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The DeKalb Observatory

Version II

In the fall of 2003, ground was broken on the new DeKalb Observatory.  [The DeKalb observatory was named for its location in DeKalb County, IN.  DeKalb county is named after General Baron Johan DeKalb, a French military officer who assisted the Americans during our revolutionary war.  More about Johan DeKalb HERE] The original observatory was an 8'x8' roll off roof structure built in 1996.  The construction page for this original observatory can be viewed HERE.  This original observatory was built to house an 8" SCT telescope.  That scope was replaced in 1999 with a 14" SCT.  This necessitated that the roof be raised 3 inches to accommodate the new scope.  At the same time, an 8'x8' "warm room" was added to the north end of the building.  In 2003 the author had a bad case of "aperture fever" and purchased  an RC Optical Systems 16" truss scope.  [RC Optical Systems]  Rather than put the new scope in an already crowded observatory, a plan was formulated to build a new 14 foot square, two story observatory to house the new scope.  

When the original observatory was built in 1996, the light pollution in this rural area was minor.  Since that time, more homes with decorative lighting and security lighting have been encroaching the observatory's location.  Therefore, the new observatory was built with a dome to minimize local light sources infiltrating into the observation area.  In the new design, the declination axis was raised 16 feet above grade in order to minimize thermal effects found near the ground.  This increase in elevation generated a 0.5 - 0.7 arcsec improvement in zenith seeing compared to the old observatory

The Construction
It was an interesting task to find a commercial builder that would undertake the construction of a private observatory.  Of course, none of the builders in our area knew anything about the requirements necessary for mounting a telescope 16 feet off-grade or how to install a fiberglass dome and mate it up with a standard constructed building.  However, one local builder, Messer Commercial Builders was willing to listen to my requirements and became quite excited about the project.  The cost was quite reasonable.  If you are of a mind to construct your own observatory, I would recommend contacting Messer for a price before you try this yourself.

Core borings were done at two locations in the area where the observatory foundation and floor slab would be poured.  These borings were made to a depth of 16 feet and were interpreted by a licensed professional civil engineer.  The results reported that the first 20" of soil was standard top soil and would need to be removed since it was of low density and would be difficult to compact.  The remainder of the 16 feet was composed of hard to very hard brown silty clays.  This type of soil requires very little compaction and can withstand a loading of 3500 lb/ft2.  This loading is grossly in excess of the forces applied by the observatory foundation.  In this part of the country, such subsoil is the results of glacial deposits and is often laden with moisture.   Luckily, no water was found to accumulate in the boring holes since the area well drained. 

Because the observatory is not heated, it was necessary to put in foundation footers to a depth of 54" below grade.  This insures that the footer base is well below the frost line for this area, which is typically 18" and a maximum of 24" 

Pouring Footers     [Notice old observatory in background]

The telescope is elevated to the 10 foot observation deck level by a 24" diameter concrete pier.  This pier sets on its own foundation consisting of a concrete 'cube' that is 5'x4' square and 4' high.  [That is a 12,000 lb mass]  The foundation was reinforced with rebar during the casting process.  Click HERE to see rebar in pier foundation.  Notice that the pier is offset on the slab.  The reason for this is, when the telescope is set onto the final mounting, the declination axis of the mount [in this case, a fork mount] will be in the center of the building, north to south.  Failure to foresee the need for this offset is a common mistake found in amateur observatories.  

The pier was poured in one casting onto rebar that had been protruding 18" from the top of the pier foundation.  An 18" diam. x 11 foot high rebar 'cage' was constructed to provide interior support to the pier.  These 15 foot rebar used in the cage were pushed 4 foot into the wet concrete of the pier foundation. [See detail of rebar HERE] The bottom of the cage was wired to the pier foundation rebar.  The rebar in the cage was wired together, not welded.  Welding will attract moisture from the concrete and will cause the rebar to rust.  This can cause spalling of the concrete and loss of strength.  

It was necessary to lift the concrete to the top of the pier form by using a large bottom-dump bucket.   Both the concrete and the form tube were vibrated at several stages during the pour in order to release any entrapped air in the concrete [My Job].  Four inch "J" bolts were placed into the top of the pier.  The bolts were placed in the proper pattern to accept a 10" welded pipe flange.  The flange will serve later as the base for the actual steel pier to which the telescope mounts.  The advantage of using this type of telescope/steel pier/concrete pier arrangement is that if a new telescope or mount is purchased in the future, a new steel pier can be fabricated inexpensively to make up the proper space between the concrete pier and the mount.  

