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 may still get dew. 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
M 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:
Cover the telescope with a plastic garbage bag to prevent glue
from sticking to it while the dew shield is being constructed.
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.
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
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).
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.
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.
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.
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.
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.
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.
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
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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Copyright 2018 by Ronald & Teresa Keating. All Rights Reserved.