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How to Make a Small Dobsonian Telescope (4 1/4)

In this series of tutorials I show you how to make a 4 1/4 inch reflecting telescope

There are of course lots of ways to make a telescope. And it can get quite in-depth. I am going to take a middle of the road approach to this. We will make a pretty standard type of telescope and we are not going to grind out the mirror ourselves. I have bought a little kit that comes with both the primary and secondary mirrors.

This approach makes the whole process easier yet it still is a lot of fun. You can grind out the mirror yourself but that is quite a few hours of work. And it is a precision task so there is a lot of testing and regrinding involved.

 

First let's take a look at the concepts of the telescope so we understand how we are making it and why. The following picture shows us the tube assembly of a newtonian telescope. It is actually a pretty simple thing. I will show you the major parts and explain what we need to do.

Note: This telescope setup is often abbreviated by calling it the OTA or Optical Tube Assembly

An illustration of the telescope tube assembly

The Tube - I haven't labeled the tube in the drawing but it is pretty easy to see. It is the overall housing for all the parts. Often times the tube is a cylindrical tube. But it doesn't have to be. It can be rectangular or octagonal.

Three types of telescope tubes

This picture shows the three most common telescope tube types. Keep this in mind if you are making a telescope. You can go with any of these types. They are from top to bottom the round tube (most common), the square tube, and the hexagonal tube. The hexagonal tube is kind of a compromise between the circle and square tubes.

The easiest to make is definitely the square tube. Unless you buy a round tube.

 

The Primary Mirror and Mirror Mount - The primary mirror is the main mirror. That is the one that gathers all the light. And the bigger this mirror the more light that is gathered. There are two important things to think about when it comes to this mirror.

1. You want to cover as little of the surface as possible. More of the mirror that is open the better.

The mirror clampsWe typically do this by clamping the mirror with three small clamps that just overlap the surface of the mirror a little bit. This way the mirror is held nice and tight yet only a small amount of the surface is blocked. It's important that the mirror is clamped in tight because the telescope moves a lot. The mirror could be lying flat or be vertical depending on what part of the sky we are looking at.

 

Illustration of the mirror mountThe mirror is attached to a mounting system and this system is important.

The mirror is a very finely created optical instrument and it will have to be adjusted within your telescope tube assembly. This is done occasionally or before you use the scope. The mount is made so that you just adjust three screws. These three screws change the angle of the mirror. Typically there are three bolts, three wing nuts and three springs. This process of adjusting the mirror is called collimating the telescope. It's an easy process and we will talk more about this. All you have to know for now is that we need to make a mirror mount that makes collimating the mirror easy.

The Secondary Mirror and Mount

Illustration of the secondary mirror mount The purpose of the secondary mirror mount is to hold the mirror in just the right spot in the center of the tube so the light is bounced at a 90 degree angle from the primary mirror to the eyepiece.

There are a couple of ways we can do this. We can use a vane of four metal legs as shown at the top of the illustration. Or we can use a thicker single leg mount as shown in the bottom illustration.

The single leg mount is probably easier but the top vane setup is a little better. It blocks less light and it is easier to adjust. You can turn the four screws to move the location of the mirror. With the single vane setup it is difficult to move the mirror.

Typically we don't have to tinker with this mirror too much. If so, then it is just another part of the collimation process.

 

I also have a tutorial where I take you through the process of actually making a 4.25 inch newtonian telescope. That tutorial with an included video is here:

New Project . I have started the 4 1/4" reflector telescope project. I made a temporary tube assembly to test it all out. Worked great. And now I have completed the tube assembly. You can follow along with that project here: How to make a Newtonian Reflector Telescope

 

 

 

 

Resources: Dale Keller's Newt Software page for calculating the size of your newtonian telescope

 

 

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