A Simple Observing Chair

Featured in the December 2016 issue of Sky & Telescope Magazine


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Chair with slats
In the December 2016 "Astronomer's Workbench" column I wrote about a simple, easy to build observing chair based on a design I got from Oregon ATMer Frank Szczepanski. Since the article came out I've had some people write to ask for dimensions and building tips. Happy to oblige! Click on any of these photos for larger versions.

Few if any of these dimensions are vital. You can make your chair wider, taller, heftier, whatever works. The only really precise element required is the spacing and placement of the two wooden dowels that support the seat. See below for more on that.

Note that the rear upright support is much shorter than the front one (31.5" versus 36"). That allows it to ride forward farther than if both supports were the same length, yet it's still at enough of an angle to prevent tipping over backward if you lean back in the chair. This angle is governed by the length of the horizontal front-to-back brace at the bottom. Don't make this brace out of rope or anything else flexible. If you do you'll find that when you lean forward, the back rail will scoot forward, too, and when you lean back again you'll fall over.

The bolts that hold the front-to-back brace in place are just deck screws driven through a piece of dowel (drill a pilot hole first or you'll split the dowel) and covered with rubber hose for grip. I had to glue the screws into the dowels so they wouldn't unscrew while I was tightening them into the chair legs. You could just as easily use T-nuts in the legs and use bolts to hold the brace in place. Spin a couple of nuts onto the bolt and jam them tight to the head, then cover the nuts and bolt head with a section of rubber hose for grip. Or buy a fancy knob that fits the T-nut, but where's the fun in that?

The seat is padded with closed-cell foam and covered with canvas. White canvas doesn't dew up nearly as fast as black vinyl, and it's easier to see in the dark.

The back upright section needs some cross-braces for stability, especially near the bottom. (See detail below.)

The bolts at top are just carriage bolts with capture nuts snugged tightly enough to hold the chair together without flexing, but not tight enough to bind when you open and close it.

Chair with flat rail
I've included the version with the smooth front rail, even though I'm not as happy with it. You might like it better, though. You might need sandpaper or rubberized covers on the dowels or some other more grippy surface than just varnished wood. Experiment and see.

The advantage to this design is that it's adjustable in smaller steps and you can do it without getting up completely off the seat. The disadvantage is that the chair can wobble its way down to the bottom like a toy woodpecker if you nudge it wrong. A little felt or Velcro on the sides of the seat supports will help prevent that, but it's difficult to get the thickness just right. Too much and the seat becomes difficult to move; too little and you've got a toy woodpecker again.

Here's the back of the chair showing the cross braces at top, partway down, and at the bottom. The top cross-brace makes a nice handle for carrying.

Here's a close-up of the outrigger. Note that it only touches the ground at the outer ends. The vertical rails stop at the cross-piece. This keeps the chair from wobbling if there's high ground between the ends of the outrigger.

The thick brace above the outrigger is bigger than the others to help keep the upright rails from bending or breaking the screws if you lean hard to the side. I used a pretty good sized block of 3/4" plywood. It measures 2.5" x 5.5"

Chair dimensions
The spacing of the front slats will determine the coarseness of your seat's adjustment ability. My chair is adjustable in 2.75" increments.  That seems to be okay, but if you want tighter increments you can do that by making thinner slats. Just make sure you leave enough of a gap between them to allow the seat support dowels to rest solidly on the rails.

Note that the back rails slip inside the front rails when the chair is folded up. Make sure you leave enough space for them to do that smoothly. (This will be governed by the length of the cross-braces you put on the back rails.)

Note also that the front-to-back brace that holds the chair open can be stored inside the inner rails when the chair is closed.

Those little 2.25" nubs at the end of the cross-brace are there to help keep the chair stable on bumpy ground. Without them, the chair will wobble if the cross brace is resting on a rock.

Closeup of seat
Leave room between the back of the seat and the slats so the seat won't hit the slats when you tilt it backward to adjust it for height.

I made the seat supports out of 3/4" appleply. This part of the seat receives a lot of stress, and plywood is much stronger than a plain board.

I angled the front of the seat supports. That's entirely optional. The edge of the seat hides the front of the supports in this photo, but they're blunt in front, with about an inch of wood before the angle starts.

The placement of the dowels is the only really critical measurement of this project. Those will determine whether or not your seat rests flat or at an angle, so do make the effort to get those right. Their placement depends upon the angle of the front rail, so the best way to make sure you get it right is to build the rest of the chair first, then put the rear dowel in place and see where the other one needs to go in order to hold the seat level. Note in the photos that the dowels are not necessarily evenly spaced between the top and bottom of the seat supports. The rear one is being pushed upward by the back of the support, while the front one is being pushed downward by your weight, so it makes sense to offset them a little and give them more wood to push against. That also puts them a little farther apart than if they were centered in the seat supports. Both design elements will make the chair stronger.

You may be tempted to use bolts instead of dowels for the cross supports. That will work, but bolts will dig into the rails more than the dowels will (unless you use some gargantuan bolts!)

That's all I can think of for design notes. Feel free to email with questions if you have any.

How to contact me

email graphicI'd love to hear from people who are interested in this chair design. Please feel free to email me at the address on the right. (Sorry you can't click on it or copy and paste it; it's a graphic file to thwart spambots that search the internet for addresses to send junk mail to.) I have no idea how much mail this idea will generate, so I can't guarantee a response, but I'll do my best to answer everyone who writes with a genuine question or comment about the design.


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