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.
We use this free web application to
calculate tube dimensions and component placement, and to check
for vignetting. Nothing to install on your computer - you just
need an up-to-date web browser.
Dobsonian Bearing
Materials A discussion of why certain materials are
used for Dobsonian Bearings, mechanical issues for Dob bearings,
and where to obtain these materials.
Recommended Books on Building Dobsonians
Here are books we learned from, and you might find useful for
additional ideas and techniques.
Build
Your Own Telescope
- Richard Berry -
[Berry94] The classic small Dobsonian construction book, by the
person who was a major force in popularizing this design in
the 1980's.
The
Dobsonian Telescope
- David Kriege & Richard Berry -
[Kriege97] Most of this book focuses on large aperture truss tube
design, but there is an excellent section on building and
8-inch Dobsonian also.
A Newtonian Optical Tube Assembly
Shown resting on a rusting hay mower, our completed 6-inch
f/7.5 Newtonain Optical Tube Assembly. Optional tube rings
and "red-dot" finder are shown. A nicely painted concreet
form tube can look pretty good!
