An Inexpensive, Waterproof, Durable and Easy DIY Table-Top Dew Hutch
Submitted: Wednesday, 7th November 2007 by Scott Tannehill
Dew is one of the banes of amateur astronomy. Dew occurs when an object’s temperature falls below the dew point, and is similar to the phenomenon that makes a cold beer bottle sweat on a hot day. An object’s temperature falls at night because it loses more heat (by radiating in the infrared) than it gains. Dew forms more commonly when the sky is cloudless, because the soon-to-be dewed object in question radiates heat more readily in the absence of cloud cover; clouds act as an insulator to keep heat in. Dew can form on cloudy nights, but it’s not our concern then, obviously. Low heat capacity materials radiate heat (collect dew) more readily than high heat capacity substances. So your metal tube – which radiates heat more easily just as it conducts electricity more easily – will dew up before your Pyrex primary mirror. But secondary mirrors, corrector plates, eyepieces, finders and other items will dew up more readily for several reasons, primarily because they have less mass and can drop their temperature faster, and also because they are more exposed to the night sky. The primary mirror of a Newtonian reflector, in addition to having more (thermal) mass, has that long tube acting as a barrier (insulator) against radiative heat loss. Primary mirrors rarely dew up for this reason. Dew shields work by limiting the amount of sky the object (secondary, corrector plate, lens, etc) can “see” and to which it can radiate heat. Such a shield isn’t proof against dew, it just delays the process of radiative heat loss.
I have concluded, after living here for eight months, that Melbourne, Australia is Dew Hell. My home state of Wisconsin in the U.S. is, at best, Dew Purgatory. The only reason we have fewer dewed-out nights in Wisconsin is because four to five months of the year is below zero; anyone out observing isn’t fretting about dew, but rather about having left all of his or her eyelashes frozen to the eyepiece.
I have not been out a single night here where some piece of equipment did not suffer some consequence of dew. One cold night in Snake Valley we were struck down by the double indignity of terrible dew followed by below-freezing temperatures; all of our dewed-up gear frosted. My conclusion (and I’m a foreigner and could be wrong, I’ll admit) is that with the exception of Tasmanians, Australians are unaccustomed to frost on anything except a very cold beer. I reckon some of the guys from that night are still in counseling.
I have a Dew Box back home. This is a nice product made by a fellow in Milton, Wisconsin. I cannot recall his name, nor is there any website. A friend bought it for me at a star party in Wisconsin. It was – at the time - $40 USD. It is black fluted plastic. Corflute™ is one trademark name for this material. It had folding panels and Velcro ties, and when assembled it is a waterproof cover that sits on a table, under which you can place items you want to guard against dew. It’s no different than simply putting a box top over the object to insulate them against radiative heat loss. Nothing ever dewed up when stored under my Dew Box back. There is a good reason the Dew Box man in Milton chose fluted plastic. It’s waterproof, light, cheap, and durable. It’s much better than cardboard (which I used for a prototype hutch); cardboard is flimsy, fragile, and prone to disintegrate when damp. See Figure 1 for a close-up of the fluted plastic material.
I decided to make my own dew hutch. Dennis Simmons’ dew hutch article here on IIS is wonderfully presented; he deploys a dew hutch that is heavier duty and long on deluxe features. It’s also, accordingly, a fair bit more effort, and furthermore beyond my skills. I had simple tools and limited time and I chose to stick with simple materials: fluted plastic and duct tape.
For $20 AUD I bought a 1.2 x 2.4 meter sheet of white (only color in stock) Corflute™ at Aspect Packaging in Braeside, a Melbourne suburb. At Clark Rubber I bought some water-proof cloth tape (only in-stock color was red); some self-adhesive Velcro strips; and some rubber stripping material that I could slide onto the bottom edge of the dew hutch to help keep it on the table and keep it from getting chewed up over time. Total project cost was ~$45 AUD. I already had a piece of no-slip silicon mat for the table-top to prevent things from sliding off the slippery plastic; this no-slip material is the same stuff used under area rugs to keep them from skittering around on a hardwood or tile floor. I highly recommend this for those slippery plastic tables; it keeps those expensive metal eyepieces from shooting off like a hockey puck.
I measured the sizes of each panel, cut the fluted plastic with a utility knife and duct–taped the panels together to achieve the shape and structure I needed. You need to be thoughtful in how you tape the panels together so thing lays flat when disassembled, but it’s easy to figure out once you start. Figure 2 is an illustration of the simple panel design.
I applied Velcro at key locations to facilitate assembly into the assembled hutch, and so it would collapse down fairly flat for transport. I also used Velcro on the table, so the hutch could be secured to the table itself to prevent it from blowing off in wind. Any damaged panels on the hutch can be replaced (you’ll have spare Corflute™ if you buy the same size sheet!) in minutes with new cloth tape. However, I’ve yet to do any repairs; after a hard week of use there is no damage except some superficial creases in the Corflute™ which have no functional impact. Not surprising, since this is used to package delicate equipment for shipping. Most importantly, I took this to Coonabarabran for a week and had NO dew effects under the hutch.
See Figure 3-5 for several photos of the hutch assembled and folded flat. It takes about 60 seconds to assemble or disassemble the hutch. It folds up nice and flat and can be stowed quickly. Corflute™ is so durable that I’ve put 40 kg of gear atop the folded hutch without ill effects. When assembled the height is high enough to accommodate my eyepiece case lid, and I don’t have to hunch down to see my laptop while sitting at the table. Despite this height and the long span along the length of the table, the hutch is strong and stable. One nice feature is that you can Velcro small red lights on the inside of the top, to create a soft faint red glow under the hutch for reading atlases, seeing your keyboard, selecting eyepieces, etc, without having to hold the light in your hands.
And the top roof panel of the hutch, while not terribly rigid, is surprisingly strong. I put a variety of light and durable things here where they are out of the way but still easily visible for quick retrieval. See Figure 6. The ceiling panel bends, but is far from tearing or collapsing into a crease even when over-loading as shown. While I would not advocate loading on this much weight in practice, the demonstration does illustrate the strength of this material.
Clear Skies and Dry Gear.