Homemade Dew Shields The first line of defense against dew for SCT or MCT users is a dew shield. A metal or plastic extension for the end of the tube provides some protection for the dew-attracting corrector plate. The only problem with dew shields is their cost. A dew shield is such an easy thing to make, though, that there's really no excuse for buying one instead of rolling your own. The most convenient style of dew shield is of the flexible variety, a flat piece of heavy-duty plastic that is formed into a tube, fastened together in that shape, and slid over the end of the telescope. The hardest part of making one is finding a suitable sheet of plastic.
The perfect material is a brand of plastic sheeting called Kydex, which is used for wall covering and similar applications. This material can be obtained in a range of colors, with flat black being readily available—if any Kydex at all can be found, that is. The best bet is a local plastics distributor. No Kydex? A trip to the hardware or home improvement store will turn up a good alternative. Plastic sheeting designed to be used for bagging leaves is a common item in lawn and garden departments. This stuff is intended to keep a trash bag open and standing on its own when leaves are being dumped into it, is the right size to make a dew shield for a large CAT, and it's easy to cut down for use with smaller apertures. The leaf-bagger material has a natural spring-curl to it, but that doesn't hurt anything and may actually help when it is being secured to the end of the telescope tube.
Before purchasing a sheet of dew shield material of any kind, ascertain how big a piece will be required. It should be long enough to go around the circumference of the tube with 1 or 2-inches of overlap for fastening. The other dimension, the length of the finished dew shield, is determined by the size of the primary mirror. It should extend at least the width of the primary mirror in front of the corrector plate for adequate protection. Longer is even better. Velcro will be used to hold the sheet in the shape of a cylinder so that it can be slid over the end of the scope. Purchase strips of self-adhesive Velcro if possible, but small squares can be used if long strips can't be found. It's usually possible to purchase "industrial strength" strips, and these are preferable, since the adhesive on their backings seems stronger.
Carefully cut the sheet of Kydex or other material into dimensions appropriate for the telescope. It's a good idea to "measure twice and cut once," since screwing up and having to buy more plastic will eat some of the savings that are the reason for homebrewing a dew shield in the first place. Set up the SCT inside the house and remove its aperture cover. Wrap the material around the end of the tube experimentally to make sure it fits and fasten it in place with a few pieces of masking tape. The fit should be snug enough that the end of the shield does not droop. It's a good idea to make the sky end of the dew shield slightly smaller than the scope end for best performance. Don't make the sky end so small as to "stop down" the telescope's aperture, of course. Mark the overlap point with a soft pencil to provide a reference for positioning the Velcro strips.
Pull the dew shield off the scope, remove the masking tape that held it together, and apply two strips of Velcro (one "hooks," one "fuzz") so they join the shield together at the overlap point. One length will face up, mounting on the top of the edge that is overlapped, and one piece will face down, being applied on the underside of the edge that's "on top." Allow the adhesive to set for several hours before trying the new dew shield. The sticky glue used on self-adhering Velcro may not hold well unless it's allowed to "cure" for a little while. It is also a good idea to clean both surfaces with alcohol before applying the Velcro, to ensure a good bond. Once the strips are secure, wrap the dew shield around the end of the scope, fastening it into place with the Velcro. Looks great, works great!
There is another material that may work even better than plastic sheeting for dew protection. Being employed as an engineer at a shipyard as my day job, I try to keep my eyes open for telescope ideas as I walk around ships and materials warehouses. One day I was looking at some water chiller air conditioning equipment and noticed the black rubber-like foam used to insulate pipes. Observing buddy Pat Rochford and
I were soon hunting down this material at an air conditioning equipment supplier and making dew shields from it.
What is not good about this material? Special and somewhat vile-smelling glue is required to fasten this material together in the shape of a tube. The glue will be carried by any place that sells this insulating foam. Why not just use Velcro to hold it in a cylinder shape? The foam does not lend itself to being unfastened and stored flat, and it's easier just to glue it together. When shaped into a tube of a size appropriate for my C11, the foam tended to sag at the end. Solution? Pat cut a ring of plastic from some leaf-bagger sheeting and glued that onto the dew shield's end. With this necessary stiffening ring on the end of the dew shield, it couldn't be unfastened if Velcro were used. To install on the scope, the shield, which we made slightly smaller in diameter than the corrector assembly of the scope, is stretched slightly and pulled over the end.
