Construction of a telescope:

Glossary

Sagitta (the)

[French : la flèche]. The sagitta of a concave mirror is the difference in height between the edge and the center of the mirror (ie. its depth). It can be determined by the formula D²/8R where R is the curvature radius and D is the diameter.

Silicon carbide

[French : carbure de silicium]. Chemical formula : SiC. Its natural mineral is rare. The synthetic form is called Carborundum (trade mark) is used as an abrasive thanks to hardness.

Wet (a)

[French : une séchée]. When grinding the tool against the blank glass disk, with a wet abrasive (carbo) in between, the grains of the abrasive get eroded, mix with particles of glass, and eventually form a grey mud that tends to dry out. At this point, the process of grinding is stopped in order to rince both tool and mirror and replace the abrasive. This is called a wet.