Second Observatory Construction Notes

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Introduction 
Tools 
Construction Hints 
Preliminaries 
Arc Cut outs 
Floor Construction 
Base Framework 
Masonite Installation on Base 
Installation of Casters 
Dome Framework 
Masonite Installation on the Dome 
Shutter Tracks 
Shutter 
Weather Skirts 
Securing the Dome 
Conclusion 
Materials List


Introduction

The photos accompanying the plans will be useful when difficulties arise that neither these notes nor the plans could explain. Modifications to suit personal taste is always possible. I know because I have seen a few plans and they did not have all the design characteristics that I was searching for. The result is the observatory here that combines low cost with quick construction and durability.

From the first saw cut to the first observing session from within the observatory, you will have spent $800.00 (1981 Cdn.) and 400 man-hours. Much of the labour involves cutting out the approximately 2-foot long arcs that will generate the curved superstructure of the dome and the base. Some 186 arcs will be needed. This will require many screws and tubes of wood glue to form the rings, arches and ribs.

Tools

It is possible to trace out all the arcs on the plywood and have a local carpenter cut them out. But will add a substantial additional expense. Instead a standard jigsaw could be rented. Two would be better as one would cool as the other is cutting out the arcs. Other tools that will be required are: hammer, level, saw, drill with provision for driving in screws, square, countersink, assorted drill bits, measuring tape, and patients!

Construction Hints

Before you order your wood and begin construction, you should go through the plans and photos a few times. When you do get the lumber, choose it carefully. Select only the best of stock and be sure that it is dry. In fact, kiln dried wood should be used. If wet wood is used there is a risk that it might warp during the drying period after the observatory is constructed. The result may be a crooked observatory. Provide some protection for the observatory during construction.  I used one of those garden screen houses.

 A friend is essential for some aspects of construction and half a dozen are needed to lift the dome onto the base.​ I must stress that it is important to use the best available paint for the exterior, and stain/preservative for the interior of the observatory. The masonite  needs the best possible protection. As such, it must be kept dry during construction. I built the dome under the protection of a gazebo typically used on patios.  At present my observatory is located in humid eastern Canada and the masonite is holding up very well. 

Preliminaries

Before actual construction of the observatory can take place, the pier for the telescope should be put in place. A hole at least 4 feet deep was dug, centred where the observatory will go. Also, some type of foundation should be provided for the observatory. This keeps the observatory away from humid, wet ground and also provides more comfort for the observer. A concrete foundation would add considerable expense, so a concrete pad or a set of patio stones may be substituted. If your area is subject to high winds, then I recommend anchoring the base to the foundation.  A conduit containing various power, network and other cables should be brought to the observatory underground, under the foundation and then up the pier.

Arc Cut Outs

The superstructure of the observatory is made of curved plywood arcs.  To make the most efficient use of  the plywood, it is best to build the arcs from many short pieces rather than single long pieces.  To speed up the work, templates should be made from masonite or some other durable pattern material to be used as patterns to trace out the cutting curves.  When cutting out the arcs, do not confuse them. Label the arcs as they are cut out.  Some will generate the dome and others will generate the base. There are actually 4 different types of arcs (refer to Sketch L).

These are:

  • 4 inch wide to generate the curve for the wall base which has a 3 foot 10 inch radius
  • 4 inch wide to generate the curve for the dome bottom ring (that will run on the casters) which has a 4 foot radius
  • 3 inch wide arcs (in the end I made these 4 inches wide, contrary to Sketch 'K') to generate the curve for the dome arches, the main supports or beams, which have a 3 foot 10-1/4" inch radius. IMPORTANT - the actual radius generated for these arcs depends on how wide you make the dome slit.  Each "foot" of an arch sits on the dome bottom ring, and the distance between the feet of an arch is measured as a chord across the dome bottom ring.  Half of this chord distance is the radius used to generate the arcs.  Recall that the arches are offset from the centre of the dome, so that they would have a radius of LESS THAN 4 feet.
  • 3 inch wide arcs to generate the curve for the remaining dome rib members which have a radius of 4 feet.

