The Practical Observatory
by Dennis Allen

My family owns property up in west-central Michigan. This is an area known for it's relative dark skies. A place I go to hunt, fish, and enjoy the occasional clear night. Early this spring, I was treated to a flock of clear nights. One problem: Too much snow on the ground. There was simply no place to set up my telescope. So in the year 1994, I vowed to build an observatory.

My original idea was to create a peaked roll-off roof. This building would have a 12' square wood floor and 4' walls. Wide enough to leave plenty of room for my 13.1" reflector. Whenever I got a bigger telescope, something requiring more stability, I could always pour a small concrete pad. I wanted something simple, practical, and durable. But I didn't want to spend years planning and months building.

I kept my design simple: A one piece roof, rolling to the north. Three inch caster wheels would extend down from each truss and would ride on aluminum channel. To keep the roof light, I'd use corrugated sheet metal. The south wall would have a standard 3' by 7' door, cut off at the 4' mark. The upper 3' section would hang from the southern gable.

Step one was to build a scale model. Most people do not know what a roll-off looks like. A one inch to the foot scale model helps illustrate your intentions. You can obtain materials from any model airplane shop.

As it happens, my father is a carpenter. I told him my plans and showed him my model. I kinda knew he'd help. He quickly drew up a list of materials. To keep snow off the roof, he suggested a 6/12 pitch roof. To maintain head clearance, he suggested using church trusses. With 12' church trusses, the bottom 2"-by-4" doesn't go straight across. Instead, two 6' horizontal 2"-by-4" pieces connect to a vertical 2-1/2' 2"-by-4", creating an interior 3/12 pitch.

As soon as the snow melted, I contracted a bulldozer to clear and level the top of my hill. My dad ordered the trusses, custom made, from the local lumber company. One regular 12' truss (for the northern gable), and three of the 12' church trusses. Meanwhile, I ordered four 16' sections of 1-3/4"-by-3/4" aluminum channel from a local sheet metal shop.

By the time I was ready to build, several people told me a light concrete truck could make it up the hill. I always wanted a concrete floor. Concrete makes for a solid foundation, and is less expensive than treated wood. With a concrete floor, my building could house a bigger telescope. To house an eight-foot long telescope, for example, I'd simply locate it's base a few feet north of center. I had considered the thermal problem of concrete. But this is a roll-off, after all. Once opened, the heat should dissipate quickly.

There was one drawback, however. A concrete floor meant a permanent structure. Such a structure would require a special building permit from the local township board. I would have to hire a surveyor to obtain the exact location of the structure. Finally, I would be required to withdraw that location from the Commercial Forest Act of Michigan.

While acquiring the permits, I decided to upgrade my design. I opted for a 12' by 14' building with 5' walls. I would have liked a 14' square building, but I already had the 12' trusses. These trusses were designed for 48" centers. So my dad made a fifth truss, using the other trusses as a pattern, to give me 40" centers.

By the time I got my permits, it was almost the end of June. But with help from my dad and brothers, I knew it wouldn't take long to build. In fact, it didn't take an hour and we already had the forms in the ground. Once the forms were down, I had the local cement company bring in three yards of concrete. We went with a 4" thick floor, 10" edges. We used 5-gallon buckets, open at both ends, as forms for the outside rail posts. The whole process took only a half day. There was plenty of leftover concrete, though no extra forms. We should have poured an outside viewing pad. Something you might want to keep in mind if you decide to pour concrete.

Actual construction started a couple of days later. On the first day of construction, we threw up the walls and the rails. The walls were built out of simple 2"-by-4"s, 16" centers. We used treated pieces of 4"-by-4" for the top of the walls, the bottom of the trusses, and the outside rails. To connect the rails to the walls, each piece of 4"-by-4" had a 2" square notch at the end.

On the second day of construction, we put up the plywood. Originally I thought of using cheap particle wood, covered with vinyl siding. My dad, however, talked me into using fake rough-cut 7/16" plywood. This material looks like rough-cut pieces of 2"-by-8". As it turns out, this material is stronger than particle wood and already had a gray primer coat.

We brought 13 sheets of plywood. The sheets were cut with a 2" inch overhang on the bottom and a 6" overhang on top. The top overhang turned out to be a blessing. It would end up overlapping the roof's 4"-by-4", covering the caster wheels completely, thus keeping the elements out. As a bonus, this top overhang would serve to keep the roof rolling in a straight line.

We brought a full size door, cut at the 5' mark. So to finish the day, we hung the bottom section. We made this section of door swing to the outside, thus preventing people from kicking it down. If you hang a door this way, however, remember to use special outdoor hinges.

On the third day of construction, the roof went up. We mounted the ten 3" caster wheels on the two 14' pieces of 4"-by-4". The caster wheels were spaced so that each wheel would rest under a truss. The channel was used to make sure the wheels were lined up correctly. This channel was already counter-tapped, so we quickly screwed it onto the rails.

One suggestion: Keep your location in mind. But for a portable generator, we had no electricity. So try to have as much of your material prepared off-site, as possible.

