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. Tony Gondola of  BigEye Optics makes custom mirrors, too.

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:

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 outside diameter than your objective. Therefore:

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

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

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

A 4.5 inch diameter objective mirror requires a 6.5 inch outside diameter tube.

Do not fret if these outside tube diameters are not exactly 2" bigger than your objective mirror--it is best, however, to err on the large side.

Materials List

• Cardboard tube ("Sonotube") (1):Construction, specifically concrete construction supply houses usually carry these tubes, which are used for forming concrete columns. Get the supply house to cut your tube rough, that is, longer than you need by, let's say, six inches or so. To "finish cut" your tube square: Tape several 8-1/2" x 11" pieces of paper together end for end--enough to wrap around the circumference of the tube, and do just that... Make the ends come together squarely; and mark the edge you want to cut. Proceed with a hand saw or Jigsaw...
• Exterior--or interior (smoother) grade plywood: 4' x 8' x 3/4" thick (or 1/2" thick in you are making a 4.5" scope). For an eight or ten inch telescope, one sheet will be plenty--six and 4.5" scopes use less than a half a sheet--you might be able to scrounge up scrap from your friendly local cabinetmaker. An alternative to "exterior grade plywood" would be "shop grade"; not much more expensive, a MUCH smoother finish is possible. Pre-finished plywood is fine, too.
• (Optional) Six-Eight feet of Douglas Fir 2"X 2": Cut these into small lengths and glue to inside of Rocker Box and Tube Box corners--this will strengthen these joints considerably.
• Paint and painting supplies: Flat black  for inside the tube; any dark color is fine for the outside of the tube. White is not recommended--it takes longer for a white tube to cool down to ambient (outside temperature).
• Sheet Metal Screws: Panhead, size #8, 3/4" long. Get at least a dozen.
• Nails or Screws: Assorted sizes. Hot-dipped galvanized box nails work well. Though the plans consistently refer to nailing the various parts of our scope together, using screws instead is highly recommended. 1-1/2" flat head wood screws work great. Be sure to pre-drill and countersink before driving the screws.
• Machine Bolts (3): Three bolts, 1" long; 3/8" in diameter.
• Lag Screw with matching washer (1): One lag screw, 3" long; 1/2" in diameter. (2" long if making a 4.5" scope).
• Record (1): One phonograph record—33-1/3 LP rpm size (A "used" record is fine.) Or visit your local cabinetmaker for some free "scrap" Plastic Laminate ("Formica" is a brand of Plastic Laminate)--you won't need much--just enough to cover the bottom of your Rocker Box (see   Section "C" of these plans) and line the outside edges of your Altitude Bearings. Do not use "gloss" Plastic Laminate, however--just the rougher textured stuff. You may also want to search the internet (''google") for  "kits" of  Plastic Laminate and Teflon from enterprising--though often elusive--amateur telescope makers.
• Chrome-plated Brass Tubing: Washbasin drainline trap—1-1/2" outside diameter: We 'll need two pieces: one about 1-1/2" long, for the eyepiece holder, and one about 6" long, for the aligning tube. (Available from a plumber's scrap bin.)
• Cedar Shim Shingles: Three pieces, about 1-1/2" to 2" wide. Shingles break easily, so it's a good idea to keep a few extra shingles on hand. They are most often sold in packages.
• Wooden Dowel: One piece, about 3" long. Usually sold as "closet pole" or "hand rail stock"— Approximately 1-3/4" in diameter.
• Cardboard "Mailing" Tube: One piece, 1-1/2" inside diameter, about 2" long (Grocery stores have this tube in the produce department—used for dispensing plastic bags.)
• Thumbtacks (3)
• Leather Scrap: Three small pieces—about 1/2" square. Old belt leather works fine.
• Sticker or Decal (1): About 1/2" in diameter. I like to use "hole reinforcements" stickers, for three-ring notebook paper. A "gold star" also works well. Visit your stationary store, your teacher, or raid your "junk drawer."
• Masonite: One rectangle of 1/8" thick Masonite board about 3" x 4" (1/4" thick is also O.K.) with a 1-1/2" hole drilled in the center; and three pieces about 1" square.
• Teflon: 7 pieces, approximately 1" x 1" square, and 1/4" thick. Three pieces will be used for the lower Rocker Box bearings and four for the Cradle Board bearings. Try a local electronics surplus house; otherwise "google" for it. Teflon is sometimes hard to find (and more expensive than you may think)--do not accept anything else! Teflon is the key ingredient for SMOOTH movements!
• Furring nails (4) If you can't find furring nails, don't fret; I like to use rubber furniture glides (the kind you just nail in--this serves the same purpose as the furring nails: namely preventing our primary mirror from falling forward.
• Glue: White glue works fine. In addition, I like to use 100% black silicone glue on selected parts (like focuser construction and diagonal mirror to diagonal holder adhesion.
• Telescope Objective Mirror (1): See above "A word concerning focal ratio.." and "Sources" below.
• Cardboard: The back of a cardboard breakfast cereal box works nicely.
• Telescope Diagonal Mirror (1): Order when primary f/ratio is decided upon.
• Eyepiece (1): Eyepieces may be purchased from telescope supply houses (see "Sources"), or you can salvage one out of an old pair of 7 x 35 binoculars (binoculars should be labeled "fully coated optics").
• Optional Modifications: Commercial focusers, primary mirror cells, diagonal holders and spiders, one-power finders, additional eyepieces. . . should be factored into your plans, or can be later added as "upgrades" as you see fit.

