Overview
In part one we covered
how to build John Dobson's low-cost, non-equatorial plywood telescope mount. Dobson's clever invention makes it possible for astronomers on a budget to build some pretty spectacular homemade reflector
telescopes.
Instead of spending as much or more on the mount, you can put your money into the optics of your scope. A fancy clock-driven equatorial mount is all well and good, but the cost of one can force you to buy a scope that's smaller than you really wanted - and who doesn't want a giant instrument?
Ahem.......
Dobson's invention lets us spend our cash on stuff for the biggest, light-sucking telescope you ever saw. To go with
the mount in part 1 of this series, we will give measurements for a 10-inch telescope of the relatively simple Newtonian design.
Materials
 |
| Sonotubes painted black |
- 12 inch Sonotube 8 feet long (available at a concrete supply
store)
- 10 inch primary telescope mirror
- 10 inch mirror mounting cell
- 10 inch Spider mount for the secondary mirror
- Secondary mirror
- 2 inch focuser
- Eyepiece
Tools:
- Saber Saw
- Drill and bits
- Hole saw
- Screwdrivers
- 1-inch screws, 1 box
- Wood glue
Building the Scope:
 |
| Mounted cell and eyepiece |
Step 1
First we mount the mirror onto the mirror mounting cell. It should come with instructions unless you've cannibalized a mirror from an old telescope and then, it's probably already mounted on the cell. Follow the instructions that come with the mounting cell. Test fit the mirror mounting cell in the lower end of the sonotube and mark the tube where the screw holes go for the mount. Drill the holes, fit the mounted mirror into the end of the tube with the mirror facing inward.
Mount the telescope mirror on the mirror-mounting cell
following instructions that came with the cell.
Drill holes in the lower end of the Sonotube to match the mounting
screws and screw the mount into the end of the tube. Use flat washers to protect the integrity of the tube around the holes.
Step 2
 |
| Focuser |
Check your mirror specifications (comes with the mirror). Subtract 6 inches plus the length of the focuser from the
focal length of the mirror. Measure from the center of the
mirror and mark the side of the Sonotube at that distance. The six inches is for the radius of the tube. Added to the length of the fully partially retracted focuser (right) it shows you where the secondary mirror needs to be in order to put the focal point of the mirror within the eyepiece when it's mounted in the focuser. When you've marked the spot, then measure an additional 4 inches and cut off the rest of
the Sonotube at that point. Be careful to hold the sonotube
upside down with the mirror on top while cutting. That way you don't get dust on the primary mirror.
Step 3
Drill a 2-inch hole in the side of the tube with the hole saw. the center of the hole will be where marked
the adjusted focal length. Don't forget to hold the sonotube upside down to carry off the dust. When you are finished wipe the inside of the tube with a damp cloth to get up any extra dust particles. Dust is the enemy of the astronomer.
Step 4
Next you'll mount the
secondary mirror in the spider mount (right shown assembled). I bought my already assembled. You can hand build them, but that's a whole other blog.
Mount the spider across the open upper end of the tube so that the
secondary mirror is directly below the 2-inch hole with its center exactly at the adjusted focal length (focal length minus 6 inches plus half the length of the focuser half extended. Once I figure out where the legs of the secondary spider will attach to the side of the sonotube, I drill holes for the screws. Then I cut a bit above and below the holes in line with the tube so it makes a slot so that I can adjust the legs of the secondary slightly. You'll need that later he you collimate the mirror, secondary and eyepiece. Also, this spider doesn't show it, but I use flat washers to protect the integrity of the tube around the drilled and enlarged holes.
Step 5
Screw the
focuser assembly into the tube directly over the 2
inch hole. You'll need to collimate or align the three main optical
elements - the primary mirror, the secondary mirror and the eyepiece.
Again, that's a whole other blog, but when the three elements are
mounted, it will look like this:
 |
| Alignment of the primary, secondary and eyepiece. |
Step 6
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| Mirror cell mount and adjusting screws. |
The body of the focuser will align the eyepiece. When you remove the eyepiece and look down through the focuser, you should be able to see an image of the primary mirror in the secondary mirror. If you don't see the primary and the shape of the open end of the scope centered there, adjust the secondary's mounting screws up or down to align the image. Once it looks right in the focuser hole, the image will center in the eyepiece. If you did your measurements correctly, the focus point should be findable in your eyepiece by adjusting the focuser up or down. If the primary is out of whack, you'll have to twiddle with the collimating screws on the bottom of the mirror cell mount. Don't force the screws. Let off very gently on the screws on the side you need to tilt the image toward and very very gently tighten the screws on the opposite side. Do this in very small increments. If you tighten a screw too tightly, you can crack your very expensive mirror.
Step 7
 |
Handy right angle
finder scope
shown without mount. |
Mount the finder scope. You want to put it about a quarter of the way around the tube from the
focuser. Far enough that you don't bump into it with your head, but no so far that looking through it requires you to circumnavigate the telescope every time you move the scope to reacquire the image. With Dobs, you do have to move the scope by hand, so you want to be able to switch easily between the finder scope and the eyepiece.
Step 8
Calibrate the finder scope. It may have instructions, but if not, here's how I do it. Note, I'm giving you this before you mount the scope on the Dobson mount. Really it's kind of pointless to adjust the finder till the telescope is mounted, but it's kind of a logical step in setting up the optics so here goes.
In daylight, point the telescope at a distant building or object (Mt. Ranier works up here in my neck of the woods because you can see it from everywhere). Adjust the telescope so the object is in the center. Look through the finder scope and see where the object is. There are adjustment screws on the side of the finder scope mount that can be tightened or loosened. Carefully so as not to move the telescope off target, adjust the mount screws on the finder scope till the object is centered on the cross-hairs of the finder. The telescope image should be centered on the exact spot. That way, when you are looking for something at night, you can push the tube around till the finder scope is lined up on what you see and then the same thing will be in the eyepiece of the telescope.
Summary:
That's pretty much it. It's really not hard except for getting your measurements right. Having that focal length right is the key. If for some reason you don't have the focal length for your primary, try the method for calculating the focal length described at this link.
Reference:
Mother Earth News: A Homemade Telescope
Larry Brown: Homemade Astronomy
Scopemaking: Plans
for a Homemade Dobsonian Telescope
Howdy Ya Dewit: A
Homemade Telescope: A Quick Run-Through
