Some not-too-technical advice before you begin...


This Webpage contains complete instructions for constructing a Sidewalk (Dobsonian) telescope using a six, eight, or ten inch (diameter) purchased objective mirror. If you are interested in grinding and polishing your own mirror, we suggest you pick up John Dobson's video (listed under "Sources" below) as a start. Also, Victor's Telescope Making Page has step by step instructions for mirror making.  All About Telescopes, by Sam Brown (found at Orion Telescope and Binocular Center) also has good instructions on this art.

How much is this going to cost? Well... an 8" f/7 mirror from Coulter with diagonal costs $219.95, plus $15.00 for shipping. If you can build an 8" scope for twice the cost of this, consider yourself lucky! You can buy a "Big Three" scope for only a little more; you can also buy used for less... Something else must be motivating you to "build your own." This Webpage is for you... and your daughter and/or son!

You will need to purchase one objective ("primary") mirror and one diagonal flat, ("secondary) mirror, in order to build the telescope. Mirrors may be purchased from mail-order telescope supply houses. Coulter Optical is an excellent, dependable source for good-quality, inexpensive mirrors, so we have included their address in the "Sources" list below. Orion Telescope and Binocular Center also sell mirrors, as well as alot of other stuff.

REMEMBER! TELESCOPE MIRRORS ARE POWERFUL CONCENTRATORS OF LIGHT.

Sunlight reflected off the face of a telescope mirror can cause BLINDNESS or START A FIRE! Always handle your mirror indoors or in the shade! The telescope described in these plans is for NIGHT USE ONLY. NEVER set up your telescope in a location where it may be reached by sunlight, and:

NEVER LOOK AT THE SUN THROUGH YOUR TELESCOPE!

 

The plywood cutout patterns on the next few pages are for the construction of telescopes with six, eight, and ten inch objective mirrors, but you can use the same design for smaller telescopes (4.5", for example) or larger telescopes with objective mirrors of up to 15" in diameter. Just remember that the tube of your telescope needs to be at least 1-1/2" wider than the diameter of the objective mirror. Then increase (or decrease) the tube box and rocker dimensions proportionately. THE DIAMETER OF YOUR TUBE DETERMINES ALL OTHER DIMENSIONS.

For telescopes with mirror diameters 16" and larger, a different tube box design and mirror support system is necessary. (Again, John Dobson's telescope-making video—listed in "Sources" shows the construction of a 16" telescope with this modified tube box and support system). A more popular method of construction nowadays for large Dobs is the truss design, which allows the telescope to be "broken down" for transport and storage.  See my Vanity Page for examples I have built; recommendations, and resources.

Objective and Diagonal Mirrors

What we describe as a Sidewalk Telescope, or Dobsonian Telescope, is a simple Newtonian reflecting telescope in a sturdy, wooden, alt-azimuth mount or rocker. The telescope consists of a concave (actually parabolic) objective (or Primary) mirror, which is mounted in the bottom of the tube. This objective gathers light from the object under observation and brings the light to a focus; forming an image of the object in what is called the focal plane or image plane, at the upper end of the tube.

A small, flat, front-surface mirror called the diagonal (or secondary) mirror is mounted inside the telescope tube near the front end. This mirror is mounted at a 45 degree angle to the tube's axis—hence its name. It deflects light from the objective to the side of the tube where the image may be more easily examined with an eyepiece.

The size of the diagonal mirror is dependent on the size and focal ratio of the objective mirror. So, when you order your mirrors, make sure to ask your supplier to tell you the correct size diagonal mirror to order. Specify that you will be using a low-profile focuser. To determine more accurately the size of the diagonal, peruse the following email correspondence:

balzaccom@aol.com (Paul Balzac) writes:

>By the way, I tried to find the equation you mentioned in the archives, but
>couldn't. Anyone help?

The equation is found in Richard Berry's Build Your Own Telescope, pgs 26-28. However, there are a couple of errors on those pages: "E" on page 27 should be changed to "D" (this makes more sense with the drawing). Also, in the final example he uses, the "6" and "8" are transposed; switch them around, in other words.

But to cut to the chase, the formula is:

d = df +([D-df]/F) X Lde

Where
"d" is the minor axis of the diagonal,

"df" is the focal length of your primary multiplied by: the result of the amount of fully illuminated field you want divided by 57.3 (radians in a degree). In other words,
F X (x/57.3) where "F" is focal length and "x" is the amount of fully illuminated field you desire. ("df," is, in fact, the amount of fully illuminated field).

"D" is the diameter of your primary,

"F" is the focal length of your primary,

"Lde" is the distance between the diagonal and the field stop of your eyepiece.