The total mass of the pier and the pier foundation places a force of 880 lb/ft2 on the subsoil at the base of the pier foundation.  This is well below the rated compacting value for the soil.

Pouring the Pier

Note that the bottom of the pier form is well below the level of the floor slab.  After the concrete form was removed, an anti-vibration cushion was wrapped around the pier at its base to insulate it from the slab before the slab was poured.  Also, pea gravel was used as a substrate below the slab to  totally isolate the slab from direct contact with the pier.   The idea here was to make sure that no vibrations could be transmitted from the slab or foundation to the pier.

Plastic sheeting was placed over the pea gravel to keep water from infiltrating up through the concrete.  Wire netting was placed on top of the plastic sheeting to help reinforce the concrete slab.  The slab was poured directly over the foundation walls and was anchored to the walls via rebar that had been inserted into the foundation while it was still curing.  "J" bolts were placed into the perimeter of the concrete slab to facilitate the later attachment of the wall plate to the slab.  All concrete work was completed on Nov. 20, 2003.  The concrete was striped of all forms and allowed to sit undisturbed  until March 2004.

Finished concrete work

Construction of the exterior walls was begun in March of 2004.  The construction is basic stick-frame using 2x6 wall members on 16" centers for the exterior walls and 2x12 floor joists on 16" centers to support the observation deck.  The wall frames were covered with 1/2" chipboard [MDF] exterior sheeting.  The flooring for the observation deck is 2 layers of 3/4" marine plywood.  The reason for the two layers is to minimize flexing of the floor.  Marine plywood is waterproof and will tolerate the inevitable leaky dome or infiltration of blowing snow.  The observation deck was eventually covered with a good grade of indoor-outdoor carpeting.  The carpeting acts as a sound absorber [domes tend to be plagued with echo] and saves the occasional dropped eyepiece. 

The walls contain no insulation in order that the structure might achieve temperature equilibrium as rapidly as possible in the evening.   The interior of the building is paneled with perforated Masonite [pegboard] to allow the walls to 'breathe'.  The exterior of the building is covered with PVC siding.  PVC is a common siding material used in this part of the country.  [As required by my wife, it also matches the siding on the house]  It requires no maintenance and is flame-retardant.  

Access to the upper floor is accomplished via a 1/2-spiral metal staircase.  This is a very compact method of reaching the observation deck.  The footprint of these particular stairs is 79" x 42", although several different design are available.  The top of the stairs opens into the E-W centerline of the observation deck.  This allows a clearance greater than 6' from the center of each stair tread to the overhead dome support structure.  This was a requirement of the local building code.  

Spiral Stairs viewed from Ground Floor         Spiral Stairs viewed from Observation Level

There is also a 32" square hatch in the floor of the observation deck.  This hatch allows larger articles to be lifted to the observation deck level as opposed to being carried up the spiral stairs.  

The Dome
The dome 'kit' was purchased from Technical Innovations. [Technical Innovations]  This is a 10' diameter fiberglass construction that is shipped in sections directly from their factory in Maryland.  The dome was assembled in a warehouse owned by the author and located 12 miles from observatory site.  The dome was transported by flatbed truck to the observatory site and was raised onto the observatory using a construction fork lift.  The process was very simple.  

It was necessary to cross-brace the interior of the dome base ring before loading the dome onto the truck.  This kept the dome from becoming out-of-round during the transportation and lifting process.  It is much easier to get the dome into the round configuration in the warehouse than on the observatory.  After the dome was fastened to the observatory, the interior bracing was removed.  The actual construction of the dome took about 2 weeks [40 hours total labor].  The most difficult part of the dome construction was drilling the holes in the fiberglass members.  The fiberglass/polyester dust is very irritating and hazardous to breathe.  I would suggest purchasing the dome kit with the holes pre-drilled.  There is an up-charge for this option...and I am sure that it is worth every penny!

Dome Under Construction              ....and finished

NOTE:  Caulk!  Use only good grade acetoxy or oxime cure silicone caulk on the fiberglass dome.  Clean the areas to be caulked using isopropyl alcohol before applying the caulk.  When setting the dome ring onto the observatory, clean the surfaces with alcohol and apply a liberal, continuous bead of caulk before bring the two surfaces together.  My construction crew used a water- based 'siliconized' caulk between the roof flange and the dome base ring.  It leaked everywhere!  I ended up 'digging out' all of the old caulk, re-cleaning the surfaces with alcohol and replacing it with a good grade of silicone caulk.   No more leaks.

