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[3/4] 488 mm F4.85 Dobson

Mechanical Parts of the Telescope

Main Mirror Cell




20x10mm stainless steel tubes are welded together to form the frame for the main mirror cell. There are also three 10mm nuts for the collimation screws. In order to save some time and because lack of some tools, in this project I have decided to order some demanding parts from different professional shops. For example the 5mm thick aluminium triangles are laser cut with high precision. On each tip of the triangles I first put some silicon pads but had some trouble to keep them in place. Therefore I later changed those to felt bads.

The aluminium triangles are mounted on three aluminium U-profile bars using small 5mm screws. The connection is slightly loose so the triangles are able to tilt in order to have uniform contact between the pads and the mirror back surface. For this purpose I mounted a small O-ring between the triangles and the aluminium profiles. In centre of the aluminium profiles there is a hole for collimation screw. Also this connection is designed loose so the entire structure is floating in order to minimize any deformations of the mirror.

When the telescope is pointing directly to zenith the weight of the mirror is evenly distributed between 18 support points. However, when pointed towards horizon there is also a wide sling supporting the mirror. First I made the sling out of seat belt but during the final measurements I realised that it was not stable enough since it seemed to stretch when exposed to moisture. Therefore I decided to use a very thin 0,5mm laser cut aluminium sling instead. Another advantage of using a metallic sling is that it can be locked in its positions and is not trickling off from the mirror edge like soft slings tend to do. Between the mirror and the metallic surface of the sling there is fabric tape to prevent any scratches on the glass.

In the backside of the main mirror cell I mounted an aluminium plate that holds the mirror cooling unit and switches both for the primary ventilator and secondary mirror heating unit. The fan is mechanically isolated from the aluminium plate using some O-rings. Hence any vibrations caused by the fan are not transferred into the mirror cell. The aluminium black plate is powder coated with bright red colour while the rest of the mirror cell is painted black with spray to prevent any reflections from the structure. The mass of the main mirror cell including the mirror is around 23kg.

Secondary Mirror Holder






The secondary mirror holder turned out to be a quite demanding part to design and construct. The holder must be as light as possible without losing any of its rigidity. The central parts are mainly made of plywood. The central part I constructed using 9mm plywood on which 45į angle the secondary mirror was later mounted. I sandpapered this part round by putting it into a drilling machine.

Underneath the 88mm secondary mirror an automatic heater was mounted. This dew remover measures the ambient outside temperature and is comparing it with the secondary mirror temperature. If the secondary temperature is too close to ambient temperature the heater will be automatically turned on and prevent the mirror to get foggy. Because there were lot of electronics on the electrical circuit I needed to shape the central part according to it. This is why there are some weird holes drilled on the angled plate. There is also a small LED indicator which shows when the heater is working. I mounted this light deep into a small hole back of the secondary holder. This way the light can be seen only when looked directly on it and is not disturbing any observations. In the bottom of the central part there is a thin aluminium sheet to prevent collimation screws sink into the soft wooden part. Around the central part I mounted a black plastic cylinder to make the appearance more neat.

The part that holds the angled part and spider legs is also made of thick plywood. The part is kind of star shaped in which four spikes the spider legs are glued with epoxy. In order to minimize the weight of the holder I ordered some 50x1mm carbon fibre rim from France. At the other side of the spider legs aluminium slots are glued. In these slots there are also threaded M5 holes for mounting the secondary holder. Because the secondary holder is directly front of the primary mirror its obstruction should be minimized. Therefore I didnít want to have additional electrical wires running to the secondary heater. I decided to use the carbon fiber spider legs themselves to provide voltage for the heater. So the current goes to the heather along two carbon fiber rims (+ and -).

The secondary mirror is glued to the angled part with three silicone-like flexible drop of glue (Penosil Seal and Fix). I used the same glue to attach the dew heather back of the secondary. The secondary mirror is aligned using three M5 collimation screws which are running through a triangular shaped aluminium part glued between the angled and the star shaped part. The angled and the star shaped part is mounted together using a 8mm bolt and a special low profile spring that I order from Holland (ok, maybe this special 30Ä spring went a bit too far). All the wooden parts are varnished very carefully many times before painting them black. This is to minimize some deformations the wood can undergo when exposed to humid environment. The weight of the secondary holder together with the mirror is around 800g whereof the mirror alone weights around 300g.

Upper Secondary Cage







The construction of the upper secondary cage began by routing of two 12mm plywood rings. The inner- and outer diameters of the rings are 528mm and 612mm, respectively. On the inner face of the ring I milled a groove where the electrical wires are running for the secondary dew heater. So the electrical wires are mostly hidden inside structures.

