The Mirror Cell 2

The mirror must not be pinched in its cell, as even slight flexure may distort its figure. But if not secured in some manner, it is free to rotate, and occasionally does so from being jarred, throwing its axis out of alignment. To prevent this, a small hole into which the point of a nail or screw may fit should be drilled into the back of the mirror. A nail can be used for a drill. Cut the head off squarely, and insert the pointed end in the drill press. A hole about 1/16" deep, drilled with No. 120 carbo and water, will be ample. It can be seen from Fig. 57 how the mirror is then anchored.

Finally, blacken all interior parts of the cell with a flat black enamel. The cell is held in the tube by the three 1" screws, entering triangle b through the tube sides.
To bring the holes in the tube in the same plane, wrap a large sheet of paper around it with the paper's straight edge overlapping in that plane, and mark
off the locations of the holes there, 120° apart.

After the holes in the tube are made, drill a hole in one corner of the wooden
triangle and insert a screw into it through the tube. Steadying the assembly with one hand, bring a second corner opposite one of the other holes in the tube, and mark through the hole on a center line which has previously been inscribed on the triangle.

On removing the triangle, this second hole is now drilled, after which the triangle is replaced in the tube, engaged by two screws this time, and the third hole spotted and similarly drilled. In this way, the holes in the tube are transferred accurately to the corners of the triangle.

A refinement which is not essential will provide a much more stable and durable bearing area. After the holes are located in the triangle, drill them out to 1/4" size; then insert short lengths of metal rod of that diameter which have first been centrally bored out and tapped for No. 10 screws.

A word of caution is necessary about the making of the wooden triangles. Be very precise about cutting off the corners to the exact diameter, and be sure they are kept concentric, or the mirror cannot be centered in the tube and the efforts to simplify alignment will have been wasted. For easy collimation of the mirror's axis,
the cell should be oriented in the tube as shown in Fig. 9, left.

Next- The Tube