Build a Dobsonian Telescope

Introduction

Our Dobsonian sits in the garden at dusk cooling down for the nights observing. The yellow Optical Tube Assembly is a 6-inch f/7.5 Newtonian. The plywood Dobsonian mount is simple to build and very functional. Our 6" f/7.5 Newtonian-Dobsonian

In this section we will show you how to build a complete Dobsonian Telescope. The sample we will build is a 6-inch f/7.5, however the plans can be scaled down to 4-inch scopes and up to 10- or 12-inch scopes of various focal ratios. At about the 12-inch size, it is more practical to switch to a truss-tube design, which we will not cover at this time (see [Kriege97] for very good plans on building large aperture truss-tube Dobsonians).

There are many ways to build a Dobsonian telescope, with many design decisions to be made. Our goal here is to guide you through building a basic Dobsonian that will function well, and our choices reflect that. Other plans may make other perfectly valid choices, or may have different design goals.

A Dobsonian Telescope is a optically a Newtonian Reflector mounted on a Alt-Az mount with a low and stable center of gravity and Teflon-Laminate bearings. It was popularized by John Dobson of the San Francisco Sidewalk Astronomers, who is also an honorary member of the Springfield Telescope Makers. John does not like or encourage the term Dobsonian, but his wishes have not been followed and everyone uses the term. He reminds people that cannons have been mounted this way for years.

Motivation & Results In August 2010, we held the 75th Stellafane Convention, and our president, Jeff Lowe, urged all of the Springfield Telescope Makers to bring one of their home made telescopes to Breezy Hill to honor our traditions. I have been meaning to put Dobsonian plans up on our web site for several years, and the 75th Convention has moved me to action. So on a snowy day in February not too far from Stellafane, I started this project, and had both the scope and these pages finished before convention. The scope we show on these web pages was proudly displayed on Breezy Hill by the Stellafane Clubhouse in August 2010 at the convention. The scope was entered in the optical and mechanical competitions. It won an Innovative Component Award for the adjustable cradle design. It just missed placing in the optical competition. The optical judges told me on their second visit I had the best collimated scope and the easiest to point and hold Dobsonian mount on the filed that night - so if you have any doubts about the curved vane spider or mount design, rest assured they work well. Ken Slater, Webmaster

2010-Aug-10

We break the project into two major pieces: the OTA and the Mount, and each piece is to some extent a separate project. The Newtonian OTA will be built in the classic Dobsonian-style cardboard concrete form tube, and while we plan to mate it to our Dobsonian Mount, it could certainly be attached to other types of mounts if desired by making a suitable set of tube rings or a tube cradle. Similarly, the Dobsonian mount we describe could be sized to fit an existing OTA, so if you have a telescope tube and want to mount it as a Dobsonian, you can just start at our mount project.

Optics: The mirror we will use in our sample scope was made at the Stellafane Mirror Class by the author, and we certainly encourage people to make their own telescope optics. However this project can use any suitable mirror you have, be it made by you, picked up on eBay or our at swap tables, or bought from a commercial supplier. Whatever source that works for you will work in this telescope.

Components: In most cases, when parts like spiders, diagonal holders, mirror cells and focusers are required, we will tell you how to build your own, or if you choose, what purchase if you don't want to make that component. You can mix and match purchased and home made components to suit you desires and budget. We will purchase a diagonal mirror, and we plan to use a commercially produced focuser. Everything else we will build ourselves, and we will show you a simple focuser you can build out of plumbing parts.

Tools: For this project, we will strive to give you complete instructions to complete this project with basic woodworking hand tools. However, we do expect you have an electric drill. We will also describe how power hand tools might make the job faster, or simpler, or in a few cases provide more accuracy. Having a power jig saw, circular saw, router or power sander can help or be more convenient, but won't be necessary to complete this project.

Craftsmanship: Telescopes can be works of art , or they can have a rough but functional appearance. We will strive to produce something in the middle: Our Dobsonian will be neat and well finished, but not a work of art. Of course you many choose to move up or down this scale as your skills and interests dictate.

Order of Construction: We have arranged the the steps in a logical progression below, and that is how we will proceed to built this telescope. There are dependencies: for instance you need to know the diameter and focal length of your mirror before you can size the tube, and you need to know the size and balance point of your tube before you can size your mount. To avoid rework and scrap, please proceed in the order we present the plans.