Dual telescope mounting Uses the Vixen large mounting plae to mount the VC200L and a Skywatcher ED80 along side. There's just enough room, both between the scopes and on the VC200 mounting doveplate, to move teh visac forward far enough to balance. Fibreglass shutter Fibreglass is a very strong plastic composite material you can 'lay-up' or mould in the garage. I had some minor previous experience with this technique but none with moulding or building large scale moulds. I also had trouble finding noddy guides answering the practical questions of what materiels to use and their preparation for releasing mouldings from moulds. So I wrote it down . Hope its useful. By the way - I discovered a local GRP one-man-band while I was part-way through. He had a mould segment hanging from the roof of a 24' mosque onion dome. He had also made a 3m hemisphere early that year and throuwn the mould away due to lack of storage. If you want to make your own observatory - find your local supplier and ASK!. He also said that the hemisphere had cost £41/2 GBP (April 2005 ). I could have had it for nothing had I asked at the right time.

Building a new dome shutter

I needed to make a new fibreglass shutter for the dome because the original blew out in a storm the previous year. The lightweight shutter rides on rollers front and rear and along ridges on the dome sides to keep it aligned. The shutter I replaced it with was a entirely satisfactory - it was plywood sides with thin MDF surfacing and foam filler for rigidity between the front and back faces. However after saturating with thinned resin and cladding with GRP for long-term waterproofing, it weighed a ton, metaphorically speaking. Som much that it distorts the rest of the grp dome depending on its deployed position. Then after moving house and refacing the dome halves and replacing hthe joining section it became clear that the size of the joining section had increased by a few millimeters sufficient to make the shutter no longer slide or roll - at all. SO I needed a new shutter for the observtory - to be able to observe at all. The key requirements were : light to very light strong rigid coloured good cosmetic finish good sealing with lower shutter for complete rain exclusion ability to reuse existing hardware and mechanise - see light. SO I made a mould in plywood based on the size of the clad plywood shutter, allowing extra width forthe the increased width of the joining section and incorporating a moulded lip to allow sealilng over the existing lower shutter. The mould was made from 3/4" plywood with a 4mm plywood inlay providing the curved shutter flat surface. The depth of the inlay controlled the depth of the shutter edges. The curves were marked out using a long compass ( spar of wood, pencil and nail at either end ). A curve for the inlay was routed out using a 6mm straight cutter on a home-made compass arm for the router. The two sides of the mould were then assembled using battens to hold the sides the correct distance apart while the flat sheet was slid down the router groove from top to bottom. This bit is not easy. I've done it twice now - once for each shutter. The first one I used candle wax to grease the grooves for easier sliding but I couldn't do that for the second one because I wanted to paint over it to seal it. There are really only two options, feed the sheet into to a rigid frame and hope it fits all the way or spring it into a less rigid frame and stiffen the frame after to prvent it springin out again. i found the former the only way to proceed. ANything that can spring in can spring out again before you can apply stiffness. Using a combination of pushing the sheet along the routed grooves using a wooden mallet and spare spar to protect the end, and pulling it using a a g-cramp to sandwhich the end and provide a handle for pulling worked very effectively. I also found that I had to run a spar between the sheet and frame stiffeners to prevent the sheet from bowing and springing of its own accord. The end of the mould with the overlap moulding required the use of some lightweigth battens fastened across the end of the sheet and some more hand-routed curves into which these lightweight battens could be sprung in sections. These were then nailed to the support spars of the framework underneath and the curves filled out with bodyfiller. The entire woodwork of the mould surface was then sanded back before giving it a good varnish with Ronseal exterior hard varnish. This I assumed was a one-part polyurethane varnish. I followed that up with a surface sanding before coating again, sanded again, final coating of varnish and final sand. Following further discussions about release agents and the practicalities of removing the GRP component from the mould, I then applied a further layer of 2-part polyurethane gloss paint, as often found in marine chandleries. Nasty isocyanate hadeners and more... but finishes to a great hard gloss coat. The reason for this was that the varnish when tested reacted against the polyester resin used in GRP by wrinkling. If the GRP and varnish react then all my efforts would be for nought. This two-part paint is strange stuff, in the tub its quite runny, as soon as its applied its very stiff to work and then when you think you have it suitably spread out it starts running again. I had a great finish in some areas and some major runs in others. However my paint brush when applying the varnish hadn't been as clean as it ought and so there were parts of the varnish poisoned by a different chemical which subsequently reacted with the two-pack gloss coat and caused serious wrinkling and bubbling where the poisoning had occurred. Everywhere else the paint had a good hard grip on the mould and varnish. Where the varnish was poisoned, it was a soft skin you could peel off and bubbles you could rub off the paint with your thumb back to the wood. The only solution was to sand back those posioned areas and major runs to wood and body fill to match the varnish/paint levels. After that I gave up messing about and applied two coats of PVA release solution, the first by brush and the second by sponge. The sponge layer comes up much better - its more even if you can avoid bubbles and also is much less prone to streaking. Then it was time to lay up the glass fibre. I mixed a litre at a time mostly. I did a coloured gel-coat first consisting of 500ml resin-A, 50ml polyester colourant and catalyst. this resin went on as thickly and evenly as I could get it to try and get a thoroughly opaque and even white coloouring on the shuter outsides. Then after this had catalysed, ( took all night it was so cold ) a layer of glass tissue was resined in place with some medium-weight CSM glass on top. As I worked through the CSM layers I started with 4% catalyst and eventually moved to 2% to achieve working time in 1 litre pots, even in the 5°C working temperature. After 3 layers of 350g/m2 CSM I then finished with another layer of tissue - running out of tissue and CSM as I went. Important lessons learnt were : A ribbed consolidating roller worked very well, kept in a pot of acetone between uses. Applying a width of CSM across the shutter at a time and staggering joins worked very well at keeping time per layer down. I'd already body-fillered into the corners to provide a smooth chamfer curve When laying up so I had no problems with CSM in corners. When CSM extends beyond the mould edges, cut to about an inc protrusion and resin it all the way up so it joins the previous layers. This means the edges are well supported and makes a throughly resined edge for later trimming which is far easier to handle as a single surface rather than scrappy ends of glass mat. It also means you trim and sand the edge back to your mould edges later. DO rough trimming at the 'green' stage and full trimming prior to remiving from teh mould. I did the rough trimming with the glass scissors and the neat trimming after cure with a hacksaw blade. the blade should be pulled towards ou against the support of the mould edges to prevent break-out of glass fibres. Removing from teh mould. The mould is all flat surfaces. This either makes it quite hard or quite easy depending on circumstance. There's large areas to grab but no complex shapes to 'pop'. Starting from the end I pushed with a piece of wood to start lifting the centre and spread it to the edges. Then I went along the edges of the shutter pushing off the grp from the mould with a wooden chisel and block hammer. repeating this all the way around and using a long chisel for lifting the shutter surace all the way down the mould eventually lifted it after half and hour. The PVA surface was like a layer of cling film starting to peel off. A little of the gloss paint had come off the mould in places but could be rubbed off the shutter with a thumb. Finally, I went around all the edges of the shutter sanding with a coarse sander back to the line of the gel coat. this gives a nice clean finish all the way around and cleans off all the extra glass left by the neat trimming. And the result ? - Wow its light!. Its also a bit more flexible than I expected and thinner - about 3mm wall-thickness. If anything this needs beefing up a bit. By the time I fix the roller hardware on it will be much stiffer. All in all a All in all a very useful experience . My key points for a next project would be : Varnish IS fine as a mould sealant - use a one pack polyu and ample PVA release. I applied PVA release direct to the porous body filler and it still came off fine. I guess even the hardware store ordinary varnish is fine as long as you have a good application of PVA. Keep your brushes clean and single-use only. ( Don't mix chemistries.. ) Think about stiffness and use foam sheet. ITs very light but stiff, takes resin and it's easy to laminate into the finished product. Apply two layers of coloured gelcoat. Mine was too thin in areas. A layer of glass tissue on both outer surfaces makes handling the finished product a lot nicer afterwards. My next projects are going to be : Making a geodesic dome in GRP using 2mm insulation polystyrene sheet (roll ) as the former. This should be fine with a lightweight cladding of tissue left to go cure before applying the heavier CSM and further tissue. The only issue here is to protect the polystyrene with emulsion paint or latex to prevent resin attack by the polyester resin. Make a GRP mould for a 4' dome based on the same design as the skybadger dome. This would be purely for robotic small telescopes not requiring human intervention. Focusers and mounting plates Observatory shutter hardware Magnetometer data TIR array data Artificial stars - attempts and results. I have recently been playing with artificial stars as collimation sources for my telescopes. The rationale for creating and using artificial stars for telescope collimation looks like this