It is best to identify each arc as it is cut out with a number or colour if the plywood is quartered into four 2'x4' sections it will be easier to handle.

When putting the arcs together be careful to maintain a circular form.  A helpful guide is to scribe the correct circles for the different radii on a wood or concrete floor where assembly takes place or, alternatively, the two sheets of waferboard braced together can serve as this. Note in the above photo that I scribed such lines into a wood floor that was painted green.  For added strength a standard, waterproof wood glue should be added between the arc laminations. The drill with a screw driver attachment is useful for putting in the numerous screws.  Drill and countersink the arcs to accept the screws. Ten screws will be required to put 3 arcs together.

For a complete ring, a double lamination will require a little over 24 arcs of which only 12 need be drilled and countersunk to accept the screws (# 10 x 1" screws). A triple lamination (dome arches, dome caster ring, and ribs perpendicular to arches) of 36 arcs will be required for a complete ring.  The two ribs perpendicular to the arches are "quarter" rings so only 9 arcs are needed for each of these ribs. The dome caster ring requires 36 arcs. For the triple rings, 2/3 of the arcs which make up any given member must be drilled and countersunk to accept the # 10 x 1-1/2" inch screws. The central lamination does not require to be drilled, etc.  The double laminations for the remaining ribs, 8 in all, require 6 arcs of which only half need be drilled etc. (refer to Sketch 'A' and Sketch 'B')

Floor Construction

Now that all the arcs are cut out and all the full, half and quarter rings are generated, the floor may be cut out. The two sheets of waferboard should be laid flat and a slightly undersized circular floor can be cut out. It should be just under the required inside base diameter so as not to bind when installing the base. The two sheets of waferboard give a 8' x 8' foot surface so you have plenty to work with to give a circular floor 7 feet 2 inches in diameter The floor is in two halves (two sheets of waferboard).  The central cut out in the floor should be a little larger than your pier so as to avoid transferring floor vibrations to the pier.  The underside of the floor is reinforced with 2" x 3" lumber.  Since the floor is in two halves, there is a central common joist so that the two floor halves can be held rigidly together with screws (refer to Sketch 'A').

Base Framework

Cut the 2" x  4" x  10' kiln dried studs in half. You will now have 20 studs which will provide a very rigid framework. The base rings, one on top and the other at the bottom, each double laminated, along with 3" x  3" angle brackets and 3" long screws through the rings and into the ends of the studs at top and bottom will increase rigidity.  Actually only 17 studs are used with double studding at the doorway (refer to Sketch 'A'). The remaining studs are used elsewhere in construction.  The brackets are fastened with # 8 x  1" screws. Use a level and square to make sure all is true.

The base may now be put in place to enclose the floor. A row of bricks may be used to elevate the base so as to make the bottom base ring even with the floor which is about 3-1/2 inches higher

Masonite Installation on Base

Since the base is five feet high, there will be a seam four feet up on the observatory wall.  The bottom four feet will require almost three sheets of 4' X 8' masonite  and the remaining top foot will require almost three strips of 1' X  8' masonite  cut out from an additional 4' X  8' sheet.  Addendum - In 1988 I replaced the masonite on the base wall with corrugated steel that has a "baked on" white enamel finish.  The masonite near the bottom edge of the base finally succumbed to moisture damage after 7 years. 
 

Note on the inside of the base that there are curved shelves between the base studding. These shelves/braces support the seem at the four foot wall level. Making the base five feet high instead of four gives better door head clearance and the much needed shelf space.