Tools Needed

• Hammer
• Saw (Table Saw, and/or Jigsaw is/are helpful but not essential).
• Electric Drill and 1/4", 7/16",1/2",and 3/32" Drill Bits, in particular.
• Tape Measure
• Compass
• Screwdrivers
• Nail Set
• Crescent (adjustable end) Wrench
• Awl
• Hole Cutter or 1-1/2" diameter doorhandle drill bit.
• Carpenter's Framing Square (helpful but not essential); Combination Square
• Pencil!

SOURCES:

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

• Coulter Optical, Inc -- now part of: Murnaghan Instruments

          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.

• BigEyeOptics  Tony Gondola makes custom optics.

• You may also check the "previously owned" market at: http://www.astromart.com/.

Other Accessories, and Miscellaneous Parts:

• The ATM Resource List. The definitive, up-to-date list for the Amateur Telescope Maker. If you can't find it here, I can't help you!

• Never forget to use www.google.com or some such search engine when looking to find parts!

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

• John Dobson's Telescope-Building Video

  This 90 minute, full color video is John Dobson's personal guide to making astronomical telescopes—8 inch to 16 inch apertures and larger. Especially strong in the mirror making department and for large (read 16" Dobsonians). Free color flier available on request.

  Price per tape: $39.95

  Shipping per tape: $3.50

  CA residents add $3.40 sales tax.

  Total per tape (except CA) $43.45

  Total per tape (CA only) $46.85

  Make check or money order payable to: Dobson Astro Initiatives

  Remember to include your shipping address!

  Mail to: Dobson Astro Initiatives

  P. O. Box 460915

  San Francisco, CA 94146-0915

   

  Victor's Telescope Making Page.

  •  

  Sources for Mirror Kits and other Mirror Making Supplies:

   

  Willmann-Bell; P.O. Box 35025; Richmond, VA 23235

  (804) 320-7016

  $1 Catalog

   

  Newport Glass Works, LTD; 2044-D Placienta Ave; Costa Mesa, CA 92627

  (714) 642-9980

   

  • If you can't print--for whatever reason--from these pages, I suggest you contact the Los Angeles Sidewalk Astronomers: last I heard, they are still sending out hard copies for $2.00 from this address:

    The Sidewalk Astronomers
    1946 Vedanta Place
    Hollywood, CA. 90068

    If you are requesting these plans be mailed to another country, the price may be higher.

    The plans on this Webpage are much improved from the ones the LA folks send out--I recommend you try to print the plans from these pages.

    I do not send any plans through the mail, nor do I have any control over those that do (concerning promptness--or anything else, that is)!

     

    To join The Sidewalk Astronomers and receive our quarterly newsletter, send $15 to:

    The Sidewalk Astronomers

    1946 Vedanta Place

    Hollywood, CA 90068

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  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