint the dew shield interior with flat black 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 prime it by spraying a light coat of Super 77 spray adhesive and allow it to dry. You may then spray flat black paint onto the interior. The exterior may be left unpainted.
  12. 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. image
  13. 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.
  14. 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. image  image
  15. 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:
image image
If you would like the dew shield to roll up as compactly as possible, the 3" and 5" strips can have notches cut in them to allow the assembly to roll up tightly. 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.