Before applying the pine trim at the masonite seams and at each stud position, a coat of primer paint should be applied.  Make sure the gloss surface on the masonite is sanded off for better paint adhesion. The unpainted pine trim is fastened where there is a stud and also at the four foot high seam on the base. The vertical trim can be fastened with nails but the horizontal trim at the seam, and also at the bottom of the base, should be fastened with # 6 screws, being under stress from the bending to conform to the base diameter.  White acrylic caulking is applied under the pine trim wherever there are seams.  Not shown in any of the photos is an additional application of pine trim at the top part of the base. This is used to support the inner weather skirt (vinyl siding) as shown in Sketch 'I'.  The horizontal trim at the four foot seam is beveled along the top edge to promote water run off.  The trim can now be primed and the whole base can then be given a coat of high gloss white paint.

Fitting the access door is tedious because it is curved to follow the base diameter. The frame is made from a single 2' x  4' cut down the middle and the top and bottom of the door is made from scrap curved plywood the same radius as the base. There is an additional curved support at the middle of the door.  The doorway will be just under 29 inches wide.  Brass should be used for the hardware since any rust from metal hardware (hinges hasp, padlock) will leave streaks running down the nicely painted white base surface. Protection of the door threshold is important so a sheet of aluminium or galvanized metal can be installed. A couple of slide bolts, one at the top and the other at the bottom of the door, will keep the door securely closed.

Installation of Casters

The nine, 2 inch diameter casters (roller skate wheels) are spaced equally around the base to evenly distribute the dome weight (refer to Sketch 'F').  It is important that each axis of the caster is perpendicular to the tangent at the base edge. The casters themselves are fastened with pieces of threaded rod, washers and nuts so that each caster can be levelled relative to every other caster and thereby providing smooth dome rotation.

The 1-1/2 inch casters which maintain the centering of the dome as it rotates, are installed using special 2 inch wide aluminium angle brackets (refer to Sketch 'G' and Sketch 'H'). An installed caster is shown in the image below.

It is best to install the radial casters mounted on their brackets after the finished dome is put in place. There is a slight spring to the aluminium angle brackets which presses against the inside diameter of the bottom dome ring to keep the dome properly centred during rotation. 

 

Dome Framework

It is important to construct the dome on a level surface so that it does not become warped or crooked.  The dome bottom ring, which will be in contact with the 1-1/2 inch casters, is a triple lamination of plywood. This also applies for the two main arches and the two ribs perpendicular to these arches (refer to Sketch 'B').

When joining the ribs and arches to each other and to the bottom dome ring, right angle brackets, 3" X 3", may be used where they meet at right angles. But, for the other angles, where the ribs meet the arches, door hinges are used.  The hinges conform to the differing angles. Don't try to use right angle brackets by  bending them as this will weaken them. It may sound odd to use door hinges to give a rigid joint but when the door hinge is screwed to the arch and then to the rib, there is produced a very sturdy joint.  Additional rigidity is obtained by screwing a 3 inch # 10 screw through the arch and into the end of each rib. Each rib and arch is attached to the dome bottom ring with four 3" x  3" angle brackets.  There is bracing at the back of the dome. These braces are made from left over 2" x  4" lumber. The spacing of the brace closest to the top of the dome is such that viewing of the zenith is possible. The amount of offset of the top brace should be calculated for each type of scope. Offset also depends on the aperture of the telescope as well as the type of mounting (wedge-fork, German., etc.).  Finally, the offset depends on the latitude of the observer. The tilt of the equatorial mount will shift the telescope further back or forward, depending on the hemisphere of residence.  To keep the framework and the slit space true and square during construction, a temporary brace is inserted at the top of the dome  between the arches.  In the above photo the odd looking shapes, four of them where the arches meet the dome bottom ring, are for fitting of the arches only. They are not permanent members of the dome. Also in the above photo there can be seen blocks where the ribs perpendicular to the arches are nut upon. These blocks are used to elevate the ribs to get the right angle to cut the ribs where they are to join the arches. Once the framework is complete, it is time to place it on the base to make sure everything is turning properly and true.

Masonite Installation on the Dome

This is the most difficult part of dome construction and it requires the help of a friend. There is really no pattern to go by (although there is a formula) to cut out the dome gores; the pie shaped masonite sheets between the framework members. You don't want to have a significant gap between each gore. Each gore must therefore be custom fitted.

