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[ Mariner Home | Introduction | Optics | Plans | Final |
Construction I | Construction II | Construction III | Construction IV ]
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Construction IV


Truss

Before putting the truss together, I needed a rough idea of how long the poles were going to be. To do this, I built a rail system to hold the mirror box and the secondary cage on a sliding sled, then set the whole thing up horizontally to look at a distant mountain. I used the geometric optics formula (1/o + 1/i = 1/f) to make sure I was looking at a distant enough object ("o") as to not make a significant difference in the image location when compared to the stars.

truss thumb truss thumb truss thumb truss thumb
truss thumb truss thumb truss thumb truss thumb

Ultimately, I found the outcome of this endeavour to be unsatisfactory for a perfect estimate of the truss length. I wanted to make sure all my eyepieces could focus. So after this experiment, I set the whole thing up outside one night, looking straight up, and slowly cut off 1/8" to 1/16" segments of my truss poles until I had the focus behaviour I wanted. As it turns out, the horizontal test gave me the same number. :-)

Following the advice in Kriege & Berry, I decided I needed to be using 1.5" tubing for the truss. I couldn't find anyone who made 1.5" truss hardware and had resigned myself to making my own (as I did on Equinox), but at the Oregon Star Party in 2010, Tom Clark suggested Aurora Precision. Their stuff is great! I use their Quick Release Cage Clamps and their Truss Clamps. The only modification I made to them was I changed the clamping knob. They come with round, fluted edge knobs (McMaster Carr #6479K54 -- 1-1/4" diameter with 1/4-20 threaded insert). I found them hard to get a good grip on, especially with gloves on, so I replaced them with the knobs I use regularly on my other scopes, with 4 arms (McMaster Carr #5993K45 -- 4-arm 1-1/2" diameter with 1/4-20 threaded insert).

I also needed aluminum poles with the right thickness for the machined patter on the Aurora cage clamps, so I ordered it online from Aircraft Spruce. The upper cage clamp assembly holds two poles together with a bracket insert that is secured with threaded inserts.

truss thumb truss thumb truss thumb truss thumb
truss thumb truss thumb truss thumb truss thumb

Covering the poles was not too bad. Finding snug polyethelyne foam that fit 1.5" poles was hard. I toyed around with sticking two smaller pieces together, and also with other kinds of wrapping, but eventually went to a plumbing supply store and bought foam for a 1-5/8" pipe. I cut a small strip off, used the remaining sticky tape to try and bind the foam closed around the poles, then added velcro ties to keep it closed. It works pretty good!

While everything is, in principle, interchangable, it is my habit to mark things like poles and always put them in the same place to minimize the chanes I experience when I take the scope apart and put it back together again. I use a 4-label system on all parts (it's also written on hidden parts of wooden pieces): GND for parts on the face of the Dob that tips to the ground, FAR for parts that are on the far side of the scope from the eyepiece, SKY for parts on the face of the scope that is closest to the sky, and EYE for parts on the same side as the eyepiece (for me, this is the side where I can look through the eyepiece, and grab the top of the cage with my left hand for guiding).

truss thumb truss thumb truss thumb
truss thumb truss thumb truss thumb

For the shroud, I thought about having one made --- there is good chatter about Shrouds by Heather, who is affiliated with Teeter Telescopes, and they look pretty sharp (particularly with the outside colors), but I did not want spandex. Spandex gets tight when its hot (that's why it looks good as exercise clothes!), and expands when it gets cold (that's why it sags on your scope at night!).

On Equinox my mother-in-law had sewn my shroud for me out of t-shirt material. I asked her to do this shroud and she agreed. I was in Alaska for the Venus Transit and the AAS Meeting, so she and I spent some time trolling around and found what we needed. This shroud is big, so we bought JoAnn Fabrics out of all they had. :-) I used Nite Ize reflective camp cord for the drawstrings.

truss thumb truss thumb truss thumb truss thumb

Once the truss was in place, I could work out the balance point. I put everything in/on the scope, then laid the truss across a large diameter dowel, looking for the balance point. Wherever that point was, is where the center of the bearing had to sit.

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Finishing

I've never been fond of the pure finished birch look that is so common to modern truss dobsonians. When I built Equinox, I chose a green stain on much of the birch plywood, complemented by some unstained birch. When I started planning Mariner I had thought I might choose a different color, but couldn't find one I liked. In the end, I went back to the same green as on Equinox. Fortunately I had some of the stain left from 10 years ago; it is apparently no longer available, but I had enough to finish. I'm hosed on the next telescope, however.

truss thumb truss thumb truss thumb

The longest part of the finishing process was coating everything in polyurethane. I used Helmsman Spar Urethane. Everything got three coats, with a round of sanding with 220 grit sandpaper between each coat. I set up an assembly line, so I could give the entire scope a coat at a time. I had to build jigs to hold the bearing upright, allowing me to do both sides at once. The inside of the mirror box was painted flat black, after a single coat of varnish to seal it.

truss thumb truss thumb truss thumb

As I've noted elsewhere, many of the parts are labled to go on a particular side of the scope, allowing me to take it apart and put it back together the same without too much confusion; I just varnished over the labels, so they are permanent. Once the finishing was done, hardware went on (such as the truss hardware), and the various parts were mated together (like the bearings to mirror box).

truss thumb truss thumb truss thumb truss thumb

There are two plaques on the scope, etched on black, anodized aluminum. One is the telescope nameplate (on the mirror box), and the other (over the eyepiece) is a quote from van Gogh that I particularly like.

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[ Mariner Home | Introduction | Optics | Plans | Final |
Construction I | Construction II | Construction III | Construction IV ]
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Last Updated: 25 September 2012