SkyNest 2 Observatory: The Build
In the intro to my Website I recounted how the ever worsening light pollution eventually led to my decision to relocate my observatory to a little town situated in the semi-arid region of the Klein Karoo, South Africa.
SkyNest 1 refered to in the intro was a stand-alone observatory. It had a simple arrangement with a cover which had to be removed manually from the outside. I had to rebuild this dome for SkyNest 2 by adapting it resulting in a wider and longer aperture with two shutters opening and closing on a horizontal axis. Lengthening the aperture now allows for imaging past the meridian and the widened shutters results in less frequent rotating of the dome to compensate for the Earth's rotation.
In this article I will explain how I constructed this dome as this might be of interest to DIY-ers wanting to embark on such build.
SkyNest 2 dome rebuild in progress
It is not my intention for this to be a construction article but more a general description of the techniques I used and to provide a few pointers. The thought of doing this yourself might be daunting at first but it is in fact not very difficult and does not require any special tooling or materials that are not easily available.
I decided on a Geodesic dome design which in essence is nothing but a bunch of triangles of two sizes stitched together to form the dome. The stitch and glue method is often used in the construction of light weight marine ply boats and lots of how to do articles exist on the WEB.
For calculating the dimensions of the triangles go to www.desertdomes.com/domecalc.html select 2V enter the required dome radius and ‘voila’ the calculator comes up with the exact dimensions of the triangles required. My dome without the skirt measures 2200mm in diameter.
I decided to use 4mm marine ply for the dome construction. I used this relatively thin plywood because it was important that the weight be kept down as the end result had to be manhandled into position on both sites as I would not have been be able to hire cranes for this job. At the same time it had to be able to withstand the harsh climate of the Karoo.
Up she goes!
The end product is very strong in spite of the light materials used. The outside of the dome is covered in fiber glass cloth which weather proofs the dome and further adds to rigidity. The bottom of the dome is a flat surface cut by using a plunge router fixed to a length of wood forming a large compass anchored at one point with the router at the other end. The semi-circle cut pieces when put together form a circle of plywood like a donut with a big center hole and a narrow edge. The individual pieces are then laminated together. I did cut two sets and laminated one on top of the other ending up with a circle 10mm thick 100mm wide with an inside radius of 1100 mm. This construction method results in a very strong lamination and forms the base that runs on 10 small rubber casters allowing the dome to be rotated by hand to allow for the Earth’s rotation.
Supporting Casters Dome positioning Casters
To the circle I epoxied a skirt 100 mm wide. This was done by bending a 100mm wide marine ply strip around this circle so forming the skirt. Here again I laminated a second strip on top of the first one for strength. This skirt hangs over the observatory walls shielding the inside of the observatory against the elements and is also the surface against which four casters press to keep the rolling castors in position thus preventing the dome from slipping sideways as the dome is rotated.
The finished round ‘half channel’ is then stitched to the dome and epoxied in place.
The half channel epoxied in place
The opening to be covered by the shutters was cut out with a jigsaw after the dome was completed. I then stitched and epoxied 75 mm sides to the opening. The curves of the sides were cut with the router compass. Care was taken so that no distortion occurred in order for the opening to be square.
The shutters were formed by bending marine ply to follow the curve of the sides of the opening. The sides of the shutters, also cut with the router compass, were then stitched and epoxied into position in ‘situ’. I finished the shutters in glass fiber cloth whilst in the formed position to ensure that the curve was retained. I used plastic film as a barrier to prevent inadvertently epoxying the shutters to the dome surface or to each other.
The completed dome ready to be trailered 1200km to its new home
The shutter mechanism is essentially 30mm stainless steel pipe bolted to the dome top and bottom and the shutters slide on nylon bearings pressed into 40 mm stainless steel pipes. The 40mm pipe is bolted to the shutters. The opening and closing is achieved by an arrangement of pulleys and a rope system.
The rope and pulley system
This arrangement will not win any design prizes but was the best I could come up with after hours and hours of racking my brains to come up with something that could be home built. It is crude but it does work. The proviso is that the bottom and top linear bearing systems must be absolutely parallel otherwise binding will occur.
Important aspects to consider are:
1 The triangles must be accurately cut otherwise stitching them together will be difficult if not impossible. I used a jigsaw to cut the triangles to size. I then stacked them in sets and finished to exact size with a belt sander.
2 Use SP 106 epoxy resin and a slow hardener. This is a quality epoxy resin with superior adhesion qualities.
3 Use armature (copper) wire to stitch the panels together.
4 Use glass fiber tape and epoxy on all the joints inside and outside after stitching. This will permanently glue together the triangles and other pieces making up the dome.
5 It will be easier to first stitch 5 triangles together forming a pentacon and then stitch the sets of 5 triangles together forming the dome. Make sure that you are stitching the correct triangles together. This job will be much easier having a helper to hold the sets in place while you do the stitching.
6 Work on a flat and true surface to ensure that the finished dome is true in all its planes.
7 Take care to ensure that the bottom and skirt part of the dome is absolutely round and that the surfaces that will run on the casters are flat and true. This is important to ensure that the casters keeping the dome in position push equally against the sides of the skirt and that all the castors make contact with the flat part of the skirt once installed.(my dome is not perfect in this regard and not all the casters make contact at all times as the dome is rotated)
8 The success of the rope and pulley system will depend on how accurately the nylon bearings in relationship to the linear pipe bearing surface are. If the shutters bind during opening and closing they will get stuck in that position and will have to be loosened by hand.
9 A design flaw was that my linear bearing system is exposed to the elements. This had the result that dust settled on the bearing surfaces and that the lubricant became tacky over time requiring frequent maintenance. Rather than re-designing the concept I fitted UV resistant and waterproof booties over the bearing surfaces and these have solved the problem. I expect that the booties will degrade in the sun and will have to be replaced every 6 months or so.
10 In forming the skirt and the shutters cut the plywood in such a way that bending will happen with the grain. Damping the plywood with hot water will also assist in bending. The required curves are not that extreme in a dome that is 2200mm wide and bending the plywood is surprisingly easy. Having lots of c-clamps will make this job easier.
11 Measure and then measure again before cutting. The base of the dome has to fit the skirt accurately. The measurement of the dome base determines the radius of the compass router assembly. This is a critical part of the dome construction and you do not want to do all this work and then find out that the skirt does not fit the dome.
Skynest 2 is now entering its second year of operation and has survived rain, hail and high winds. Having access to my observatory through my living room and being able to image whilst being in my home has added a new and pleasant dimension to my hobby.