Place a whole sheet of 4' X  8' masonite  on the dome between two ribs. With your friend holding the sheet on the outside, and tacking the sheet in a few places to make sure it doesn't shift, trace out the gore from the inside. Remove the sheet of masonite and extend the perimeter of the traced line by 50% of the rib width at the sides,  50% of the dome bottom ring at the bottom, and 50% of the arch width at the top. It will now be possible to cut out the gore from the sheet of masonite and install it. Work in a clockwise or counter clockwise manner around the dome, fitting each gore as above. Each joint will be tight and very well fitting. The gores are fastened with ardox nails so that they will not pull out under stress from the masonite being formed over the dome radius.

The dome may now be primed. The slit lip is installed next (refer to Sketch 'C' and Sketch 'K'). This lip will take the weather strip that will seal the done slit against the elements.  The lip should have caulking under it to seal out and water that might seep in and it can be nailed in place. The inside radius of the lip is equal to outside radius of the dome (4 feet) and can easily be made from scrap plywood.  The lip extends to within 1-3/4 inches of the bottom of the dome, leaving enough room for the dome skirt mounting.

The dome pine trim that covers the gore seams can now be installed. Again, caulking is used it every joint where the masonite gores butt-up to each other (they are not overlapped). The vertical pine trim starts at the bottom of the dome and is brought up to butt-up against the slit lip. Because of stress these vertical trims are fastened with # 6 x 1-1/2 inch screws. The pine trim around the bottom of the dome is also fastened with # 6 x 1-1/2 inch screws. These however, are bevelled at the top edge to facilitate water run-off.  The vertical trim is only 1 inch wide while the horizontal trim is 1-3/4 inches wide..

Shutter Tracks

The dome shutter rides on 2 parallel aluminium angle tracks.  Each 4 foot length of 2"  x  2" x  1/4" aluminium angle is simply screwed down. The top track is simply screwed down to the uppermost slit brace (refer to Sketch 'C' and Sketch 'J'). The lower track must be fastened to a specially cut mount which fits the radius of the dome (refer to Sketch 'D' and Sketch 'E'). The mount is made from a length of left over 2" x 4" stud and has a curve cut out of it that corresponds to the outside radius of the dome, 4 feet.  The aluminium angle is then fastened and a piece of pine trim is also fastened on the outside of the track to make sure that the shutter doesn't fly off when rolled on the track. At the end of each track is a small bolt that provides a shutter stop.

Shutter

The shutter is made from 1-1/2 inch wide arcs and is 2 ply (refer to Sketch 'M' and Sketch 'N'). The length of the shutter depends on the slit length which in turn depends on telescope type, as discussed in Dome Framework above. The bracing in the shutter can be made from either plywood or left over studs . After covering the shutter with masonite,  an application of pine trim can be made around the edge with a bevelled piece at the bottom and top to promote water runoff .  Also in these sketches you will notice placement of the four casters to roll the shutter on the aluminium tracks. Be careful as to the orientation of the casters, as shown in Sketch 'N'.  It is important to keep the shutter true and square during construction. It can stick when opened or closed just like a crooked door.

The underside of the shutter can be faced with the weather strip of rubber/aluminium construction and the outward face of the dome slit can be faced as well (refer to Sketch 'K').


  
Weather Skirts

Two skirts made from lengths of vinyl siding (you may have to trim off some protrusions) are attached to the observatory. One is hung from the dome and the other is attached to the top of the base. This gives the familiar "S" trap to keep out blowing snow and rain (refer to Sketch 'I'). The dome skirt cannot be attached until the dome is put on the base. Brass screws are used to fasten the skirts in place. The reason here is to avoid rust streaking on the white skirt surface common from weathering metal screws.  A small skirt must also be attached to the bottom of the shutter.  Addendum - After experiencing a few snow blizzards that drove fine, blowing snow under and over the skirts, two additional skirts were installed in 1999.  The first skirt is made from landscape fabric and is 6 inches wide and 25 feet long.  It hangs from the inside edge of the dome bottom ring, fastened with a staple gun.  I chose fabric so that the bottom edge of this skirt "rides" on the top of the base but doesn't bind.  The fabric also allows some air circulation.  An additional, 4th skirt, also made from vinyl siding, is attached to the inside edge at the top of the base.  The added benefit of this skirt is that it prevents your hands from getting caught in the casters when you rotate the dome.