With all these benefits what possible drawbacks could there be ? Well - you need to guarantee that the target is bright enough, sufficiently far enough away to be not resolved by your telescope and illuminated by a source of suitable size again not to be resolved ( when using a reflective artificial star) The most common source used for illuminating the target is the sun, where the target is a spherical surface, say a ball bearing or even christmas tree decoration. As long as these targets are smaller than a given size at a given distance ( mathematically described by Suiter in 'Star-testing Astronomical Telescopes' then they can be used as irresolveable sources just like stars. A rule of thumb often quoted for minimum target distance is 20x focal length of the telescope to be tested. I tended to find that using the sun as a source in Good Old Blighty is not reliable enough and my garden orients in the wrong direction for good illumination of the ball - I get glancing illunination only so I wanted something a bitmore controllable. So the next step was to take a diode laser as found in a pen-pointer or similar and try that. I rigged up a mounting in wood, where the target was again a ball-bearing and the laser was 12" away in the direction of the observer so get a good, almost direct-incidence reflection. I also added a transistor heatsink to the laser to ease the heatload on what is really a cheap diode not meant for always-on use. This solution was tested in the dark of the evening across 20m of garden and found to be OK for coarse alignment but no use for fine collimation. This was because the artificial stars linear size on the ball bearing was too big and oriented as a horizontal or vertical bar. Indeed, close inspection of the lasers spot output showed a 2mm or so bar with 3 bright spots along its length. The resulted diffraction pattern was too confusing to be of use and the out of focus pattern only of little use. with these results I moved designs and went to a fibre-optic source. The first stage was to use a piece of polyethylene strand from my childrens' toy wand, such as also found on 'fibre-optic christmas trees'. These are quite coarse, large diameter (0.5 to 1.0mm ) strands but all I had. I attempted to feed the laser into this and didn't get a good result. So I changed the design to use a mini-maglight torch as the light source and made an adapter with a fine drill and a few shiny washers and some hot-glue to fix it all together. This was much better - the light output was better, the output size was smaller and I could position the fibre anywhere I liked to hide the glow from the torch from being visile in the telescope. The result in the eyepiece was much better too. Enough to expose the limitations of my testing environment. for the sake of comfort and environmental control I was using the telescope indoors through double glazing. Now in itself this is not too big a deal, plate glass is quite flat enough for gross testing but clearly not good enough for detailed testing . however , opening the doors means there is a large degree of cold air causing thermal optical effects so I'm back to testing at thermal equilibrium outdoors. In the meantime, our good friends at ukastroimaging provided me with a real optical fibre pigtail for testing so more in that shortly.