There are not really many cases where a 5 point cell is preferable to a 6 point cell. I present them here mainly to promote the idea that the old sequence, 3, 9, 18, 27, is not the last word. I particularly want to steer atm's toward the 6 and 12 point designs as alternatives to 9 point cells.
The 5-point cells described here all use five support points arranged equidistant around a circle. Four of the points are located on pivoted bars. Each bar's pivot point is midway between the two mirror support points.
Diameter = Mirror Blank Diameter in Inches
Thickness = Mirror Blank Edge Thickness in Inches
f/ = Focal Length in Inches / Diameter
Obs. Dia. = Central Obstruction Diameter in Inches (This is usually the diagonal mirror minor axis.)
r1 = Radius of circle on which bar pivot point lies as a fraction of mirror blank radius.
d = Distance separating the two support points on the bar as a fraction of mirror blank radius.
r2 = Radius of circle on which support points lie as a fraction of mirror blank radius
P-V = Peak to Valley Mirror Deformation with this cell in Waves (1 wave = 500nm)
RMS = Root Mean Square Mirror Deformation with this cell in Waves (1 wave = 500nm)
| Pyrex | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Diameter | Thickness | f/ | Obs. Dia. | r1 | d | r2 | P-V | RMS | Focus Shift |
| 12 | 1 | 5.3 | 2.36 | 0.465 | 0.646 | 0.575 | 1/38 | 1/198 | -0.046mm |
| Plate Glass | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Diameter | Thickness | f/ | Obs. Dia. | r1 | d | r2 | P-V | RMS | Focus Shift |
| 8 | 0.75 | 6 | 1.52 | 0.460 | 0.669 | 0.568 | 1/84 | 1/465 | -0.024mm |
© 2003 Mark D. Holm