The two 14' pieces of 4"-by-4" were dropped into each channel and the trusses placed on top. We used 14' pieces of 2"-by-4" to connect the trusses. After some adjustments to the trusses and caster wheels, we could roll the roof back and forth.

Originally, we ordered 16' pieces of 2"-by-4" to mount the corrugated sheet metal. We didn't stop, however, just because we were stuck with 14' pieces. To get our north/south overhangs, we simply used scrap pieces of 2"-by-4". This added a little weight to the roof, but hey: If you stop construction for every minor inconvenience, you'll never get any work done.

For the roof, we used 11 panels of 8' White McElroy. These sheet metal panels went up in only a couple hours. We did have to cut one end piece. For that, however, a roofing knife did the trick. Simply run a straight edge with the knife and flex the sections until they split. But whatever you do, be careful! While all the panels have an edge, the cut pieces are razor sharp.

Oh, another suggestion. When you install your panels, do both sides at the same time. Each time you have enough panels, put a section of cap on. When we installed our panels, we left the cap to last. Being light weight, I had to perform a high-wire act just to get the caps nailed down.

On the fourth day of construction, we worked on the gables. We were running short of plywood, so we brought three more sheets. Which, as it turned out, was about how many sheets worth of scrap we had left over.

The northern gable was easy. The fake rough-cut plywood was measured and cut to butt right up to the corrugated. Notched out for the 2"-by-4" slacks. To keep out the elements, we left a few inches of overhang on the bottom of the gable.

The southern gable was a different story. I wanted 3'-by-7' of clearance for the door. To achieve that, we couldn't place a piece of 4"-by-4" across the threshold. The fake rough-cut strengthen the walls considerably, but the southern wall was still the weakest. So for more strength, I decided to add tables to each corner on the southern wall.

For the upper section of door, we built a 2"-by-4" frame. For strength, we used a couple of 2"-by-4"s to connect the lower gable corners to the next adjoining truss. We placed our hinges at the top of the upper door, so that it would swing inward. When I want to move the roof, I simply prop the upper door with an extra piece of plywood. To lock the upper door, I mounted I-bolts and drilled two holes into the 2"-by-4" frame.

To roll the roof off, there couldn't be any plywood overhang on the southern gable. So we used 1"-by-6" trim, nailed to the southern wall, to cover the crack. We also used this material around each section of door.

To keep the roof from blowing off, I installed chain binders to each corner of the building. These chain binders hook to big eye-screws, which are screwed into the roof's 4"-by-4" pieces.

And that's it! Since then, most of the work has been minor. I've added corner tables to strengthen the southern wall. For security, I installed a latch guard on the bottom door and a 12' cattle gate at the bottom of the hill. They may not stop anybody from breaking in, but they should make people think twice.

I added 40" strips of 4" square foam between the trusses and the pieces of 4"-by-4". They keep the elements out, as well as animals and insects. This last month, we've had lots of rain in Michigan. The building, however, has remained bone dry.

As an added touch, I installed a 12'-by-14' piece of outdoor carpeting. The carpet helps protect your telescope from the corrosive effects of concrete (not to mention saving that occasionally dropped eyepiece).

Were there mistakes? Most certainly. When the cement truck left, he had to dump the extra concrete. That concrete could have been used for another viewing pad.

We could have reduced the weight of the roof if we had single 16' strips of 2"-by-4". In fact, we probably have gotten away with 16' 2"-by-2" strips. Though, you'd have to check to be certain it's within code.

If I had to do it over, I'd have used 4" caster wheels instead of 3" wheels. The 3" wheels have already developed a fine film of rubber. Probably due to wear and tear. At some point, I may end up replacing those 3" wheels.

But there were pleasant surprises. The plywood overhang does cover the caster wheels rather well. They made building the roof easier. In addition, I don't have to insert foam strips between the caster wheels to keep the weather out.

The church trusses make the inside look like a cathedral. Had I known I'd have that much head room, I'd have stuck with 4' foot walls.

I was a little worried about the channel. The caster wheels are 1-1/2" wide, while the channel is less than 1-3/4" wide at the ID. I figured for sure the wheels were going to bind. As it turns out, however, the tight channel keeps the roof running in a straight line. And there is no need for side casters.

At first the roof was very hard to roll. I was already thinking I might have to rig up a block-and-tackle system. But as time went on, the rolling became easier. The plywood overhang tends to swell, so I've been inserting wooden shims to keep it peeled back. Applying silicon spray to the caster wheels also helps reduce friction.


The entire building cost about $1500 in materials. Less than I expected. The success of this project goes in large part to having a carpenter supervise the construction. I'm very lucky to have one for a father. I'm also lucky to have brothers willing to lend a hand. Don't have a relative in construction? Why not hire one? You'll cut down on the building time and you'll end up with a better observatory. You know the old saying: Penny wise and pound foolish. If you need to save money, get your friends and family to help with the grunt work.

The future

In the future, I might replace those 3" caster wheels with 4" wheels. But new wheels call for new channel. For the moment, I'll just keep the wheels cleaned and greased.