Securing the Dome

You will need at least 3 (preferably 5) other people to help lift the completed dome into position on the base. The dome rides freely on the nine, 2 inch casters. The smaller 1-1/2 inch casters working against the dome bottom ring keep the dome centred during rotation. 

During periods when the dome is not used, a securing system comprising four turnbuckles to hold the dome down against high wind is used.  Additionally, four door hooks are used to secure the dome shutter and an additional hook is used to secure the access door when the observatory is in use. 
 

Conclusion

There were many hours put into the construction of the observatory, but they are all paid back by the reduction of telescope set-up time and the elimination of polar alignment. The observatory means that observations can take place during windy nights without the wind vibrating the telescope. Consequently, astrophotos will have a higher resolution and better overall quality. Most of all, the observer is protected from the elements, and dewing is practically eliminated. Stray light is also reduced considerably, especially by the application of a dark stain on the inside surfaces. The best outcome of having your own observatory is the prestige, even though small, that comes with it. You are now identified with the person who has the observatory in the back yard.

I wish you success with your new facility.

Clear Skies & Good Observing!

 


Materials List

NOTE: this list should be adjusted in consideration of the addenda made to the construction notes.

Quantity  - description/size/other

6        sheets plywood, 4' x  8' x  3/4" sanded one side 
10      sheets masonite/hardboard, 4' x 8' x  1/8" tempered (less sheets required if corrugated steel used for base wall) 
10      kiln dried studs, pine, 2" x  4" x  10' 
2        sheets waferboard, 4' x  8" x  1/2" 
12      studs, pine, 2" x  3" x  81 
6        500ml  tubes wood glue 
1        wood preservative, clear, 1 gallon 
5        acrylic caulking, white, 300ml 
100     # 10 screws, 3" long 
600     # 10 screws, 1" long 
400     # 8 screws, 1" long 
500     # 6 screws, 1-1/2" long 
1 lb.   galvanized nails, 2" long 
1 lb.   finishing nails, 1-1/2" long 
3 lbs  ardox nails, 1-1/2" long 
120    angle brackets,  3" x  3" x  1/2" 
8        hinges,  2-1/2" x  2-1/2" 
2        hinges, brass,   3" x  3" 
9        rubber casters,  2" dia roller skate wheels 
9        rubber casters,  1-1/2" dia wheel 
9        aluminium brackets, 4" X  8" X  2" X  1/8" 
4        threaded rod, 1/4" X  #20 X  3' 
144    nuts, 1/4" 
72      flat washers, 1/4" 
18      nuts, 1/8" 
18      bolts, 1/8" x 1" long, 
5        white vinyl siding, 8" x 12'  (2 additional lengths required for 4th inside skirt) 
200     brass screws, # 6 x 1" long 
2        aluminium angle,  2" x 2" x 4' x 1/4" thick 
1        white primer, 1 gallon 
1        white gloss paint, 1 gallon 
1        flat black stain, 1 gallon 
1        pine trim, 80 feet 1/4" x 1" 
1        pine trim, 150 feet 1/4" x 1-3/4" 
1        pine trim, bevelled one edge, 100 feet 1/4" x 1-3/4" 
1        door weather strip, rubber and aluminium construction, 28 feet 
1        hasp, brass 
1        padlock or other, brass 
1        galvanized sheet, 12" x 24" x 26 gauge 
4        turnbuckles 
4        screw eyebolts 
5        door hooks 
2        slide bolts 
patio stones to make observatory base 8' x 8' 
bricks to make observatory even with observatory floor