Dome Rotation:
The rotation motors supplied by TI are adequate for doing the job but are VERY noisy.  They run at 12 VDC and at a very high rpm.  I was inside a friend's HD-10 dome during a rotation and nearly soiled myself!
  My observatory is close enough to other homes that I'm sure it would generate complaints about the noise.

The solution was to change over to a quieter motor.  I opted for a right angle 120 VAC gear motor supplied by Grainger.  The part number is #4Z278-4.  See details at www.grainger.com .  This is a 1/12 HP motor that has an output of 173 rpm. They are a bit pricey at $223 each. With this setup, the dome rotates at 1 rpm.  See the completed drive assembly by clicking on the image below.   Dome Drive Motor Small.jpg (42457 bytes)You also can see that I also replaced the TI tension mechanism with a threaded brass rod and a spring. This pushes on the bottom lip of the ring to hold the drive mechanism against the bottom of the dome. The motor is reversible and it is driven it through a set of three 12VDC relays.   The three relay system keeps the dome from getting confused if both directional contacts are closed at the same time and also allows simple switch closure from any low voltage system [e.g. computer relays] to control the dome.   


The Electrical
The observatory is wired  for a 40 amp, 110 volt service.  There are 36 outlets on the two floors of the observatory and two GFD outlets outside.  My philosophy is, "You can't have too many outlets."  There are both red and white lamps installed in can-light housings in each of the four corners of the dome support structure of observation deck.  [See Picture of lamps HERE] This provides indirect lighting during setup and observing.  The lights are on separate dimmers so that their level can be tailored as required.  A two lamp 600 watt halogen work light hang from the dome and provides sufficient light during maintenance operations at the observation deck level. There are both red and white lamps in a total of 13 can-light fixtures in the lower level.  These lamps are also on separate dimmers.  The lower level houses the computers, a work area and a storage area for tools and telescope accessories.

The observatory is also wired for telephone, an alarm system and a LAN that attaches the computers in the observatory the computer in the house [for remote observing] and also to the Internet.  Connection to the house for the LAN and power is through two 1-1/2" PVC conduits.

The Ventilation
In order to keep the observatory from building up too much heat during the day, a thermostatically controlled 18" fan is located in the wall at the  observation deck level.  The fan is sized to turn over the air in the observatory every 5 minutes.  It can also be turned on manually and is variable speed controlled.  This allows the fan to be run at a reduced speed when there is a marked differential between the warmer observatory interior and the cooler outside temperature.  This condition causes 'dome seeing'.  When this condition exists, warm air from the interior of the observatory rises out through the dome slit in front of the scope and causes a degradation in the seeing conditions.  Running the fan at a reduced speed pulls air into the dome through the slit and reduces the effect.  A 20" square inlet louver near the floor in the lower level also lets air into the observatory.  There is a replaceable air filter behind the louver to keep dust and insects from being pulled into the observatory by the exhaust fan.  In the winter, the filter is replaced with a plywood plate to block out any wind/snow infiltration.

The Finished Product
Here is a picture of the RC-16 in it's new home.  The mount is a modified Mathis Instrument Model 750.  The RC-16 is mounted to the Dec axis using Losmandy dove tail adapters.  These adapters fit into plates that were originally the top and bottom of the RC-16. 

RC-16 on the Mathis 750 Mount.  Like most telescopes these days, the RC-16 has a severe case of "cable-itis".

The lower level of the observatory acts as a workshop and 'warm room'.  This is a misnomer since there is no heat in this area.  But when the Indiana winter winds start to blow, it's warmer than being up in the dome.

North end of Warm Room

The computers and monitors are all wall mounted wall in order to keep the clutter off of the counter top and also to keep the connections to the scope and mount as short as possible.  Pegboard wall covering allow the area between the wall studs to cool rapidly and also encourages the proper storage of astro-related gear.  The concrete floor has two coats of a custom epoxy coating that contains a small amount of large bore silica.  This keeps it from becoming slippery when wet but still facilitates easy cleaning. 

_MG_7516_small.jpg (353916 bytes)   This makes it all worth it!
Image Credit: Jamie Starkey Photography

Click on the images below for full sized drawings of the elevation and floor plan of the new observatory.

Observatory Elevation rev20.GIF (79007 bytes)Observatory Floor Plan.jpg (350561 bytes)






Elevation                                                       Floor Plan