Except for your head, the 5' walls provide a good protection against the wind. In the future, I might make a couple of 3'-by-3' wind panels. These panels will have 2"-by-4" pegs about 2' long. They should work like side rails you put on a truck bed. In whatever direction the wind blows, I'd just put up panels to block it.

At some point, I should pour an outside viewing pad. I've already had people come out for a visit. Unfortunately, they had to set up their telescopes in the sand.

1995 Events

It's been ten months since we built the observatory. So far the building appears in good shape. The inside stayed dry all winter. The outside rails, however, did need some work. The 4-by-4 wood was a little green and the west rail twisted on me. So I shimmed the center post and added a few reinforcing trusses (something I should have done in the first place).

I did notice one other problem. Since last fall, the roof was getting harder and harder to roll. Straightening out the outside rails helped. But then I noticed the distance between the east and west channel wasn't built even. In fact, the mid-section of the building loses about 3/4". I also noticed the rollers on the east side appeared to be staggered against the lips of the aluminum channel. The rollers are 1-1/2" wide, while the channel is only 1-3/4" I.D. Not much room for error. So I decided to replace the east side with 3" wide aluminum channel. That worked! Rolling is much easier now.

1996 Events

Another year went gone by. The building is still in real good shape. I had problems with wasps and yellow jackets, but a no-pest strip took take of them.

This year I decided to replace the west side with wide 3" channel. I was a little worried, but the east-west plywood overhang has kept the roof rolling in a straight line. In the future, I want to install 4" caster wheels. They'll have the same width as the 3" wheels, and will only raise the roof one inch. I'll still have east-west plywood overhang.

In the future, I might put down a wood floor. You see, my new 24" telescope is somewhat allergic to heat. Unlike my smaller 13" telescope, the 24" scope takes a couple of hours to cool down enough to view low-horizon objects like Jupiter. It should be easy enough to install a wood floor over the concrete. I'll lay 2"-by-4" studs flat with 16" centers. I'll insulate between the studs and reinforce the center. Finally, I'll cover the entire floor with 1/2" plywood.

2002 Events

Wow, time does fly! The building is still in remarkable shape. I've had to do very little maintenance. The 3" caster wheels are slowly wearing down. Soon I'll be forced to install new casters, else the top of my door will no longer clear the bottom.

Once a year I have to spring-clean my building, mostly to get rid of Japanese ladybugs. Those ladybugs get into everything! Every nook and cranny, every part of my telescope, even into my radio! The mice were also chewing my telescope shroud, so now I store everything in a big self-sealing container.

Decided not to add a wood floor. Between the outdoor carpeting and the telescope shroud, I see little need for a wood floor. I still want an outside viewing pad, however. I also want to add barnyard soffit. Foam strips between the trusses are nice, but the wind can blow them out. As long as overlap is short of the building, the soffit should work fine. What I don't want is wooden panels. If the wind blows higher than twelve miles an hour, my 24" telescope turns into a weather vane and I have to quit taking pictures. Wooden panels wouldn't help.

2004 Events

Haven't been up since last August (not after that big fight with a hornet nest). No wasps, but the mice sure did tear up my chair (will need a new one). They also started working on the soffit foam strips. If they do more damage, I'll replace the foam altogether with regular barnyard soffit.

Cleaned the observatory and gave it a fresh coat of paint. On my next trip up, rather than replacing those caster wheels I think it'd be easier to elevate the channel. Maybe some 1/4" plywood. Otherwise, the place is in great shape.

2005 Events

I took a half sheet of 5.6mm plywood and cut 3" strips. Screwed these strips under my 3" channel. My roof is raised just right. The upper door clears the lower door, no problem. Now I don't need to replace my dolly wheels. Sometimes a simple solution is the best.

I see the mice are still working on the foam soffit strips. Yes, I have to replace the foam with regular barnyard soffit.

2014 Events

After our record cold winter, the mice finally tore up most of the foam soffit strips. I decided to remove every last inch of foam from my building.  Replaced the foam with 5.5" strips of 1/8" plywood. Not an airtight fit, but keeps the wind out. Due to the cold winter, had to replace the deep cycle battery (shouldn't have left it there over the winter). Also had to paint the door and a few spots in the plywood where animals started chewing on it. The building itself remains in surprisingly great shape, considering 20+ year old homebuilt observatories are rarely still standing.

2016 Events

When the local astronomy club rebuilt the big 16'x20' roll-off roof, I moved my 24" telescope into it.  Although my observatory is now empty, I plan to put back my 13.1" Coulter telescope.

A thought. Over the summer we rebuilt the club dome as a roll-off. This new roll-off is about the same size as my observatory, but much easier to open. Unlike my building, the new roll-off has 4.5' walls and 5" caster wheels. So I have an idea: Let's lower the walls on my observatory about sixteen inches, the height of two layers of brick. And let's finally replace the 3" caster wheels with 5" wheels. That should make the roof easier to open. If I bring back the 24" telescope, lower walls also means the scope won't need to sit on two layers of brick. The biggest challenge I see is adjusting the frame headers, since the wall panels are nailed not screwed. Might have to put in new paneling. Now I don't know if I'll rebuild next year or wait until I get a chance to clear-cut the trees on top of my hill, so check this article for further updates.



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