Plans for a homemade
|Since you are reading this, you are probably considering to build
your own Dobsonian telescope.
This page provides detailed plans and instructions how to build one.
You can buy some ready made parts, or you can make everything on your own, including the mirrors and eyepieces.
You can follow the instructions strictly, or make modifications to suit your needs, available materials, and skills.
The design described here works very well for more than 40 telescopes I have built either for myself, fellow astronomy club members or educational projects.
What is an Dobsonian?
It is an Newtonian optical tube, mounted on a very simple, yet very stable and easy to use alt azimuth mount. The mount works via friction, just little enough to easily move the telescope, but yet enough that the telescope remains pointed. It is held together by gravity alone. This mount was invented by John Dobson, a member of the San Francisco Sidewalk Astronomers. It was first referred to as the "sidewalk telescope". Its main goal was to make astronomical observations affordable to anybody, by using this cheap, easy to make telescope.
What size should I choose?
are the 2 most common sizes, 150 mm (6") and 200 mm (8") primary
Besides the primary mirror diameter, there is another important parameter : the focal length.
Commonly available are 150 mm primary mirrors at f8 (1200 mm), f 6.7 (1000 mm) F5 (750 mm),
and 200 mm mirrors at f6 (1200 mm) and f5 (1000 mm)
The mount presented here can not accommodate the 200 mm f 8 (1600 mm FL) tube.
Most popular are the 150 mm F8 and the 200 mm F6. Both are universal telescopes, equally good both for planetary or deep sky observations. Going for a shorter focal length will give you a shorter, more compact tube, easier to transport. On the other hand, the faster focal ratio will want more expensive eyepieces, and is more demanding on collimation. The 150 mm will be cheaper and lighter, but the 200 mm gathers more light and will show more deep sky objects.. for a higher price, and more bulk and weight.
Materials and tools needed:
important components are the optics, primary and secondary
mirrors. Buy those first. For a 200 mm F6 primary, a 40 mm minor
axis secondary is needed. For the 150 mm F8, a 32 mm
You will also need at least one eyepiece, and a finder scope.
Besides that, the rest of the telescope are boards, a tube, a handful of screws and couple more easy to find items. You might buy a ready made focuser, spider and primary cell... or make your own. Same with eyepiece's and finder scope.
A drill, jigsaw, and some simple hand tools is all that is needed.
Lets start with the MOUNT:
The mount is made from standard 19 mm particleboard. It is cheap and commonly available. In most shops, you can bring a cutting list, and you receive ready cut, square pieces. Pick any color you want, or, you can buy the small cutting leftovers very cheaply. You will get boards in a few colors and patterns... but all it takes is one can of spray paint to paint it. You have to paint it anyway, to make it moisture resistant. Out there, at night, it can get very wet. You can also make this mount out of 19 mm plywood, or laminated wood.
Cutting lists :
150 mm mount
200 mm mount
At this point, you can also buy the tube, and the altitude
bearings. For this telescope, you need a PVC drain tube.
If you have a 150 mm F8 mirror, a 200 mm OD tube is needed. Unfortunately, the standard sizes are 1 and 2 meters in length.
If you can find a scrap piece about 120 cm long, that would be just right. The f6.7 and the f5 focal ratio fit the 1 meter tube.
Same goes for the 200 mm mirror, only here you need the 250 mm OD tube. Also, around 120 cm long. The f5 will (barely) fit the 1 meter tube.
Assembling the altitude bearing box
Lets start with the easiest part, the altitude bearing box. Simply, it is a frame that is holding the tube, and the altitude bearings are mounted on it. First, mark your drilling holes. Since the board is 19 mm thick, draw 19 strips on the wider side of the board :
Mark 3 drilling holes per strip, 30 mm from the edges, and one in the middle. Use a 4 mm bit for the holes. You also need to sink in the screw heads. There is a special tool for this... but you can do it with a 10 mm bit, with your hand. Do the same on both boards. The even sided boards do not need drilling.
