Now the fun part begins.
This project is all well and good as a telescope base (albeit one that opens up and has the potential to dump my yard cannon on the ground in a shattered mess)....but I designed it as a hiss drive primarily, and a telescope mobility platform secondly. It’s time to get drivin’.
Going off what I found on the internet I gathered the following supplies:
One inner-tube (a 14” non-highway trailer inner-tube from Menard’s I think).
One hand pump for said inner-tube.
Two packs of aquarium air hose (8 ft each, just to be on the safe side…hey, it’s only $1.50).
A package of two 3-way aquarium air hose connectors and a one-way check valve (again for aquarium use).
And the all important escapement valve—-in my case, a 2 1/2 way aquarium air “junction”. It was the only thing I found at Wallyworld that had a needle valve on it that I could adjust to customize how fast the air is let out of the inner-tube. A trip to Petco may be in my future…this thing is kind a awkward and I’m only using two of the 5 ports on it…
Right. First I had to figure out the order of assembly. The idea of the hiss drive is simple. For a little historical overview, click here. Basically it was thought up by a gentleman named Tom Fangrow back in 1985. You take a box, slice it in half at a angle that matches your latitude and put a hinge on that angle, so that the upper part of the box “rotates” around the hinge—-which is lined up with the earth’s rotation. Your box now effectively counteracts the rotation of the earth. To control said rotation, he put an inner-tube in between the two halves of the box, connected air lines to it and used a valve to very very slowly release air from the inner-tube. Once locked on a celestial target, the releasing air allowed the upper part of the box to slowly rotate around the hinge, canceling the effect of the earth’s rotation and letting Tom view his target without the infamous “bump n nudge” so integral to Dobsonian telescopes.
Will it be the next greatest invention and replace computerized GoTo SCTs or Dobs that with the click of a button will slew to a target and track it all night? Not likely. But for the poor schmuck like me who can’t afford to drop $800 bucks on a scope that’s smaller than what he already has…this could be the next best thing since sliced bread.
How well I put the dang thing together will determine that. I do like my bread, so topping the already sliced thing will be tough.
Okay. It seems simple. I need a way to hook the pump to the inner-tube and inflate it. To make it really useful, the pump needs to stay attached in the field so if the air runs out (I have NO idea how long it will last…internet reports are from a few minutes up to an hour or more, depending on the type and size of the inner-tube or other inflatable thingy) I can just fill ‘er up without having to dismantle everything and pull the telescope off the base.
To complicate things, if you don’t have a one-way check valve, when you stop pumping, the inner-tube will simply deflate. So, that check valve needs to go in line before the pump. Online, I found that most guys were cutting the pump hose in half—-leaving the connector…well, connected…to the inner-tube. It seemed to work, so that’s what I did:
To my relief (it was only a $10 pump, but man was that not easy to just slice open a brand new pump) the hose from the pump was almost perfectly sized to allow the fish tank hose to slide right in!
I hooked up the other end of the 8 foot fish tank in-line to the connector part of the pump hose…
And then locked it in place.
Next I came up from the pump about a foot and cut the aquarium hose and installed the check valve:
From what I can tell it doesn’t matter where you put the check valve, as long as it’s closer to the pump than the three-way connector. After all, we want the air to come out of the inner-tube, we just want it to go to the needle valve so we can control the escape velocity. Once that was installed, I came up another foot or so and added the 3-way connector.
I had to cut about half the air tubing off at this point to make sure I had enough for me to stand at the eyepiece and control the air flow. From the splitter, the air line goes (1) to the inner-tube and (2) to the needle valve.
Well. That about does it, I think. Next was the big test. I tightened the valves and started pumping. The inner-tube filled up on about 7 pumps—-not bad! It didn’t deflate on it’s own…so everything was holding. I turned the knob on my escapement valve and heard nothing. I turned so more and hard a very faint hiss of air escaping. Then I opened it up wide open and saw the inner-tube deflate.
It looks pretty ungainly sitting there on the floor. Let’s get ‘er installed.
Once out in the shop again, I quickly realized that the connector on the inner-tube sticks up at a funny angle and prevents the upper half of the platform from smoothly compressing the inner-tube. A kiss from the drill and there was a nice hole for it to snake through, giving me no problems. Then I added a zip tie to keep it secure (no too tight…don’t want to pinch the air line). I then added another zip tie to loosely hold the inner-tube in position in the middle of the platform (as soon as you let go, sitting at 45º, the inner-tube wants to slalom down the platform to the ground).
Ah but there’s one more thing to do. There was a problem I read about that I had to address (from inventor Tom Fangrow’s site):
“A drawback to the design is that the telescope rests on a very springy cushion, and light breezes may bounce the tube. But Fangrow dampens these oscillations in a simple manner. When the base folds around the inner tube, an air pocket is trapped inside the “doughnut hole”. Several small perforations, drilled through the center of the lower prism’s hypotenuse, allow this air to pass through the drive base. The resistance of air passing through these small holes keeps the telescope from vibrating.”
Hmm. Sounds easy enough:
Now, things are getting interesting. To test it, I put everything together and added some weight (my drill and impact driver) to the platform. In a few pumps the inner-tube was fully inflated, everything was holding steady and ready. I slowly released the air and nearly squealed in delight as the upper platform slowly rotated around the hinge and closed, squeezing the air out of the inner tube.
It’s aaaallliiiiiiivee bwahahahahaaaa!!!!
Ahem. Okay, so excitement building, I still had to mount the telescope to this thing. That required getting some holes cut to fit the feet on the base of my scope.
Easy enough with some Forstner bits. As an unexpected benefit (not even sure it’s necessary), the supports for the upper base were positioned perfectly so that when I drilled holes for the telescope feet, two of the three were drilled through the plywood and tubafore, making an nice secure…footing…for the feet. And as Jack Nicholson said, “Two outta three ain’t bad.”
Then I put the telescope base on for a glamour shot:
Everything fits nice and tight and secure, no wobblies, no worries. At last, after over a year of dreaming and planning and buying materials and then false starts and aborted attempts, we have the rover completed (almost)!
Alas, there is still plenty left to do—-and with the way it suddenly turned cloudy (of course) the last few days, I may get this thing 100% finished before the test drive. Here’s a list:
1. Trim the locking dowel on the lever to size and cap (if desired).
2. Round off the end of the lever for comfort, install cross for ease of use.
3. Make and install a “lock” to keep the platform stable when the drive isn’t in use.
4. Round off/sand all sharp edges—-no splinters or cuts in the dark please.
5. Attach the powerstrip from the old base for ease of use of the cooling fan, netbook for imaging, etc.
6. Tidy up the air hoses and find a spot to conveniently store them all and the pump (inside the base).
But for now…I really can’t wait for a clear sky to test this contraption out!
-- Steve http://vaughtwoodworks.wordpress.com