After varnishing the rings I connected them using 30mm carbon fiber tube. By using carbon fiber instead of aluminium I was able to made the upper part around 150g lighter. It might sound less but 150g produces quite a lot of moment when placed end of a 1,6m long arm. One metre of this carbon fiber tube cost me 50Ä.

Between the two rings I also glued two 9mm thick mounting plates for the finder scopes and focuser. For the focuser I used the Steeltrack from Baader. It is quite heavy but very rigid and good quality. For the finder scopes I used not only a standard 50mm optical finder but also additional Laser Finderscope from Celestron. During my previous observations I have found it a bit tricky to get the target directly into my optical finder scope without first roughly aligning the telescope from the bottom. Therefore I can now stay at the eyepiece, align the telescope first using the laser beam and then continue with the optical finder scope.

To the bottom of lower ring four upper truss tube clamps are mounted. I ordered these cast aluminium clamps from Obsession Telescopes (USA). Into each four camps two truss tubes are tightened by means of a wedge, pin and quick- release. Through one clamp there is a cable running for the secondary heater. When the telescope is mounted this cable is plugged into one of the truss tubes in where another cable is taking the connection down to the mirror box.

The two plywood rings are also connected with a thin laser cut 0,5mm aluminium sheet. The laser also cut the openings for the secondary mirror holder. This is why routing the inside diameter of the rings was very crucial in order to get the openings in right positions. This is why I had to route one of the rings again after finding out the inner diameter was about 1mm off. The sheet is powder coated red and the inside surface is sprayed matte black. The shroud is glued to the rings with a flexible glue.

Now when the upper secondary cage was finished I mounted the secondary holder. For the first time I aligned the primary mirror on its test stand and the finished upper secondary cage front of my shop window. I aligned the optics towards the nearby light pole and was able to focus the image. This was the very first ďfirst lightĒ and at least now I knew that the mirror was not figured terribly wrong. I also got the first impression how big the telescope will be, huh!

Side Bearings






In my previous telescopes I routed all the side bearings by myself but I found it not a very pleasant job. Therefore I decided to water cut nice bearings out of 21mm birch plywood. To the backside of the bearings I milled some grooves to hold aluminium triangles. The purpose of these laser cut and powder coated triangles is purely aesthetic and they serve no other functionality.

After the final finish of the triangles they are glued into the grooves and sealed with a 9mm thick plywood back cover. The diameter of this cover is slightly larger than the diameter of the bearing rings. This way the mirror box will automatically be centred on its Dobsonian mount and there will be no contact between the sides of the mirror box and the inner sides of the mount. This contact can easily happen if the two bearings are not perfectly aligned respect to each other. The back cover is attached with some glue and small hidden screws. I think this way I managed to make very nice and neat looking side bearings for my scope.

Because the side bearings are simply sliding on Teflon pads the friction between the pads and sliding surfaces must be minimized. I found some laminate ribbon (texture: granite) from Bauhaus that felt quite feasible for my purpose. There was already a layer of glue behind the ribbon and it was attached to the bearings by heating it with an iron. After the ribbon was attached I removed some excess material and finished the edges very carefully with a small hand-tool and some sandpaper. This work was quite nerve taking since the front surface of the bearing was already finished.

Mirror Box and Truss Clamping Blocks


For the mirror box I ordered some ready cut plywood sheets. I rounded one corner from two sheets and cut large circular openings for the supporting top and middle sheets. The sides of the box are made of 12mm thick plywood while the supporting sheets are made of a bit thinner 9mm birch plywood. The box is assembled so that no screw heads are visible from outside. In the inside corners of the box there are some wooden support profiles in order to join the sides together with some screws and wood glue. It was quite a challenge to assemble this size mirror box alone since the glue was setting fast and there was no much time to align all the parts perfectly.





Making the lower wood clamping blocks for the truss poles was not much fun. I needed eight of them and each block needed several work steps. I made these clamping blocks out of laminated veneer lumber that is a bit softer than normal plywood. First I sawed the blocks using my small band saw. Then the deep holes for the truss poles were drilled. It was very difficult to find a drill bit with a right diameter because 30mm was too small for my 30mm poles and the next size 32mm bit makes the hole too loose. Therefore I decided to drill with the 30mm bit and later sandpaper the hole bigger using some special tools.

When the holes were finished I opened the back side so the tightening slot is able to bend more easily. I also drilled a 8mm through hole for the tightening screw and filed some wood from this region so that the tightening effect is possible. There are also small holes for mounting screws in each corner of the blocks.

