Rocker box: All of the rocker box is made from 9mm birch ply. Webs were bonded to the perimeter of the hexagon to give greater lateral stability in supporting the scope. These buttresses work well.
I have used the double ring ground board format for the base. This method is quickly gaining popularity as it eliminates the need for heavy material in the centre without compromising on strength, stiffness of functionality. The scope does not rotate on a pivot bolt but ‘moves round’ bearings placed on the inside radius.
However, instead of roller bearings, I have used Teflon and Laminate bonded to the inside radius of the rocker box. As there is no real weight on these bearing, this method serves extremely well.
This underside of the rocker box has a flat ring of Textured laminate bonded to it for the azimuth-bearing surface.
The sides of the rocker box have 4mm ply altitude bearing pads bonded to them. I have made the pads elliptical in shape to minimise the starting force required to move the scope. The surfaces were then radiused by sticking sandpaper to the outside of the altitude bearing and rotating the scope. The pads were varnished and the Teflon sheet was stuck down with double-sided sticky tape.
The surfaces of the rocker are varnished wood and flat black paint.
Mirror Box: is a fairly simple affair. It is constructed from 9mm birch ply. The box is 17.5cm high and the sides are long enough to give a hexagon with 15mm clearance from the edge of the mirror. I bonded the sides by cutting 30degree angles on their edges and fixing them with cable ties round the perimeter while the glue dried.
The box has holes cut out on each of the faces to further lighten the design. To strengthen the upper end of the box I have attached a hexagon of 4mm ply with its centre cut out. This effectively webs the hexagon and makes the profile strong. The bottom end of the mirror box accepts the mirror cell, which strengthens the profile of the bottom end.
The bearings are attached by location on 4 aluminium dowels on each side inset into the wood and locked in placed with wing nuts that run through the centre of the dowels.
The truss poles are attached by location on 1 dowel and 1 magnet. I have used very strong neodymium 2.5Kg pull magnets bonded into the mirror box. The truss poles have iron screws in them at the bottom to attach to the magnets. This arrangement is very stable and strong. The truss tubes just snap in and out for assembly / disassembly – no fixers, extra parts to lose or drop on the mirror!
Mirror cell: is constructed from two layers of 9mm birch ply bonded together. 18mm is solid and supports the mirror well. The bottom layer of ply is a slightly larger hexagon than the top. This allowed the cell to slot into the mirror box before it was all bonded up.
The collimaton bolts are M5 Machine screws. I have drilled three points in three different radii. This was to allow me to try supporting the mirror at 40,50,60% radius to try and find out if this made any difference to the image. I suspect that my mirror is too small for this experiment to make a difference (the mirror is 8.75in x 1in thick) I have inset and bonded Nylock nuts into the frame. This prevents the collimation going off due to vibration and other movement. There is no lateral movement of the support points, which sometimes occurs with sprung cells.
The backs of the collimation bolts have large wooden discs attached to them to make turning the bolts through the Nylock nuts easy. The only concern I have about using nylon locking nuts is that the nylon may wear over time. However this can be easily remedied with a small drop of locking compound.
I have used a sling and split bolts to support the mirror.
I have also added velcro (with sticky back to the edge of the mirror). Velcro straps wrap round the back of the mirror cell during transportation to keeep the mirror safe from falling out of its cell if it is inverted. The velcro straps are loosened when using the scope.
This is a fairly complex construction. The base ring is made from 3mm balsa ply. This is plywood I made specifically for the scope (I don’t know if balsa ply is made anywhere). The load bearing areas are made from 0.7mm sheet aluminium. This ring is extremely stiff and light.
The load bearing areas are held in tension by the spider vanes. This helps to further rigidify the cage. The photos and diagrams of this part can do the construction more justice than any description so I’ll be brief.
Basically the ring is a ‘3mm balsa plywood hollow box section with drilled faces’. The ring x-section is 45mm wide and 15mm thick. The construction has been heavily treated with UPVA. I then used a contact adhesive to bond three crescents of aluminium to the top face, two quadrants to the sides of the focuser board and one ring to the face of inside diameter. The aluminium additions really stiffen the structure up as these are load bearing areas.
The truss attachment points are a 6mm aluminium dowel with magets bonded on either side which hold the trusses on.The magnets are not load bearing. The dowel holds the truss and the magnet keeps it snapped down.
All parts are painted flat black.
MK2 Truss ‘trusses’ (since replaced with carbon fibre poles. 2015)This truss was designed as a cheap replacement for ultralight carbon fiber tubes. The first incarnation of the design, although light and stiff enough, was too fragile. It was based upon a frame of balsa wood which was soaked in PVA glue.
This new design improves greatly upon the toughness of the previous, and its ability to withstand bangs or knocks. I have used the same format as before. however the cross members now have reinforcing ribs on the back face. Also more attention has been payed to grain direction. The main improvement however is the use of a thin layer of Fibreglass along the spines of the truss.
1/ I first drew out a template on a large board covered in wallpaper liner. This template formed the basis for the three sides of the triangular truss.
2/ Strips of balsa were cut and layed out on the template. The spines were laid lengthways and the crossmembers with ribs glued to the spines.
3/ The three sides were left to dry, then chamfered (sanded) with 60 degree edges.
4/ The three bonded sides were now stuck together to form a triangular section tube.
5/ The end pieces with 6mm dowel holes were then glued on. The jig had dowels inset into it, so that each truss is identical.
6/ All sharp edges were sanded.
7/ A layer of thin fiberglass was laid along the spines running the length of the truss, where the truss is most likely to be abused.
8/ The whole lot was spray painted black.
9/Iron washers were attatched over the dowel holes to act as clamps for the magnet. This is the method previously implemented.
What I have ended up with is an extremely light tube (Lenght=105cm, Weight = 60g) which is now also fairly tough. My end goal is to achieve the same performance as a carbon tube at a fraction of the cost. This design is coming very close to achieving this. For my 20in scope I am planning on the truss tubes to each weigh 250g. (2m long)It may be commented upon that this process is of course time consuming. Hopefully this is offset by the fact that they are cheap to manufacture.
The ground board is of identical dimensions to the base of the rocker box. It is also made of 9mm birch ply. However there are holes bored all round to lighten the structure. I have then stiffened the board up greatly by bonding and screwing a 15mm strip of 0.7mm aluminium to the circumference.
The azimuth bearing pads are 4mm ply. Again they are elliptical in shape to minimise the force needed to start the scope moving. They have been varnished and Teflon sheet stuck to them with double-sided sticky tape.
The feet of the scope are directly under the bearing pads to maximise stability. The feet are made from 25mm dowels sanded to a point.
The ground board incorporates the retaining bearings for the movement of the scope on the horizontal plane. I mentioned in the description of the rocker box that I have not used roller bearings – I’ve used pads with Teflon on. These pads are attached to L-shaped shims that are fixed to the underside of the ground board. The shims have a slot so that I could adjust the radius that they track. This means that the scope does not slip on the horizontal plane, but is free enough to move without any restriction.
All parts were primed, painted varnished etc.
The focuser is a standard aluminium sliding collar. I have had parts of it machined down to minimise weight. In its original state it weighed 150g. It now weighs 65g.
It is held to the focuser board by 6 M3 machine screws, and is radiused on the inside.