Building a 20 inch Dobsonian telescope

(Nederlandse versie)
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 rockerbox

The 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 cell

A 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 support

Silicone 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 astigmatism was gone (though I sometimes saw some when pointing to very low altitude). Because of the stretching of the sling and because the mirror did move sideways while using an equatorial platform, I decided to make made double metal sling, an idea of Nils Olof Carlin (see linkspage).

The double sling
This doublesling gave less astigmatism then the RTV, but sometimes I still saw some. The reason could be the fact that the way the slings are attached to the mirrorbox grip the mirror too tight. In Nils Olof's opinion the problem is probably, that the upper attachments of the slings, to the adjustment bolts, are too close to the mirror. To get rid of the problem I made piano wire edge supports.

Piano wire supports
This page shows how I made the piano wire supports and here is a photograph of one of the supports in my 20 incher. I never saw astigmatism since I use these supports, they do not stretch or deform and the mirror cannot move sideways. The supports are placed at 90 degrees, at the lower corners (when pointed at the horizon) of the mirrorbox. They touch the mirror's edge at the plane of the center of gravity of the mirror and point to the center of the mirror. I have the idea of this type of lateral support from Frederic Gea's homepage. There's a link to his very interesting pages on my linkspage. There's also a link to Nils Olof Carlin's article about edge supports. His study shows, that piano wires, if placed accurately, work very well. They are even slightly less critical to accurate placement then a sling. As I see it, given the need for very accurate placement, a big advantage of using piano wires as compared to a sling, is that piano wires stay in the correct position and a (traditional) sling does not.

Rollerbearing supports
The piano wires work very well, but there is one (minor) problem. When I collimate the primary when collimation is relatively far off, the mirror tends to 'hang' a bit in the piano wires. Pointing the scope in vertical direction lets the wires 'jump' to neutral position again, after which the collimation can be finetuned, so it's no big deal. But after I motorized my telescope (Mel Bartels' stepper system), pointing the scope up was a bit more work then it used to be and I decided to look for another solution. A discussion about edge supports on the Torus mirror mailinglist and a picture of a 'rollerskate bearing' edge support from Nils Olof Carlin in the photo section of this list, made me decide to make rollerbearing edge supports.

The secondary cage

The 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 fans

To 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 focuser

A 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.


After 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.

Go to: main menu

Go to: the mirror cell Go to: secondary and spider Go to: the truss construction Go to: the double sling
Go to: piano wire supports Go to: roller bearing edge supports Go to: computerizing
Go to: A 20 inch f/3.6 computerized Dobsonian Go to: Building a trilateral computerized 20 inch f/5 Dobsonian
Go to: Project: five 12 inch lightweight Dobsonians Go to: Motorizing a 12 inch lightweight Dobsonian
Go to: 20 inch telescope Go to: Equatorial platform Go to: Scotch mount Go to: Binocular mount Go to: Dobsonian tips
Go to: Bending aluminium Go to: Collimating Go to: Making a Krupa collimator Go to: Dotting the primary
Go to: A ballhead type telrad/finder mount Go to: Mirror making log of 300 mm mirror Go to: Astronomy
Go to: Using digital finder charts at the eyepiece Go to: Astronomy links Go to: Building a spherometer
Go to: Building a Bath interferometer Go to: A Foucault-Ronchi-Lyot tester Go to: Building a mirror making machine
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Email: Jan vanGastel