Tips and Suggestions on Building A Rolloff Roof Observatory

Detailed 3D CAD drawn plans featuring step by step construction details


Written 2007 by: Jim Pennington

When I first contemplated building my own backyard observatory, about all I had to draw on were magazine articles and photos of what others had done. Since everyone's needs and budgets aren't the same and telescopes vary tremendously in size, I was left with the realization that I was going to have to pretty much design the observatory myself.

The first consideration was how much was this project going to cost. I initially figured construction costs were going to fall in the $15 to $20 per square foot range. In the end, I wasn't too far off base. The second consideration was where to build it. While much can be said for locating it at the darkest site available, other very important factors come into play. I felt the most important of these would be accessibility. If the observatory isn't accessible for frequent use, then it seemed to me that it would be located in the wrong place. Because of the nature of my work (I am a cotton farmer), I frequently work long hours which puts me arriving home pretty tired. By the time the necessary shower or bath is over and supper is eaten, there simply is not enough time to pack everything up, drive several miles to a dark site, do my business, take down the scope and drive home. When one has to arise at 5:30 in the AM to start another day's work, the decision on where to build is not a hard one to make: it would be built close to my bedroom and that means building in the backyard.

I live in a small "semi-urban" community in NE Louisiana. While not exactly being in the country, I do have a few neighbors, a yard light in one neighbor's yard and a nearby 2 lane highway and railroad to contend with. The yard light problem was easy to solve and I figured the highway could also be dealt with to some extent. The railroad turned out to be the biggest problem and a partial solution will be addressed later.

My telescope, an 8 inch Meade LX200, requires a minimum of space and I calculated that an 8' X 8' floor space would be adequate. After completion, I realized that a 10' X 10' area would have been better for this size instrument to allow ample room for a desk and observing chair. With my 64 sq. ft. current size, I am a little cramped for space and the extra room would have been welcome. A good rule of thumb is to allow a minimum of 4 feet from the scope to the wall. This will provide room for a small desk with barely enough room to maneuver around the scope.

The type of observatory was easy to decide upon. After reading horror stories about the effects of heat currents exiting the slit in dome observatories and how "seeing" would be affected, the decision to build a roll-off roof was an easy one. In addition, being in the south, we frequently experience thunderstroms that sometimes have winds of 60mph or more. I had to devise a system whereas when the roof was in the "stowed" position, there would be no chance for high winds to lift the roof from the walls.

Another consideration that requires careful thought is the stability of the scope mounting. Since my primary interest is in astrophotography, I had to have an extremely solid mounting. I settled on a 6 inch schedule 40 steel pipe (pipe walls are 3/8" thick) that would be concreted into the ground to a depth of 4 feet. One should consider the "environment" the pier will experience. That is, what is the normal depth of the freeze line in the soil? In very cold climates, the soil can freeze to several inches deep and this would require the pier to be placed a bit deeper to keep the pier from moving due to the pressure of frozen soil. The type of soil at the site is another consideration. Sand is more stable than clay soils and rocky soils have other requirements. If not sure, consult an engineer familiar with conditions in your area. In my case, 4 feet would be adequate with 15 bags of ready mix concrete poured around the pier. If the scope is only going to be used on an alt-azimuth mount, the pier could be of lesser construction than it would if used on a polar mount. In any case, I would advise constructing and installing the pier with a sizeable "overkill" figured in to avoid problems.

While a 6 inch steel pier is very solid and strong, one must realize that steel, no matter how heavy, will bend to some extent under an off-center load such as would be encountered with a polar mounted Schmidt Cassegrain telescope (the larger and heavier the scope, the more this effect will be encountered). Under these conditions, the pier is in compression on one side and tension on the other. Any induced movement such as from touching the scope or from outside sources (such as the RR close by in my case) will set up a "ringing" effect in the pier. Under high magnifications, a star will rapidly "bounce" up and down until the source of the vibration has passed. I solved my problem of the ringing pier by pouring sand in the hollow pier and filling it almost to the top. Since sand has no natural harmonic frequency when not under compression, the problem was more or less solved.

I wanted my observatory to be completely isolated from the pier so that any movement from walking around would not affect the scope. An above ground floor would be just the thing since the weight of the observatory would be placed on the ground well away from the pier itself. The pier would pass through a hole in the floor with a couple of inches clearance all around. This would prevent floor vibrations that arise from moving around while working with the scope from ever getting to the pier.

An added benefit of building an elevated floor would be to facilitate the movement of air beneath the floor and thus speed up the thermal loss of the observatory from daytime heating. There also is no insulation in the walls for the same reason. You want the observatory to be as close to outside ambient temperature as possible when using the scope to minimize rising thermal currents and their effects on "seeing".