For assembling the box, right angle clamps would be handy, but not necessary. Use 4x50 mm wood screws, and some carpenters glue in-between the contacting surfaces. Done :)
You can paint the box, and mount the altitude bearings. It is important that the center of the PVC plug is mounted at the center of the even sided square of the box. Usually there is a mould mark at the center of the plug already. Drill a 4 mm hole in the center, and find the center of the even sided boards by drawing diagonal lines, corner to corner. The intersection is the center of the square. To attach the PVC plug, use 4x16 mm wood screws, 3 per plug is enough.
Preparing the sideboards
|First, we need to
draw a line across the middle of the board. This line enables us
to draw the altitude bearing cutout, and later, to align the
mount on top of the altitude bearing. PVC plugs are cheap and
work very nicely as altitude bearings. Either the 125 or 150
diameter plugs will work nicely on both mount sizes.
The alt bearing cutout should be slightly larger than the PVC plug itself, about 3 mm, to provide space for the felt pads, which the PVC plugs ride on. For a 125 mm plug, use 64 mm RADIUS.
|After making the cutout, draw a 19 mm strip , 45 cm long, on the further side of the sideboard (refer to picture above) Again, use a 4 mm drill bit, and sink in the screw heads. But, here you have one left side and one right side board. The bearing cutout is the same, drilling holes too... only sinking in the screws is done on the opposite side. Put both your board on the table, or floor like on the image:|
Now sink in the screw heads , and you will get one right side and a left side board.
azimuth bearing boards (ground boards)
|First we need to find the
centers of the boards. This is easily done by drawing diagonal lines.
The lines are not only to find the center, they will be used to align
the top piece of the mount on the azimuth bearing, so draw them in full
length. In the center, drill a 8 mm hole. It is very important that the
hole is vertical relative to the board, since this is the axis of the
azimuth bearing. A drill press would be nice, but it can be done
freehanded if you are careful. Next, we improvise a large compass.
Assembling the mount
First we need to assemble the top
part of the mount. Two sideboards, and a front board.
Again, use 4x50 mm screws and carpenters glue.
Take one azimuth bearing board, and put the upper part of the mount on top, aligning the marks on the boards you draw earlier.
Draw the outline of the upper part of the mount on the azimuth bearing board. Mark your drilling holes, as per sketch above. You also need to sink in the screws, do it well on this board, because the azimuth bearing will not be able to rotate if the screw heads are sticking out. You should sink in the screws on the OPPOSITE side of the outline drawn. To put everything together, place the upper part of the mount with the altitude cutouts downwards, and put the azimuth bearing on top. Apply some glue, align the azimuth board and screw everything together with 4x50 wood screws.
We have one last
remaining azimuth bearing board.
one side of the board, put 5 felt pads, the same one used on furniture
Put 3 on the outer edge of the disc, 120 deg apart, and 2 near the 8 mm axis hole (green dots on sketch above)
On the other side of the board, attach 3 rubber feet, near the edge, 120 deg apart.
They need to be at least 25 mm tall to provide enough clearance on uneven ground. The upper part and the ground board are rotating around a M8x60 screw that we put trough the center holes.
Use oversized washers on each side, and a self-locking (Nylock©) nut.
The bolt head should face the ground. The tension should be just a little that the boards are connected together.. but not too tight.. so they can rotate freely. This can be adjusted at any time... and will probably be necessary after a couple of months of use, because the felt pads will compress under the weight of the telescope.
|To provide a bearing surface for the
4 more felt pads are needed. (picture on the left)
This is how your brand new, finished Dobsonian mount looks like.
The hole in the front board is optional, this way it is easier to carry the mount.
are handy with the jigsaw, and want your mount to be
lighter and easier to transport, you can cut out the
mount like on the picture below. Also, installing a
handle on the altitude bearing box will make
transporting and setting up your telescope much
Now that we have a nice and stable mount, we can continue building the optical tube assembly (OTA)
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