New Butterfly Hood on Raynox 2.2X Telex

Started Jul 17, 2007 | Discussions thread
Stephen McDonald
OP Stephen McDonald Forum Pro • Posts: 14,443
Re: Wow...

Rudy, since you have worked with fiberglass, you know for sure there was a lot of grinding and sanding involved. And a lot of itching and raw skin from the dust, etc. The finished hood for the DH1758 fits over the outside of the last 2 cm. of the telextender's housing. I cut the plastic carton off about 5 cm. from the bottom, using a small, serrated knife, so its diameter (at 75 cm.) would fit tightly over the last half-centmeter of the housing. It can't cover any more of the housing than that, as the fiberglass has to fit directly on the remainder of the overlap, to achieve its proper shape. If you can find a carton that is flared out at the top and is exactly 75 cm. in diameter, even better.

For the Raynox hood, I didn't cut the carton at all, but fit the bottom down inside the last quarter-inch of the telex housing. It has to be exactly 81 cm. in diameter. In both cases, there's a slight lip from either the carton or the telex housing, but this won't cause much of a problem in the contours.

I temporarily tacked the carton in place on the Sony telex, by putting 4 spots of shoe-goo on the inside, lapping over both the carton and the housing. Don't let the shoe-goo run down onto the lens. I then coated the housing and the carton with Johnson's Paste Wax and then used a Polybrush (foam-tipped) to put on a coat of liquid green PVA, as a mold-release. Get the PVA at a fiberglass store.

It dries in several minutes or help it along with a hair-dryer. It's soluble with water, for clean-up. I always use a two-part mold-release, for safety. Some other brands of mold-release wax are not compatible with PVA.

I then laid the telex on its side in a party-open vise, so it would support it and allow me to turn it, as I applied the fiberglass. I used 4 layers of 7.5 oz. fiberglass cloth and polyester laminating resin. I had to constantly pull the cloth in the places the contours were narrower, to tighten it up and remove the air pockets. With the hood for the Raynox telex, there are more curves and smaller diameters, so I had to cut the cloth in 4 separate strips, each just long enough for one layer and an inch to lap over. The best cloth to use, would be 6-inch wide seaming-tape, which has edges that are sewn in and there's no loose threads to fight. I let the hood stay on the form and the telex for 36 hours after laying it up, as the fiberglass shrinks as it goes through the curing process. This minimizes the tightness around the rim of the telex.

Then comes the careful cutting-off of the excess fiberglass of the hood, just 2 cm. from the edge of the telex, so it can be popped-off. I use a small, serrated knife and saw very carefully, so I don't cut into the telex. The contour flares-out a little, at the end of the housing, so it takes some struggling and wiggling to get it free.

After you've used a rasp or a Surform Plane to to roughly shape the outside, use rough sandpaper, followed by 220-grit silicon carbide sandpaper to put a smooth surface on it.

You may have to add a layer of duct tape or a layer of one half the width of the overlap onto the rim of the telex, to give the hood a snug fit. Or, you might have to take a rasp and grind off a little of the inside diameter of the hood at the very end of the overlap, to get it to slide in place on the telex.

My hood for the DH1758 is 2 inches long, not counting the overlap part. The Raynox hood is 3-inches long, besides the overlap. You could make them longer if you wanted. To cut the contours of the lobes, I marked-off the diameter of the hood at each 90-degree point. Then I drew longitudinal lines with a marking pen at those places. I made the short lobes 2 cm. less in length. The bottom contour of the cuts between the lobes is 3 cm. lower than the top of the short lobes. I drew straight lines between the center-top of each lobe down to the bottom of the cuts between the lobes with a ruler. Then, using those as guidleines, I drew in the curves outside those lines by freehand. It takes a lot of eyeballing and fine-tuning to get them exactly right.

Then, I used a sabresaw, with a narrow, fine-toothed metal-cutting blade (36 teeth per-inch, is my favorite) and carefully cut out the shape. I use round files and sandpaper wrapped around 1/2-inch round dowels, to shape the bottoms of the cuts between the lobes. I paint them with a good brand of flat black enamel.

There was a lot of rasping and shaping the overlap contours to get them exactly right. A commercial company would make most hoods with mounting threads to fit the outer rims of lenses, but the DH1758 has no threads there. The Raynox 2.2X telex does have threads, but I haven't found any source of a hood for it. I don't believe the "flower" hood supplied with the H7 and H9 would be compatible with the DH1758, as it is smaller on its inside diameter and uses a different attachment method.

Those who don't have experience with fiberglass work, be cautioned that it is nasty, messy stuff. The resin can cause allergic reactions in some people (especially epoxy resins) and the catalyst can burn your skin or damage your eyes. I keep a glass of water near where I'm working, so if I get a splatter in an eye, I can quickly dash the water into it. Gloves and a painter's cartridge mask should be used and you must avoid breathing the dust, which is abrasive to your respiratory tract. Never sand fiberglass that has asbestos fibers in it or that has carbon fibers, as the airborne particles can be very unhealthy or even deadly.

Don't hesitate to ask me any more questions if you get into building some hoods like this. I've designed and built Kayaks and Human-Powered Vehicles for many years and have done a lot of unusual fiberglass projects. This includes VTR-adaptor bodies for pro video use and bicycle and motorcycle parts.
--
Steve McDonald
http://www.flickr.com/photos/22121562@N00/

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