How do I Design a Mirror Cell?

The mirror cell will help you see into the night sky with a telescope.

The telescope was first aimed at the sky above in the 1600s by Galileo, who saw the moon and sketched its phases. Over time, telescopes became more sophisticated as mankind's knowledge of manipulating glass grew, making it possible to see farther and farther. When you build your own telescope, one of the key parts is the mirror cell, which helps you see the images viewed through the telescope. The mirror cell must be a steady platform that remains still no matter in which direction you move your telescope. The mirror cell design must also take into account this imperative: movement of the glass due to variations in temperature. If the glass cracks or is too tightly mounted, its accuracy is lessened.

Things You'll Need

• 1/8 inch Masonite
• Mirror
• Pencil
• Rotary cutter
• 1/2 inch thick wood
• Ruler
• Drill and bit
• 1/4 inch diameter threaded brass rod
• Hack saw
• Super glue
• Three .3 inch OD (outside diameter) springs
• Three nuts
• Electrical tape

Instructions

1. Lay the mirror on a piece of Masonite and trace around it with a pencil. Cut out the circle with a rotary tool.

2. Lay the Masonite circle on a piece of 1/2 inch wood and trace around it with a pencil. Use a ruler to draw an equilateral triangle (one on which all the angles are equal). Cut out the triangle you have drawn. When you place the circle on the triangle, the angles of the triangle should just extend beyond the circle.

3. Drill three holes all the way through the corners of the triangles. Space the holes 120 degrees apart, ensuring they are placed evenly on the triangle. This means that the mirror will be supported evenly, which will help it to be more accurate. The holes should be larger than the brass rod so that the threads are unobstructed. You can check the diameter of the hole by inserting the brass rod and withdrawing it. If the rod passes through the hole without getting hung up, then you can go on to the next step. If the rod catches on the wood, then enlarge the size of the hole by drilling it with a larger drill bit.

4. Fasten the brass rod in a vise and use a hack saw to cut the rod into three 1.5 inch screws. Insert the screws through the holes in the triangle. Slip the springs (available at hardware stores or hobby shops) over the screws. Put Super Glue on the tops of the screws and place the Masonite on top. Be careful to line this up properly. Thread the nuts on the bottom of the screws.

5. Center the mirror on the Masonite and secure it with a length of electrical tape that wraps around the edges of the Masonite and the mirror. When you look at the mirror cell from the side, the tape will form a solid black bland. The tape will allow you to easily remove the mirror to clean or polish it.

6. Adjust the height and the angle of the mirror cell by moving the nuts up and down the screws. The spring will provide the resistance that will keep the mirror steady.

Tips & Warnings

• The size of the mirror is determined by the size of the telescope you are making; the triangle must be in proportion to the mirror cell. However, when making a mirror cell that is larger than eight inches in diameter, it would be better to use a different method for designing a mirror cell. Creating the steady platform is imperative and would require another method to achieve this goal when working with a larger mirror.

References

• NASA: Telescope History
• Webster Telescopes: Telescope Mirror Cell
• Schmidling: Mirror Cell, Telescope