Two of the clamping blocks are mounted inside the mirror box because the side bearings were on their way. This way the telescope also looks nicer I think. The upper ends of the truss tubes had to meet pair wise in order to clamp them together. This is why it was very critical to sandpaper the right angle for each clamping blocks. Now when the clamps were done for the first time I was able to assemble my mirror box and the upper secondary cage together. Woah, it was huge! It doesnít happen very often that you can actually go inside your telescope to tighten some screws.

After having all the holes etc. in the mirror box I sandpapered it carefully and varnished the plywood several times; first with some tinted varnish and later finished the surface with glossy one. I didnít realise that I had to thin the tinted varnish and therefore the colour of the mirror box is not very even. I learned this when I was finishing my mount. After varnishing I sprayed the interior of the box matte black. I also made a lid for the mirror box so that the box is sealed when the telescope is transported or not in use.

Next I was able to mount the side bearings. I attached the bearings by glue and using screws from inside the mirror box. The alignment of the side bearings is critical in order to have the box sitting straight in the mount. The heads of the two 8mm tightening bolts are hided inside the side bearings so they are not visible from outside.

Behind the mirror box I mounted two 12V gel batteries. It is important to use gel batteries because they donít leak when turned various positions. They are also good for intermittent use. The batteries are mounted simply by hanging them from their poles on hooks made of brass. In order to keep the batteries not moving I mounted a magnetic strip between the mirror box wall and the batteries. It is very easy to mount the batteries even when the main mirror is installed.

The batteries are first connected to the primary mirror holder that holds the switches for the cooling unit and the secondary heater. For the secondary dew remover there is one additional connection in the primary holder. From here current goes to the bottom of one of the clamping blocks where another plug is installed. The counterpiece for this plug can be found at the bottom of one of the truss tubes. A wire installed inside the tube delivers the connection up to the upper secondary cage which is connected to the tube with another plug.

So basically when assembling the telescope you only need to connect the upper secondary cage to this truss pole. Again the wires are hided inside some structures of the telescope. On the front side of the mirror box I also installed one external plug from where 12V can be obtained or batteries can be charged.

Dobsonian Mount

The low profile Dobsonian mount I made out of 21mm plywood. I first cut the necessary curves and round plates using a router. Because I didnít want to have any screws visible on the mount either I had to assemble it mainly using some glue and connecting pins. The bottom sheet is screwed to the sides, though. Because the bottom sheet will be exposed behind the main mirror I painted the interiors of the mount black. In the centre of the mount is 12mm bolt acting as an azimuth axis. All the screws, bolts, nuts and washers used in this telescope are rust free or acid-proof grades.

The mirror box is sliding on four Teflon pads. The Dobsonian mount is rotating on three large Teflon pads mounted between the bottom sheet and the round ground plate of the mount. On the bottom of the ground plate there are three rubber legs. When I first mounted the legs I realised they were too soft and were yielding when the telescope was turned. Therefore I added some additional plywood support rings around the legs.

In order to make transportation of more than 70kg telescope feasible for one person I had to make wheel handles for it. When the handles are attached to the rocker (mount) it is easy to move the telescope around and even to slide it into a transport vehicle alone. These wheel handles are removed when observations start.

The First Assembly

Finally all the parts were finished! I asked a friend of mine to help me install the primary mirror. Then we rolled this impressive Star Cannon under the sky for the first time and pointed it towards the Moon. We could not yet focus the image because the truss tubes were intentionally left a bit too long. Little by little I shortened the tubes until I was able to focus all my eyepieces. The image of the Moon was crisp and full of wonderful little details! This was a great relief to know that the optics is working and I didnít spend almost thousand hours for nothing.

The last thing was to sew a black light shroud around the truss poles to prevent some stray light. However, this turned to be quite a challenge. Iím able to build a half a meter telescope but I think I do not have enough courage to sew a light shroud by myself. Therefore it took some weeks to find first a sewing machine and then bring it together with a needlegirl. For the light shroud I bought meters of black leggings fabric. It is stretchy (can be tightened straight), nicely glossy from outside and matte from inside. To the bottom and upper part of the shroud I slam some rings that can be used to set the shroud nicely around the truss.

Even the telescope is homemade it cost me almost 4000Ä. However, it is still cheaper than commercial telescopes in this size. The good focuser, special mirror coatings, laser- and water cut parts, powder coatings etc. increased the expenses quite a lot. For sure it is possible to make a half a metre telescope with much less money. By making a telescope by yourself you can have it exactly as you like it with your own little details.

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