The Ultimate Accessory - The Construction of Dunham Hill Observatory

The Ultimate Accessory The Construction of Dunham Hill Observatory Mike Lewis - January 2005

All pictures copyright Mike Lewis

As amateur astronomers, most folks think we are nuts trudging out in the cold or bug ridden night to set-up our scopes. We do this out of our love for the night skies. At some point, I think we all dream of a permanent place to pursue our passion. With this in mind, I started on a project that led to what I call “the ultimate accessory”, a home observatory.

One thing I must say at the outset-check your local and state regulations! This may save some heartache down the road. In my situation, without any neighborhood or association constraints, any outbuilding under 200 sq/ft did not require a permit or inspection, except for the electrical. My decision was to construct a 12’ x 16’ roll-off structure. (192 sq/ft). I researched just about all there is to find on the web concerning home-built observatories and since I am doing all the work myself, this was the best fit for me. I also must add that I have worked in the construction industry 25+ years. I have just under 1.5 acres to work with and picked a spot 125’ from my house. This is not a decision to take lightly! Location is important for obvious reasons. The building consists of a 12’ x 10’ observing area with a 12’ x 6’ warm-room/lab. The roof of the former rolls over the lower roof of the lab.

I decided to go with a permanent pier, so this was the beginning. The pier is offset N/S one foot north and centered E/W on the proposed observing room floor. It is an 8” reinforced concrete structure with the bottom “belled-out “ to 20”. This enabled me to excavate a smaller area. I made a bolt pattern to accommodate the mount I was going to use, bought a “sonotube”™ and mixed the concrete by hand. The bolts for the mount are 2’ long embedded in the concrete. The pier itself is reinforced with a cage of rebar running the full height and tied to L shaped bars extending into the base. It is important to take some time with this, make sure the form is plumb! When all is said and done, you will appreciate this step when polar aligning.

With the pier completed, I laid-out the actual structure and pounded stakes at the corners. The building was oriented N/S along the long axis. For construction purposes, I then off-set the corner stakes 2 feet in both directions. There are two schools of thought concerning the floor of your building, a wood, deck-like structure or a concrete slab. I opted for a slab. The wood type floor is easier to build and a slight bit cheaper but I decided that concrete would support the walls well and have considerably less vibration. I dug out all the topsoil from the building footprint, anywhere from 6” to 14” in my case, and ordered sand from a local contractor. The sand was placed in the hole and compacted. I did this all by hand, if you have access to a backhoe, it would be a lot easier. The floor slab area was then formed up with 2x4’s and leveled to grade. I also dug an 8” wide x 12” deep “rat wall” along the exterior of the slab. At this time, before the concrete pour, I installed 2 conduits from the base of the pier to the inside of the lab for cables from the lab to the scope.

The slab figured out to be about 2 cu/yds of concrete so a call to a local concrete supply firm was made, I did not want to mix that much by hand! The supplier indicated there was a 3 cu/yd minimum for delivery, oh well. (ended up fixing some sidewalk out front with the extra.) The mud was poured and struck off, I went with a light broom finish. After the concrete had hardened a bit, it was covered with plastic and kept wet for 5 days to insure proper curing.

The building itself is traditional 2x4 stud walls, 16” on center. I went with 6’ high walls and a peaked roof for the observing area and a “shed type” roof for the lab. The main roof slides off on 12 hard rubber casters ( 6 per side/3” diameter ) inside aluminum tracks over the lab roof and supported by 4x4’s place in concrete, 4’ below ground. The tracks and wheels can be purchased online, a google search turns up a myriad of suppliers with items to meet your individual needs. I found a local supplier, Grainger ™ who can also provide these items nationally. If I were to do this over, I would pay the extra and use “inverted” V tracks and a larger diameter grooved wheel. The roof over the lab is regular shingles and the larger one is metal. I went this route to minimize the weight of the one that has to move, its pretty heavy, but doable by hand. The roof of the lab area is 2” shorter to allow the movement of the larger one. At some point I may install a roof removal system. All that was left for the basic structure was siding. I chose a regular wood siding in 4’x8’ sheets. Vinyl would be another option. The siding was painted to match my house. Well, fall 2003, the inside of the lab was not done, but I enjoyed a winter of observing out of the wind in my unfinished observatory.

The next year was the fun part of the construction! I dug a trench from the house to the observatory to bury the electrical and phone cables. It is important to separate these two wires, the power supply may adversely affect data transfer, (wireless is another option). I was lucky in the fact that the previous owner had had a hot tub and an unused breaker box was already located in the breeze-way. I ran 10-3 underground (direct bury) wire from the box to another one in the observatory. Installing the wire in a conduit is another way to go. I then ran direct bury phone cable in a separate trench. In hindsight, I should have planned better, cat-5 or wireless would have been a better solution. I purchased two books on wiring and accessed the national wiring code online to help with the actual wiring of the building. I then asked the help of a friend who is in the electrical business to take a look at my work. He indicated a few mistakes and we made the corrections, the electrical inspection went well and I only had to make one minor change. One point, be sure and ground the system at the building and be generous with plugs!

Now, the only thing left was to finish the interior of the lab. I insulated the walls and ceiling of only the lab and used 3/16” wood paneling. I built a desk and put up some light fixtures. A 3’ electric baseboard heater keeps it toasty. I have one access door on the south side of the building and an interior door between the two rooms. I also have a window between the two. <>

Now the bottom line- how much did it cost and does it work! My total investment is at $2300.00. I was trying to keep the cost around two grand, so, I’m close to projected. Also, keep in mind I did ALL the work myself. OK, does it work? Absolutely!!! It’s not perfect by any means and I will continue to tweak. Was it a ton of work? You bet! But, once you personalize with desk, shelves, storage, lighting, music and all the other stuff we haul around, it is a joy to use!

My observatory now houses a Meade LX50 7” Maksutov-Cassegrain with a Borg 76mm ED riding on top. I also have an 8” Homemade DOB and a 120mm achromat. On any given night, I can be at the eyepiece in 10 minutes max!! The scopes are all at or close to ambient temperature so a 1 hr sky tour on a work night or a short notice session is a breeze. If you are contemplating any type of permanent observing structure, go for it! You will not regret it. My observatory truly is the “Ultimate Accessory”.