Since the scope was to be permanently housed in the observatory, protection from the weather was paramount in my concerns. I had to have an absolutely rainproof scheme and yet still provide adequate ventilation to prevent condensation on the inside. This was provided by ensuring that the moveable roof would have overlapping drip seams with the walls when the roof was in the stored position. On the sides this was easy: I simply made what I call a "skirt board" that overlaps the walls by a couple of inches. On one end (the end over which the roof rolls off) the gable was allowed to overhang the wall by a couple of inches. The other end was the problem since any overhang below the wall line would prevent the roof from rolling off. I solved this problem by building a short hingeable wall section (measures 4 inches tall and is as wide as the building end wall) that would be mounted to the top of the end wall with hinges. The bottom edge of this hinged wall section overlaps the top of the end wall and underlaps the gable wall of the roof. As the roof is nearly pushed all the way to the stored position, I simply let this hinged section rotate upright as the roof hits the sliding track stops. Ventilation is provided by the air space around the skirt boards and the hole in the floor through which the pier passes.

Here is something else to bear in mind: if the observatory you build is, for example, 10 feet long, then the outrigger beams and outer portion of the roof track need to be this long PLUS the width of the eaves of the roof PLUS an added foot or two to ensure that the roof gets far enough out of the way to keep from blocking the telescopes view of Polaris or anything south of Polaris. In other words, the beams supporting the roll off tracks should be about 13 feet long if the eaves are 18 inches wide and the roof is 2 feet high in the center. The observatory should be built on a north-south line with the roof rolling off to the north end. Any object blocked by the roof in the rolled off position will become visible at some time of the night depending on the time of year. Just make sure that the roof doesn't block the view of Polaris.

For tracks, I elected to use common 2 inch angle iron with 1/4" thick walls. The angle iron tracks on either side are one continuous piece. In my observatory with 8' X 8' walls, the tracks are 18 1/2 feet long. The angle iron tracks are bolted to the wall headers with 7/16" bolts that extend all the way through the header with flat washers and nuts on the bottom . The tracks are arranged with one side of the angle iron sitting flat on the header with the vertical side positioned to the inside edge of the observatory's wall. This way, any rainwater that may be blown under the side "skirt" boards of the roof will be directed toward the outside. In addition, with the tracks in this arrangement, the vertical sides of the tracks will serve as guides for the roof rollers. The roof rollers are nothing but common sealed ball bearings with a 7/8" bolts through their centers serving as "mounting" axles. The bearings don't turn on the bolts but rather turn on their inner and outer races as bearings should. The bolt simply mounts the bearings to a 4" X 10" X 3/8" plate that is mortised and bolted onto each of the four corners of the roof lower frame. I used four bearings (one on each corner) on my observatory and they are amply strong enough. The outside diameter of these bearings is 2 1/2". The bearings roll on the upper surface of the flat part of the angle iron tracks. The 7/8" bolt and bearings are positioned and shimed so that the head of the 7/8" bolt barely clears the outside surface of the vertical part of the angle iron tracks on either side. This way, the roof can do nothing but follow the tracks since no side to side movement is possible. When the roof is in the closed or "stored" position, the bearings (rollers) are situated under four short pieces of angle iron that were welded to the tracks in the upside down position. These short angle iron pieces reach over the top of the bearing rollers to prevent them from being lifted off the tracks in high winds. In other words, when in the stored positon, any high winds cannot possibly lift the roof from the observatory without litterally ripping off the roof tracks. The roof is prevented from rolling due to high wind loads by a simple latch system. If it were allowed to roll a short distance, the rollers would no longer be positioned under the short inverted angle iron pieces and the roof could be blown off. The latching system, therefore, must be designed to be 100% reliable. When the roof is rolled off, it and the roof tracks are supported by doubled up and treated 2 X 6 beams. The beams are supported at the ends by 2 1/2" steel pipes which were concreted into the ground. At each end of the roof tracks, I welded a small steel block to prevent the roof from going too far in either direction.

The floor joists of my observatory are constructed of treated 2 X 6 lumber put on 16 inch centers and flooring is 5/8" exterior grade plywood. The walls are framed with fir 2 X 4s on 16 inch centers and covered with 3/8" exterior grade plywood. The wall plywood in turn is covered with vinyl siding with no moisture barrier or insulation underneath (you don't need any). The roof gables, sofits and facias were also covered with 3/8" exterior plywood and vinyl siding. The wall heights on my observatory are 54" high from the floor level (this depends on how high you want your scope above the floor) and this provides full aperture views within 10 degrees of the horizon. The roof bottom frame is 2 X 6 treated lumber, rafters are fir 2 X 4s with a 3-12 pitch. Roof decking was 1/2" exterior plywood (to keep weight down), covered with roofing felt and then shingles. The roof weight of my observatory is about 550 lbs. It can be rolled off with the pressure of one finger.

For those interested in building an observatory similar to mine but two feet on a side larger, click the link below to get a very detailed set of CAD drawn plans with easy to follow step by step instructions that will take you from site selection to pier construction and installation to completion. There are over 60 pages of illustrations and text.

Detailed CAD drawn plans with step by step construction guide available HERE

Email any questions to Jim Pennington

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