If it proves difficult to find the above material, a good alternative is the thin rubber-like foam sold in sporting goods stores for use as a pad under sleeping bags. There's no need to stop with rubber foam, either. Use your imagination. Almost any insulating material that can be shaped into a light, durable tube will work very well as a dew shield. Much better, in fact, than the pretty but expensive metal dew shields sold by scope merchants.
Securing a Dew Shield Sometimes the Velcro that holds a flat, flexible dew shield in place around the corrector end of a CAT just doesn't cut the mustard. If that's the case, help out the Velcro by securing the shield to the corrector end of the scope with a clothes drier vent clamp. These hose clamp-like gadgets, designed to fasten an exhaust hose to a drier, work great for smaller CATs and can be linked together for use with larger aperture scopes.
Dew Protection for Red Dot Finders Everybody loves "bb gun finders." They present a non-magnified right-side-up image that makes a CAT easy to aim. They also collect dew and become useless in an hour or less in humid areas. To protect a red-dot sight (or Telrad) from dewing, cut down a worn-out sock until it fits snugly over the sight's glass window. Remove the sock only when it's time to use the finder and voila! Dew protection that doesn't cost money or use battery power.
Cool Down Even if you happen to live in the sunny southern United States where a winter's eve with a temperature below 32 degrees Fahrenheit is rare, you still have to allow some time for your CAT to adjust to outside temperature after being taken from the warm house. On weekends, that is not a problem, but this waiting period becomes a pain when you're intent on getting a spur of the moment weeknight observing run going. A possible "fix" would be to store the scope permanently in an unheated shed or garage. Alas, these locations are no good for many of us due to cleanliness and security concerns. There is a way to speed the cool-down process, nevertheless.
Owners of large Dobsonian telescopes have found small battery-operated fans located at the mirror end of the telescope greatly decrease the time it takes for a mirror to equilibrate, and CAT owners have realized they can do the same thing by arranging a fan to blow air into the scope's rear port. Devices to do this are available commercially, with the SCT Cooler from Lymax being a standout product. Not unexpectedly, they don't exactly give these things away. That's OK. Something similar is not difficult to make.
The most important item is a battery-operated fan that produces a good amount of air flow. One possibility is a muffin fan either purchased new or scrounged from somewhere (maybe from an old computer power supply). More elegant, if maybe not as effective, are little CPU fans. These 12 volt (usually) devices are designed to sit atop a computer's CPU and provide a cooling flow of air. CPU fans are available online and at most brick and mortar computer discount houses and "office" stores.
When a suitable fan has been located, procure a tube that will fit into the telescope's visual back. A piece of sink drain tube (PVC) works nicely, being the same diameter (1.25-inches) as an American standard eyepiece. The length of the tube is not critical. Next, affix the fan to the outside end of the tube, mounted so that air blows into the telescope. Some fans are reversible by reversing their power leads; others may have to be mounted facing in a particular direction. How exactly the fan is mounted to the tube will depend on its size and shape. It may be possible to glue it to PVC pipe with epoxy or Super Glue. In a pinch, duct tape works, even if it doesn't look very professional.
The fan will also need to be connected to a DC power source. If it is a 12-volt model, purchase a male cigarette lighter plug and cord and connect that to the leads coming off the fan. Solder these into place and wrap electrical tape around the connections. No switch is needed; just plug the fan in to turn it on. If a different DC voltage is required, dry cell batteries are the way to go. Obtain a battery case appropriate for the combination of batteries the fan needs, add a switch, and away ya go.
When it's done, slip the fan tube into the visual back, turn on the power, and let 'er rip. Ambient temperature air will be blown into the OTA, making cool down go quickly. There are enough gaps in the OTA—around the corrector end, for example— to allow sufficient air flow through the tube. Some folks worry about blowing dust into the tube, but this is not really a problem. If that is a concern, a small pad of filter material—perhaps from an air conditioning system filter—positioned over the fan's intake will keep dirt out of the OTA.
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