A self-serving example: I recieved my 8" f/7.06 mirror from Coulter yesterday. The common rule of thumb is to have a half (.5) degree of "fully illuminated field" for visual use. (But I will also plug in a .25 fully illuminated field, just to see how much smaller my diagonal will be...). The telescope will use a 10.5" outside diameter Sonotube, have a low profile focuser (say 2.125 inches high), and I will add 3/4 of an inch to be sure all my eyepieces will focus with
a variety of eyeballs: So my "Lde" will be: 8.125 inches: 5.25 (radius of 10" tube) + 2.125 + .75.

"df" is then,
for a .5 degree fully illuminated field: 56.5 X (.5/57.3) = .493
for a .25degree fully illuminated field: 56.5 X (.25/57.3) = .247

So lets plug these numbers in:

The formula, again is:
d = df +([D-df]/F) X Lde (be sure to multiply BEFORE you add)

.493 + .133 X 8.125 = 1.57 inches. So, a 1.57" minor axis diagonal will fully illuminate a half a degree at the eyepiece.

.247 + .137 X 8.125 = 1.36 inches. So, if I want only a .25 degree fully illuminated field to produce more contrast on the planets... I would go with a diagonal this size.

Diagonal mirrors do not come in the above sizes, of course; but one can round off--in either direction--your preference!

                       --Ray

 

A Word About Focal Length and Focal Ratio

The focal ratio of the mirror you select determines how long your telescope will be. A 10" objective mirror with an f/7 focal ratio will give you a telescope with a 70" focal length. (Multiply the "f-number" by the diameter of the objective mirror to get the focal length.) Your tube will need to be cut to the length of the focal length, so you would have a 70" long tube. An 8" objective mirror with an f/7 focal ratio would have a 56" focal length, and a 56" long tube.

(John Dobson recommends a focal ratio around f/6 or f/7)

FOCAL RATIO (f-number) x MIRROR DIAMETER = FOCAL LENGTH = LENGTH OF TUBE

When you get your mirror, the focal ratio may be exactly what you ordered, or it may be a little more, or a little less. So don't cut your tube till you receive your mirror. To measure your focal length exactly, have a friend help you: Take your mirror, a tape measure, and a piece of paper outside on any clear night and catch the light of a bright star or the Moon with your mirror and reflect it back in that direction. Using that piece of paper find where the star, or Moon forms the smallest image. Measure that distance as accurately as you can. (Instead of a piece of paper, it is often easier to reflect onto a fixed surface, such as a garage door jamb or header). Write this measurement down! This determines the length of your telescope tube, as well as where you cut a hole for your focuser. The key thing to remember is that you want this formed image (called the focal plane), to hover in the same plane as the field stop of your eyepiece. If you opt for a commercial focuser, you will undoubtedly have to cut your focuser hole in a different place than these plans call for! Do you have your eyepiece(s) yet? If that's a "yes," good: Look into your eyepiece and put your pinky finger in the other end--slowly and carefully--can you see where your finger comes into a magnified focus? Usually there is a black ring (called a field stop) at this point around the inside of the eyepiece; and usually this corresponds to where--on the outside of the eyepiece--the chrome barrel ends and the rest of the eyepiece body begins. This means this is where the eyepiece "bottoms out" when inserted into a commercial focuser. But, you don't want your commercial focuser to bottom out when focusing! Individual eyes and eyepieces are different! Always allow at least 3/4" "in travel" for your commercial focuser, when doing the arithmetic to determine where to cut your focuser hole! More is said on this subject in "Section A" of these plans online.

TUBE DIAMETER

The telescope tube should be about 2 inches wider in diameter than your objective:

A ten-inch diameter objective mirror requires a twelve-inch diameter tube.

An eight-inch diameter objective mirror requires a ten-inch diameter tube.

A six-inch diameter objective mirror requires an eight inch diameter tube.

 

Materials List

 

Tools Needed


SOURCES:

Sources for "Ready Made" Telescope Mirrors and Telescope Eyepieces:

1781 Primrose Ln.

W. Palm Beach, FL

We recommend Coulter's mirrors: quality products at very reasonable prices. Call or write for catalog and price list.

 

2540 - 17th Avenue, P. O. Box 1158

Santa Cruz, CA 95061-1158

In California (800) 443-1001

Outside California (800) 447-1001

Call toll free number for free color catalog.

 

P. O. Box 224 Main St.

Maple Shade, NJ 08052-0224

 

 

Other Accessories, and Miscellaneous Parts:

 

 

If you want to make your own mirror, may we suggest:


Onwards to: Six-inch Scope Overview with Plywood Cut Pattern

Onwards to: Eight-inch Scope Overview with Plywood Cut Pattern

Onwards to: Ten-inch Scope Overview with Plywood Cut Pattern

Back to: Homepage, Table of Contents

E-mail: Ray Cash