If you have found your way to this page, you are likely already familiar with the Telrad. This single device is probably more responsible for me staying in this hobby than any other piece of equipment that I own. I was a very frustrated beginner, not able to find anything with a finder scope, but the Telrad saved the day.
The Telrad is a wonderfully useful and simply designed piece of work. For all that was accomplished in its design, one very practical consideration was missed, protection of the the sighting window from dew. Like any other surface exposed to the night sky, under the right conditions, dew will form on the window, effectively ending its usefulness for that observing session.
After experiencing that first hand a couple of times, I went searching for a solution. There are a number of commercial products available to address this issue, and they start in price from about $20 and continue upward from there. While they will certainly do this simple job, so will this Home Built solution and for less than $3.
The materials required for the project are very simple and widely available. The shield itself is formed from craft foam. It is a closed cell foam (will not absorb water) and is available in many colors, I picked black for this purpose. This foam sheet is widely available in craft stores, art supply stores, office supply stores, even the big discount retailers. The brand I found was sold in 9x12" sheets and sold for 59 cents a sheet.
The only other material we will need is some Hook and Loop fastener (Velcro). It is available in many forms, but I found for this project, the pre-cut round variety were perfect.
As to it's shape, To be most effective the shield should extend for some distance over both the front and the back of the screen. The only other consideration is that we must accommodate the switch lever. After playing around with a couple of trials, I settled on the shape below (gray area).
I cut the template out of some yellow card stock and traced the outline onto my foam using a pencil.
Your best friend on this project is a fresh, sharp razor or hobby knife. The foam cuts very easily with a sharp implement. I have tried scissors, but have found them to leave the edges a bit rough for my liking. They are also difficult to use for cutting out the circle.
Once the piece is cut out, the next step is to apply the Velcro tabs. I used 4 per side, three along the bottom edge and the 4th on the side of the viewing window to hold the shield closer to the side of the Telrad. To place them, for the bottom tabs, I simply laid the edge of the foam along the side of the Telrad and matched the locations on the Telrad to the foam. For the tab on the side of the window, the simplest way to get the placement to match was to attach the tab to the Telrad, then attach the mating tab to that with the adhesive exposed. Next, carefully line up the foam and attach it to the three bottom tabs, then just press the foam down on the exposed adhesive of the 4th tab.
Repeat the process on the other side and the job is done.
It is very easy to modify the shape of the foam in any way you may find useful. For those interested, I am making the template that I used available for download.
The template I created was hand drawn and then scanned, as a result it is by no means perfectly symmetrical. If anybody accomplished in CAD or other Computer Drawing and would like to clean it up and send me a file, I would be happy to make it available here.
Thanks to the kind efforts of Michael Dziak of Astro Trim, we now have a clean and symmetrical template.
He cleaned up my scratchings very nicely.
A note on printing the templates. The template is a full 11 inches long and 5.25 inches wide. Due the the limitations of some printer's abilities with regard to printing to the very edge of the paper, you may find that your template seems to have a "border" around the edge. Just ignore that white space and include that space in the template. To say it another way, the only cuts you should have to make when the template is printed on a standard 8.5x11" sheet of paper is the side of the template with the curves, and the hole. The other sides are defined by the edge of the paper.
The downloadable template files are below,
RIGHT CLICK on the link, Save the file, and then open from the saved location. By doing it that way you avoid difficulties that can be introduced by your browser. Many browsers automatically resize graphics to fit the screen, or printer, or add headers and footers to prints. Obviously, any change in the size of the template will change how it fits, so we should try to avoid that. RIGHT CLICK on the link, Save the file, then open and print with a graphics program to avoid these issues.
Telrad Dew Shield - JPEG Format
Telrad Dew Shield - Bit Map Format
Telrad Dew Shield - TIFF Format
In use, I have found that this shield long delays the formation of dew, many times well past the point where I begin to have troubles with objectives and secondaries dewing up. Now if there was $3 solution for those other issues I would be all set.
©2009 Rod Nabholz