Build your own Galilean telescope
Follow our DIY project and make your own telescope just like the one used by Galileo Galilei during his most famous astronomical discoveries.
This project is a homemade, working replica of a very famous instrument. In 1609 Galileo designed and built his first telescope, and we thought it would be interesting to try and make our own telescope based on this instrument.
It’s not intended to be an exact replica, but we’ve used similar construction techniques and chosen lenses with similar specifications, so our views should be approximate to those seen by Galileo over 4 hundred years ago.
- Read an extract from the book A Conversation With Galileo.
- Download an Annotated Galilean telescope plan (PDF)
What kind of telescope did Galileo use?
Galileo’s telescope could magnify approximately 8x, but he quickly improved it to magnify objects nearly 21x.
The aperture was small and the field of view was very restricted, but despite these limitations, in the following year Galileo published some exciting celestial discoveries in Sidereus Nuncius.
These included craters on the Moon, the phases of Venus and three of the largest moons of Jupiter (which would eventually become known as the Galilean moons).
Galileo also observed the unusual shape of Saturn, but was not able to interpret that this was due to the presence of Saturn’s rings.
Our Galilean telescope consists of a main tube with separate housings at either end for the objective lens and the eyepiece. These can slide in and out to adjust the focus (longer for close objects).
The tube is formed by long, thin strips of wood joined together with a resin and is covered with red leather decorated in gold. In our telescope we have used similar strips of wood and filled the gaps with wood filler.
After rubbing down, we used a sticky-backed vinyl with a leather effect as a finish (leatherette) and decorated it with gold paint.
The original objective lens is a plano-convex type (flat one side with a bulge on the other), with the convex side facing outwards. Its specifications include a diameter of 37mm, an aperture of 15mm and a focal length of 980mm.
For our project we found a very reasonably priced 50mm-diameter double convex lens with a focal length of 1,000mm, and we reduced the aperture to 15mm with a ‘lens ring’ to replicate the original.
Galileo’s original eyepiece was lost and was replaced in the 19th century by a biconcave eyepiece (curved inwards on both sides); with a diameter of 22mm and a focal length of –47.5mm (a diverging lens).
We chose a biconcave lens with a 50mm diameter (again, we reduced the aperture) and a focal length of –50mm.
This results in a magnification of 20x; the original telescope was 20.6x so we are very close, although our tube is about 25mm longer.
If you can’t find these exact lenses, then you can adapt the design to work with other focal lengths by following the steps below.
It’s also worth noting that biconcave lenses will always have a negative focal length, however many suppliers do not include the minus sign in their listings.
We mounted our finished scope on a simple tripod to test it, and although it is tricky to point it without a finderscope, we eventually managed to observe some features on the Moon and also saw a coloured (but blurry) disc of Mars.
Throughout the project, it was awe-inspiring to consider how so much pioneering work was achieved with such an instrument.
Should you wish to see it for yourself, Galileo’s original telescope resides in the Museo Galileo in Florence, Italy.
For more projects like this, visit our DIY Astronomy page.
Images to help with your build
Build your own Galilean telescope, step-by-step
You Will Need
- Coping saw
- Small handsaw
- Craft knife
- Safety mat
- Coarse file
- 3mm x 6mm x 900mm wood (30)
- 1m-length of 32mm (1.25-inch) PVC waste pipe
- Leatherette to cover tubes
- Educational-grade lens (50mm diameter with a focal length of 1,000mm)
- Educational-grade lens (50mm diameter with a focal length of –50mm)
- Wood filler, two-part, resin-type
- Wood glue
- Masking tape
- A4 black card (3)
- Small piece of 3mm plywood (approximately A4-size) or thick card
- Matt black spray paint for inside of tubes
- Red spray
- Gold paint
Calculate the length of your main tube by adding the focal lengths of the lenses together (remember, one is negative!) and subtracting 100mm (ours was 1,000mm – 50mm – 100mm = 850mm). Use a fine-toothed saw to cut the sticks to length.
Tape sticks together and apply glue to the surface. Wrap your PVC tube in an offcut of leatherette (don’t remove the sticky back) so its diameter is a bit larger, and wrap black card round it. Wrap the glued sticks round the black card layer and hold with tape.
When the glue has dried, fill all the gaps with wood filler. When this has set, file down and sand any raised areas. Sand the ends before pushing out the PVC tube and leatherette, leaving a neat, black card lining; the tube should be strong and rigid.
Use a similar process to make the two lens holder tubes; there are two plywood rings which are glued to the PVC tube. Make sure there is space for the lens and a plywood ‘lens ring’ in the recess. Sticks and filler are used as before.
After spraying the insides and ends of the tubes matt black, add pieces of leatherette to decorate. Check the lens tubes slide inside the main tube. We masked off some sections and sprayed them red and made a rubber stamp to print on gold decoration.
When everything is dry you can install the lenses. We used a very small dab of hot glue to hold them in and then pushed in the leatherette-covered lens rings. Once completed, you will need to experiment by sliding the eyepiece holder in and out to focus.