Below I will describe how I built the telescope. For some parts I will only use some lines on this page, for other parts it will be necessary to click to another page, using the menu at the bottom of this page.
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Well, there it is: my
20 inch Dobsonian telescope, while setting it up
for an all nighter in the French Alpes, summer 2002, on the 'Col D'Izoard' at 2300 meters
altitude. I have worked on it for about eleven month's, well, not full
time of course. It can be done in less time, but this is the time I had
to wait for my mirror. Exept for the primary and secondary mirror and
the mirrorcell I built everything myself, including the secondary mount
and spider and the Crayford focuser. The primary mirror is made by
Steve Swayze, the secondary by AstroSystems. As a building guide I used
the excellent book: The Dobsonian telescope: a practical manual for
building large aperture telescopes, written by David Kriege en Richard
Berry. In addition to the book I gratefully made use of many ATM
websites on the internet and, last but not least, of the ATM
mailinglist. It takes about ten minutes to assemble the telescope and a
bit less to disassemble. Movement in altitude and azimuth is smooth and
the telescope is very stable an well balanced. A little wind has not
much effect on its performance. Below I will describe how I built the telescope. For some parts I will only use some lines on this page, for other parts it will be necessary to click to another page, using the menu at the bottom of this page. |
De mirrorbox and the rockerboxThe mirrorbox is 19.3 inches high and 25.9 inches wide (outside dimension). It is made of 0.7 inch thickness baltic birch plywood. The mirrorbox is strengthened with four corner braces. The altitude bearings are made of two layers of 0.7 inch thickness baltic birch plywood and attached to the mirrorbox with carriage bolts and locknuts. I used formica on the edge, which runs on teflon pads. De sides and bottom of the rockerbox are also made of two layers of 0.7 inch plywood. I have glued a formica layer against the bottom. The groundboard, with three teflon pads for the azimuth bearing is Y-shaped (see photo). To move the mirrorbox-rockerbox assembly around, two wheelbarrow handles can be attached to the sides of the rockerbox as this picture will show.The primary mirror cellA large mirror needs a floating cell to prevent it from deforming. I made an 18 points cell. A 12 points cell would have been good enough, but an 18 points cell is a bit more forgiving. According to PLOP the deformation caused by an 18 points cell for this mirror is about 1/580 lambda RMS, which is very good, given the fact that 1/128 lambda RMS is still acceptable. The cell and tailgate are made of stainless steel. Three pictures of the mirror cell are here.The mirror's edge supportSilicone adhesive (RTV)At first I didn't use a lateral support, but had the mirror glued to the 18 support points of the mirrorcell with 0.125 inch thickness, about 0.8 inch wide silicone blobs. The startest showed a fairly large amount of astigmatism. I couldn't check if the astigmatism was in the mirror, because I couldn't rotate the mirror. So I decided to cut the sillicone and installed a sling (made from an old car safety belt). The sling The double sling Piano wire supports Rollerbearing supports The secondary cageThe two 1.8 inch wide, 25.2 inch outside diameter rings of the secondary cage, are made of 0.6 inch thickness baltic birch plywood. The 12 inch long struts are made of 1.2 inch outside diameter (0.059 inch wall thickness) aluminium tubing. I couln't find the right type of threaded inserts to attach the rings to the struts. I glued 0.5 inch long wooden dowels in the struts and glued struts and rings together. I tried very hard to find kydex in The Netherlands, but I didn't even meet anyone who had ever heard of it. Instead of kydex I used thin plastic, attached to the inner rim of the rings with two-sided tape. On the inside of the secondary cage the plastic opposite to the focuser is covered with very flat black towelling. The two baffles opposite to the focuser (one on the upper ring and one below the lower ring) are made of some kind of foam, sometimes used as camping matrasses. The baffles, as shown in this picture are also covered with black towelling. The baffles are attached to the rings (the lower baffle also to the truss) with velcro.The fansTo cool the mirror down as quickly as possible I installed two 4 inch 12V fans: one blowing against the bottom of the mirror and one blowing across the surface. Both fans have their own on-off switch and potmeter, as shown in this picture. To prevent vibrations to show up in the eyepiece I glued (with two-sided tape) strips of a 0.4 inch thickness mousepad between the fans and the wood, as shown in this picture. I haven't noticed any vibrations in the eyepiece and, though the fans can be heard when running full speed, they are not noisy at all.The selfbuilt Crayford focuserA good 2 inch focuser is very expensive in The Netherlands. Therefore I made my own Crayford focuser, using very good information from other websites. The base is made of 0.6 inch plywood. The arc to which the four bearings are attached is made of two layers of 0.8 inch plywood. The drawtube is moved by a 1/4 inch threaded rod, covered with a piece of rubber hose. The knobs to move the rod are made of aluminium. The inside diameter of the drawtube is 2.28 inch. Someone else made the 1.25 inch adapter for me and the whole thing is anodized. Here is a photograph of the focuser. And here is a large photograph.ComputerizingAfter using a platform for some years to follow the stars, I computerized the telescope with Mel Bartels' stepper system. How I did this is show on this this page.
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Go to: main menu
Go to: the mirror cell
Go to: secondary and spider
Go to: the truss construction
Go to: the double sling
Email: Jan vanGastel |
