How to Build a 20 Inch Dobsonian Telescope: A Step-by-Step Guide for Beginners

Updated November 2026

Building a 20 inch Dobsonian telescope means assembling a large Newtonian reflector on a simple altazimuth mount that you can carry, aim, and use under a dark sky. The finished scope gathers far more light than most commercial models, so faint galaxies and nebulae become visible from a backyard. This guide covers the main decisions, the build steps, and the mistakes that trip up first-time builders.

Plan the Optics and Overall Size

Start with the primary mirror, because everything else depends on its focal ratio. A 20 inch mirror at f/4.5 has a focal length of about 90 inches, which makes a tube roughly 95 to 100 inches long once the secondary and focuser are included. Many builders choose f/4 or f/4.5 to keep the tube shorter, though a faster mirror is less forgiving of collimation errors.

Shopping for one? See our guide: 5 Best 10 Inch Dobsonian Telescope Picks for Beginners (2026).

Consider how you will transport the scope. A tube near 8 feet tall needs a truss or a collapsible design, and you should measure your car, garage, and doorways before cutting any material.

Build the Tube and Mirror Cell

1. Cut the tube sections or truss poles to length, then square each end so the mirror cell and top ring sit level.
2. Build the primary mirror cell with three or more adjustable collimation bolts so you can tilt the mirror precisely.
3. Mount the mirror in its cell using soft pads at three points, keeping the glass free of pressure from the edges.
4. Install the top end with the secondary mirror holder, aligned on the tube axis.

Dimensional accuracy matters more than cosmetic finish. If the tube bends or the mirror cell twists, the image will suffer no matter how good the optics are.

Construct the Rocker Box and Altazimuth Base

The rocker box holds the tube and lets it move up and down, while the base lets it swivel side to side. Use plywood at least 3/4 inch thick for the box, and add bracing so the walls do not flex when the tube shifts. Place Teflon or plastic bearing pads on the box sides and the base so movement stays smooth.

Balance the Tube

Balance is critical on a large scope. Adjust the tube so it stays at any angle without drifting, using counterweights or by moving the tube within its trunnions. A poorly balanced scope is tiring to use and can slip when you let go.

Test, Collimate, and Use Your Telescope

Once assembled, check that the focuser slides smoothly and that the secondary mirror centers in the focuser’s view. Collimate the mirrors using a laser collimator or a star test, adjusting the primary’s bolts until the view is symmetrical. Test on a bright star first, then move to fainter targets when the optics settle.

Common Mistakes

• Underestimating the tube length, which can make the scope too tall to fit your storage space.
• Skipping bracing on the rocker box, causing wobble and blurry views.
• Pinching the primary mirror in its cell, which can distort the optics.
• Ignoring balance, which makes fine aiming frustrating.
• Building before reading a full plan, leading to mismatched parts and wasted material.

Recommended Gear

• Laser collimator helps you align the mirrors quickly, which matters on a large reflector. View on Amazon
• Dobsonian bearing pads give the rocker box smooth movement and reduce friction-related jerkiness. View on Amazon

FAQ

Do I need woodworking experience to build a 20 inch Dobsonian?

Basic woodworking skills help, especially for square cuts and solid joints. Detailed plans make the process far more manageable for beginners.

Is a 20 inch Dobsonian too large for families?

It is heavy and tall, so it suits households with space and at least two adults for moving it. Smaller scopes are easier for younger children to handle.

How long does a build like this take?

Timing varies widely with skill and tools, but many builders spread the work over several weekends. Plan for mirror handling and collimation as separate, careful stages.

Conclusion

A well-built 20 inch Dobsonian rewards patience with a powerful view of deep-sky objects. Focus on accurate dimensions, solid bracing, and proper balance, and the scope will serve your family for many years. If you want a simpler starting point first, see Build a Dobsonian Telescope: 5 Best Picks Compared for Beginners (2026).

Related reading

• Telescope 8 Inch Dobsonian: How to Choose and Use One (7 Steps)
• 8 Inch Dobsonian Telescope Price: What to Expect and How to Save (2026)
• Used 8 Inch Dobsonian Telescope: 7 Best Picks Compared (2026)

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